Document and Entity Information (USD $)	12 Months Ended
Document and Entity Information (USD $)	Dec. 31, 2012	Feb. 28, 2013	Jun. 30, 2012
Document and Entity Information
Entity Registrant Name	MERRIMACK PHARMACEUTICALS INC
Entity Central Index Key	0001274792
Document Type	10-K
Document Period End Date	Dec. 31, 2012
Amendment Flag	false
Current Fiscal Year End Date	--12-31
Entity Well-known Seasoned Issuer	No
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Filer Category	Non-accelerated Filer
Entity Public Float			$ 630,064,672
Entity Common Stock, Shares Outstanding		95,901,025
Document Fiscal Year Focus	2012
Document Fiscal Period Focus	FY

Consolidated Balance Sheets (USD $) In Thousands, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Current assets:
Cash and cash equivalents	$ 37,714	$ 50,454
Available-for-sale securities	72,238
Restricted cash	100
Accounts receivable	9,267	7,426
Deferred financing costs	32	1,946
Prepaid expenses and other current assets	8,950	5,763
Total current assets	128,301	65,589
Restricted cash	528	381
Property and equipment, net	6,297	6,206
Other assets	1,068	23
Intangible assets, net	2,165	2,485
In-process research and development	7,010	7,010
Goodwill	3,605	3,605
Total assets	148,974	85,299
Current liabilities:
Accounts payable, accrued expenses and other	24,936	17,511
Capital lease obligations		48
Deferred revenues	9,350	7,712
Deferred rent	1,153	125
Deferred tax incentives	512	755
Derivative liability	196
Loans payable	2,373
Total current liabilities	38,520	26,151
Deferred revenues	71,114	78,033
Deferred rent	6,323	23
Deferred tax incentives	755	1,267
Convertible preferred stock warrants		1,516
Loans payable	37,482
Accrued interest	1,200
Total liabilities	155,394	106,990
Commitments and contingencies (Note 19)
Convertible preferred stock		268,225
Non-controlling interest	97	574
Stockholders' deficit:
Preferred stock, $0.01 par value: no shares and 10,000 shares authorized at December 31 2011 and 2012, respectively; no shares issued or outstanding at December 31, 2011 or 2012
Common stock, $0.01 par value: 138,500 and 200,000 shares authorized at December 31, 2011 and 2012, respectively, 11,834 and 95,825 issued and outstanding at December 31, 2011 and 2012, respectively	958	118
Additional paid-in capital	434,679	60,231
Accumulated other comprehensive loss	(38)
Accumulated deficit	(442,116)	(350,839)
Total stockholders' deficit	(6,517)	(290,490)
Total liabilities, convertible preferred stock, non-controlling interest and stockholders' deficit	$ 148,974	$ 85,299

Consolidated Balance Sheets (Parenthetical) (USD $)	Dec. 31, 2012	Dec. 31, 2011
Consolidated Balance Sheets
Preferred stock, par value (in dollars per share)	$ 0.01	$ 0.01
Preferred stock, shares authorized	10,000,000	10,000,000
Preferred stock, shares issued	0	0
Preferred stock, shares outstanding	0	0
Common stock, par value (in dollars per share)	$ 0.01	$ 0.01
Common stock, shares authorized	200,000,000	138,500,000
Common stock, shares issued	95,825,000	11,834,000
Common stock, shares outstanding	95,825,000	11,834,000

Number of shares of common stock outstanding. Common stock represent the ownership interest in a corporation.

+ References+ Details

- Definition

Face amount or stated value per share of nonredeemable preferred stock (or preferred stock redeemable solely at the option of the issuer); generally not indicative of the fair market value per share.

+ References+ Details

Consolidated Statements of Comprehensive Loss (USD $) In Thousands, except Per Share data, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Consolidated Statements of Comprehensive Loss
Collaboration revenues	$ 48,921	$ 34,215	$ 20,305
Operating expenses
Research and development	125,858	100,630	58,278
General and administrative	15,805	14,454	11,381
Contingent consideration			(178)
Total operating expenses	141,663	115,084	69,481
Loss from operations	(92,742)	(80,869)	(49,176)
Other income and expenses
Interest income	184	56	74
Interest expense	(553)	(13)	(3,726)
Other, net	1,357	1,150	2,669
Net loss	(91,754)	(79,676)	(50,159)
Less net loss attributable to non-controlling interest	(477)	(453)	(55)
Net loss attributable to Merrimack Pharmaceuticals, Inc.	(91,277)	(79,223)	(50,104)
Other comprehensive loss:
Unrealized loss on available-for-sale securities	(38)
Other comprehensive loss	(38)
Comprehensive loss	$ (91,315)	$ (79,223)	$ (50,104)
Net loss per share available to common stockholders-basic and diluted (in dollars per share)	$ (1.28)	$ (7.67)	$ (5.57)
Weighted-average common shares used in computing net loss per share available to common stockholders-basic and diluted (in shares)	72,831	11,343	10,994

Consolidated Statements of Convertible Preferred Stock, Non-controlling Interest and Stockholders' (Deficit) Equity (USD $)	Total	Common stock	Additional paid-in capital	Accumulated Other Comprehensive Loss	Accumulated deficit
Balance at Dec. 31, 2009	$ (190,762,000)	$ 17,364,000	$ 13,386,000		$ (221,512,000)
Balance (in shares) at Dec. 31, 2009		10,868,000
Increase (decrease) in stockholders' equity
Exercise of stock options	294,000	294,000
Exercise of stock options (in shares)		205,000
Stock-based compensation	4,551,000		4,551,000
Series F amount interest	12,974,000		12,974,000
Change in par value		(17,547,000)	17,547,000
Ownership change in non-controlling interest	3,083,000		3,083,000
Loss attributable to non-controlling interest	55,000				55,000
Net loss	(50,159,000)				(50,159,000)
Balance at Dec. 31, 2010	(219,964,000)	111,000	51,541,000		(271,616,000)
Balance ( in shares ) at Dec. 31, 2010		11,073,000
Increase (decrease) in stockholders' equity
Exercise of stock options	1,029,000	4,000	1,025,000
Exercise of stock options (in shares)		467,000
Exercise of common stock warrants	716,000	3,000	713,000
Exercise of common stock warrants (in shares)		294,000
Stock-based compensation	6,952,000		6,952,000
Loss attributable to non-controlling interest	453,000				453,000
Net loss	(79,676,000)				(79,676,000)
Balance at Dec. 31, 2011	(290,490,000)	118,000	60,231,000		(350,839,000)
Balance ( in shares ) at Dec. 31, 2011		11,834,000
Increase (decrease) in stockholders' equity
Exercise of stock options	5,400,000	26,000	5,374,000
Exercise of stock options (in shares)	2,622,000	2,622,000
Exercise of common stock warrants	27,000	1,000	26,000
Exercise of common stock warrants (in shares)		71,000
Stock-based compensation	6,889,000		6,889,000
Conversion of convertible preferred stock into common stock	268,225,000	663,000	267,562,000
Conversion of convertible preferred stock into common stock (in shares)		66,256,000
Initial public offering, net of offering costs	98,081,000	150,000	97,931,000
Initial public offering, net of offering costs (in shares)		15,042,000
Series B dividends declared	(4,263,000)		(4,263,000)
Conversion of convertible preferred stock warrants to common stock warrants	929,000		929,000
Other comprehensive loss	(38,000)			(38,000)
Loss attributable to non-controlling interest	477,000				477,000
Net loss	(91,754,000)				(91,754,000)
Balance at Dec. 31, 2012	$ (6,517,000)	$ 958,000	$ 434,679,000	$ (38,000)	$ (442,116,000)
Balance ( in shares ) at Dec. 31, 2012		95,825,000

Consolidated Statements of Convertible Preferred Stock, Non-controlling Interest and Stockholders' (Deficit) Equity (Temporary) (USD $) In Thousands, except Share data, unless otherwise specified	Total USD ($)	Series C preferred stock	Series F Preferred Stock	Series G Preferred Stock	Temporary equity, parent Series B-G convertible preferred stock USD ($)	Temporary equity, parent Series C preferred stock USD ($)	Temporary equity, parent Series F Preferred Stock USD ($)	Temporary equity, parent Series G Preferred Stock USD ($)	Temporary equity, non-controlling interest USD ($)
Balance at Dec. 31, 2009					$ 131,273
Balance (in shares) at Dec. 31, 2009					41,368,000
Temporary equity
Issuance of stock							59,973
Issuance of stock (in shares)							11,776,000
Issuance of stock as a result of warrant exercises						11
Issuance of stock as a result of warrant exercises (in shares)						4,000
Ownership change in non-controlling interest									1,082
Loss attributable to non-controlling interest									(55)
Balance at Dec. 31, 2010					191,257				1,027
Balance (in shares) at Dec. 31, 2010					53,148,000
Temporary equity
Issuance of stock								76,949
Issuance of stock (in shares)								11,000,000
Issuance of stock as a result of warrant exercises						19
Issuance of stock as a result of warrant exercises (in shares)						3,000
Loss attributable to non-controlling interest									(453)
Balance at Dec. 31, 2011					268,225				574
Balance (in shares) at Dec. 31, 2011	64,151	14,424	11,776	11,000	64,151,000
Temporary equity
Conversion of convertible preferred stock into common stock	268,225				(268,225)
Conversion of convertible preferred stock into common stock (in shares)					(64,151,000)
Loss attributable to non-controlling interest									(477)
Balance at Dec. 31, 2012									$ 97

Consolidated Statements of Cash Flows (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Cash flows from operating activities
Net loss	$ (91,754)	$ (79,676)	$ (50,159)
Adjustments to reconcile net loss to net cash (used in) provided by operating activities
Non-cash interest expense	78		3,673
Remeasurement of convertible preferred stock warrants	(587)	864	(104)
Depreciation and amortization	3,664	4,596	3,628
Stock-based compensation	6,889	6,952	4,551
Gain on disposal of property and equipment			(26)
Changes in operating assets and liabilities
Purchased premiums and interest on available-for-sale securities	(2,354)
Accounts receivable	(1,841)	(3,681)	(1,975)
Prepaid expenses and other current assets	(2,477)	(3,933)	(571)
Accounts payable, accrued expenses, and other	6,985	8,815	194
Deferred revenues	(5,281)	11,963	12,845
Deferred rent and tax incentives	7,892	1,264	1,567
Other assets and liabilities, net	(1,030)	19	8
Net cash used in operating activities	(79,816)	(52,817)	(26,369)
Cash flows from investing activities
Purchase of available-for-sale securities	(115,665)
Proceeds from maturities and sales of available-for-sale securities	43,880
Purchase of property and equipment	(3,189)	(3,754)	(5,025)
Proceeds from sale of property and equipment			26
Assignment of restricted cash	(628)
Release of restricted cash	381		95
Other investing activities, net		7	4
Net cash used in investing activities	(75,221)	(3,747)	(4,900)
Cash flows from financing activities
Proceeds from initial public offering, net of offering costs	100,025
Proceeds from issuance of convertible preferred stock, net of offering costs		76,949
Proceeds from issuance of common stock	5,427	1,745	294
Principal payment on capital lease obligations	(48)	(443)	(864)
Proceeds from issuance of loans payable, net of issuance costs	41,128
Payments of dividends on Series B convertible preferred stock	(4,235)
Proceeds from issuance of convertible preferred stock of Silver Creek Pharmaceuticals, Inc.			4,165
Deferred financing costs		(1,946)
Net cash provided by financing activities	142,297	76,305	3,595
Net (decrease) increase in cash and cash equivalents	(12,740)	19,741	(27,674)
Cash and cash equivalents, beginning of period	50,454	30,713	58,387
Cash and cash equivalents, end of period	37,714	50,454	30,713
Noncash financing and investing activities
Accrued interest on Series F amount relieved to additional paid-in capital (Note 14)			12,974
Issuance of shares from Series F amount (Note 14)			59,973
Conversion of convertible preferred stock to common stock	268,225
Conversion of convertible preferred stock warrants to common stock warrants	929
Issuance of derivative liability	196
Changes in property and equipment in accounts payable and accrued expenses	412
Disposals of fully depreciated assets	671
Reclassification of deferred financing costs to stockholders' equity	2,748		25
Dividends on Series B convertible preferred stock declared but not paid	28
Supplemental disclosure of cash flows
Cash paid for interest	$ 169	$ 13	$ 55

Nature of the Business	12 Months Ended
Nature of the Business	Dec. 31, 2012
Nature of the Business
Nature of the Business	1. Nature of the Business Merrimack Pharmaceuticals, Inc. (the "Company") is a biopharmaceutical company discovering, developing and preparing to commercialize innovative medicines consisting of novel therapeutics paired with companion diagnostics. The Company has six targeted therapeutic oncology candidates in clinical development (MM-398, MM-121, MM-111, MM-302, MM-151 and MM-141), multiple product candidates in preclinical development and a discovery effort advancing additional candidate medicines. The Company's discovery and development effort is driven by Network Biology, which is its proprietary systems biology-based approach to biomedical research. The Company was incorporated in the Commonwealth of Massachusetts in 1993 and reincorporated in the State of Delaware in October 2010. The Company is subject to risks and uncertainties common to companies in the biopharmaceutical industry, including, but not limited to, ability to secure additional capital to fund operations, development by competitors of new technological innovations, dependence on collaborative arrangements, protection of proprietary technology, compliance with government regulations and dependence on key personnel. Product candidates currently under development will require significant additional research and development efforts, including extensive preclinical and clinical testing and regulatory approval prior to commercialization. These efforts require significant amounts of additional capital, adequate personnel infrastructure and extensive compliance reporting capabilities. The Company has incurred significant losses and has not generated revenue from commercial sales. The accompanying consolidated financial statements have been prepared on a basis which assumes that the Company will continue as a going concern and which contemplates the realization of assets and satisfaction of liabilities and commitments in the normal course of business. In April 2012, the Company closed the initial public offering of its common stock pursuant to a registration statement on Form S-1, as amended. The Company sold an aggregate of 15,042,459 shares of common stock under the registration statement at a public offering price of $7.00 per share, including 742,459 shares pursuant to the exercise by the underwriters of an over-allotment option. Net proceeds were approximately $98.1 million, after deducting underwriting discounts and commissions and other offering expenses but prior to the payment of dividends on the Company's Series B convertible preferred stock. At the time of the initial public offering, the Company's convertible preferred stock and warrants to purchase convertible preferred stock automatically converted to common stock and warrants to purchase common stock, as discussed more fully in Note 3. On November 8, 2012, the Company entered into a Loan and Security Agreement (the "Loan Agreement"), with Hercules Technology Growth Capital, Inc. ("Hercules"). As more fully discussed in Note 12, the Loan Agreement provides for an initial term loan advance of $25.0 million, which closed on November 8, 2012, and an additional term loan of $15.0 million, which closed on December 14, 2012, which resulted in aggregate net proceeds of $39.6 million during the fourth quarter of 2012. The Company may seek additional funding through public or private debt or equity financings or through existing or new collaboration arrangements. The Company may not be able to obtain financing on acceptable terms, or at all, and the Company may not be able to enter into additional collaborative arrangements. The terms of any financing may adversely affect the holdings or the rights of the Company's stockholders. Arrangements with collaborators or others may require the Company to relinquish rights to certain of its technologies or product candidates. If the Company is unable to obtain funding, the Company could be forced to delay, reduce or eliminate its research and development programs or commercialization efforts, which could adversely affect its business prospects.

Summary of Significant Accounting Policies

12 Months Ended

Dec. 31, 2012

Summary of Significant Accounting Policies

Summary of Significant Accounting Policies

2. Summary of Significant Accounting Policies

Significant accounting policies followed by the Company in the preparation of its consolidated financial statements are as follows:

Basis of Presentation and Consolidation

The accompanying consolidated financial statements have been prepared under U.S. generally accepted accounting principles ("GAAP") and include the accounts of the Company and its wholly owned subsidiaries. The Company's wholly owned subsidiaries include Hermes BioSciences, Inc. ("Hermes"), which was merged with and into the Company during 2011, and Merrimack Pharmaceuticals (Bermuda) Ltd., which was incorporated during 2011. The Company also consolidates its 74% majority-owned subsidiary Silver Creek Pharmaceuticals, Inc. ("Silver Creek"). All intercompany transactions and balances have been eliminated in consolidation.

Use of Estimates

GAAP requires the Company's management to make estimates and judgments that may affect the reported amounts of assets, liabilities, revenues, expenses and related disclosures. The Company bases estimates and judgments on historical experience and on various other factors that it believes to be reasonable under the circumstances. The significant estimates in these consolidated financial statements include revenue recognition, periods of meaningful use of licensed products, lease accounting, useful lives with respect to long-lived assets and intangibles and the valuation of stock options, convertible preferred stock warrants, contingencies, accrued expenses, intangible assets, goodwill, in-process research and development, derivative liability and tax valuation reserves. The Company's actual results may differ from these estimates under different assumptions or conditions. The Company evaluates its estimates on an ongoing basis. Changes in estimates are reflected in reported results in the period in which they become known by the Company's management.

Segment and Geographic Information

Operating segments are defined as components of an enterprise engaging in business activities for which discrete financial information is available and regularly reviewed by the chief operating decision maker in deciding how to allocate resources and in assessing performance. The Company views its operations and manages its business in one operating segment and the Company operates in only one geographic segment.

Cash, Cash Equivalents and Restricted Cash

Cash and cash equivalents are short-term, highly liquid investments with original maturities of three months or less at the date of purchase. Investments qualifying as cash equivalents primarily consist of money market funds, commercial paper, corporate notes and bonds and certificates of deposit.

Cash accounts with any type of restriction are classified as restricted cash. If restrictions are expected to be lifted in the next twelve months, the restricted cash account is classified as current. As of December 31, 2011 and 2012, the Company recorded restricted cash of $381,000 and $628,000, respectively, which were primarily related to the Company's facility lease.

Available-for-Sale Securities

Marketable securities may consist of U.S. government agencies securities, commercial paper, corporate notes and bonds and certificates of deposit, which are maintained by an investment manager and have expected average maturity dates in excess of three months. The Company classifies these investments as available-for-sale. Available-for-sale securities are carried at fair value, with the unrealized gains and losses included in other comprehensive income (loss) as a component of stockholders' equity until realized. Realized gains and losses are recognized in interest income. Any premium or discount arising at purchase is amortized and/or accreted to interest income.

Concentration of Credit Risk

Financial instruments that subject the Company to credit risk consist primarily of cash and cash equivalents, available-for-sale securities and accounts receivable. The Company places its cash deposits in accredited financial institutions and, therefore, the Company's management believes these funds are subject to minimal credit risk. The Company invests cash equivalents and available-for-sale securities in money market funds, U.S. government agencies securities and various corporate debt securities. Credit risk in these securities is reduced as a result of the Company's investment policy to limit the amount invested in any one issue or any single issuer and to only invest in high credit quality securities. The Company has no significant off-balance sheet concentrations of credit risk such as foreign currency exchange contracts, option contracts or other hedging arrangements. For the years ended December 31, 2011 and 2012, Sanofi represented greater than 99% and 98% of collaboration revenues, respectively. As of December 31, 2011 and 2012, Sanofi represented greater than 99% of accounts receivable.

Property and Equipment

Property and equipment, including leasehold improvements, are recorded at cost and depreciated when placed into service using the straight-line method, based on their estimated useful lives as follows:

Asset classification		Estimated useful life (in years)

Lab equipment		3 - 7
IT equipment		3 - 7
Leaseholds improvements		Lesser of useful life or lease term
Furniture and fixtures		3 - 7

Costs for capital assets not yet placed into service have been capitalized as construction-in-progress and will be depreciated in accordance with the above guidelines once placed into service. Costs for repairs and maintenance are expensed as incurred, while major betterments are capitalized. The Company capitalizes interest cost incurred on funds used to construct property and equipment. The capitalized interest is recorded as part of the asset to which it relates and is depreciated over the asset's estimated useful life. Upon retirement or sale, the cost of assets disposed of and the related accumulated depreciation are removed from the accounts and any resulting gain or loss is reflected in earnings.

The Company reviews its long-lived assets for impairment whenever events or changes in business circumstances indicate that the carrying amount of assets may not be fully recoverable or that the useful lives of these assets are no longer appropriate. Each impairment test is based on a comparison of the undiscounted cash flow to the recorded value of the asset. If impairment is indicated, the asset will be written down to its estimated fair value on a discounted cash flow basis.

Non-controlling Interest

Non-controlling interest represents the non-controlling stockholders' proportionate share of preferred stock and net loss of the Company's majority-owned consolidated subsidiary Silver Creek. On August 20, 2010, the Company acquired a controlling interest in Silver Creek (Note 8). The non-controlling stockholders' proportionate share of the preferred stock in Silver Creek is reflected as non-controlling interest in the Company's consolidated balance sheets as of December 31, 2011 and 2012, respectively, as a component of mezzanine equity.

The Company's financial statement activity related to Silver Creek during these periods was as follows:

(in thousands)	Non-controlling Interest

Balance at December 31, 2009	$	—
Acquisition of Silver Creek preferred stock		1,082
Net loss attributable to Silver Creek		(55	)

Balance at December 31, 2010	$	1,027
Net loss attributable to Silver Creek		(453	)

Balance at December 31, 2011	$	574
Net loss attributable to Silver Creek		(477	)

Balance at December 31, 2012	$	97

Derivative Liability

On December 21, 2012, the Company's majority-owned subsidiary Silver Creek entered into a Note Purchase Agreement with certain lenders, as discussed more fully in Note 12. The principal and accrued interest are convertible into the next qualifying series of preferred stock at a discount or into existing preferred stock upon maturity of the notes on December 31, 2013, whichever occurs first. The Company determined that the underlying convertible note represented share-settled debt and the potential conversion of the notes into the next qualifying series of preferred stock at a discount met the definition of a derivative. The Company estimated the value of the derivative liability issued in connection with the convertible note payable at $196,000 as of both December 21, 2012 and December 31, 2012. The derivative is classified as a liability on the Company's consolidated balance sheet and will be remeasured at subsequent reporting periods with changes in fair value are recognized in earnings.

Revenue Recognition

The Company enters into biopharmaceutical product development agreements with collaborative partners for the research and development of therapeutic and diagnostic products. The terms of the agreements may include nonrefundable signing and licensing fees, funding for research, development and manufacturing, milestone payments and royalties on any product sales derived from collaborations. These multiple element arrangements are analyzed to determine whether the deliverables can be separated or whether they must be accounted for as a single unit of accounting.

In January 2011, the Company adopted new authoritative guidance on revenue recognition for multiple element arrangements. This guidance, which applies to multiple element arrangements entered into or materially modified on or after January 1, 2011, amends the criteria for separating and allocating consideration in a multiple element arrangement by modifying the fair value requirements for revenue recognition and eliminating the use of the residual method. The fair value of deliverables under the arrangement may be derived using a best estimate of selling price if vendor specific objective evidence and third party evidence are not available. Deliverables under the arrangement will be separate units of accounting provided that a delivered item has value to the customer on a stand-alone basis and if the arrangement does not include a general right of return relative to the delivered item and delivery or performance of the undelivered item is considered probable and substantially in the control of the vendor. The Company also adopted guidance that permits the recognition of revenue contingent upon the achievement of a milestone in its entirety, in the period in which the milestone is achieved, only if the milestone meets certain criteria and is considered to be substantive. The Company did not enter into any significant multiple element arrangements or materially modify any of its existing multiple element arrangements during the years ended December 31, 2011 and 2012. The Company's existing license and collaboration agreements continue to be accounted for under previously issued revenue recognition guidance for multiple element arrangements and milestone revenue recognition, as described below.

The Company recognized upfront license payments as revenue upon delivery of the license only if the license had stand-alone value and the fair value of the undelivered performance obligations could be determined. If the fair value of the undelivered performance obligations could be determined, such obligations were accounted for separately as the obligations were fulfilled. If the license was considered to either not have stand-alone value or have stand-alone value but the fair value of any of the undelivered performance obligations could not be determined, the arrangement was accounted for as a single unit of accounting and the license payments and payments for performance obligations were recognized as revenue over the estimated period of when the performance obligations would be performed.

Whenever the Company determined that an arrangement should be accounted for as a single unit of accounting, it determined the period over which the performance obligations would be performed and revenue would be recognized. If the Company could not reasonably estimate the timing and the level of effort to complete its performance obligations under the arrangement, then revenue under the arrangement was recognized on a straight-line basis over the period the Company expected to complete its performance obligations, which is reassessed at each subsequent reporting period.

The Company's collaboration agreements may include additional payments upon the achievement of performance-based milestones. As milestones are achieved, a portion of the milestone payment, equal to the percentage of the total time that the Company has performed the performance obligations to date over the total estimated time to complete the performance obligations, multiplied by the amount of the milestone payment, will be recognized as revenue upon achievement of such milestone. The remaining portion of the milestone will be recognized over the remaining performance period. Milestones that are tied to regulatory approvals are not considered probable of being achieved until such approval is received. Milestones tied to counter-party performance are not included in the Company's revenue model until the performance conditions are met.

Royalty revenue will be recognized upon the sale of the related products provided the Company has no remaining performance obligations under the arrangement.

Research and Development Expenses

Research and development expenses are charged to expense as incurred. Research and development expenses comprise costs incurred in performing research and development activities, including personnel-related costs, stock-based compensation, facilities, research-related overhead, clinical trial costs, contracted services, manufacturing, license fees and other external costs. The Company accounts for nonrefundable advance payments for goods and services that will be used in future research and development activities as expenses when the service has been performed or when the goods have been received rather than when the payment is made.

Stock-Based Compensation

The Company expenses the fair value of employee stock options over the vesting period. Compensation expense is measured using the fair value of the award at the grant date, net of estimated forfeitures, and is adjusted annually to reflect actual forfeitures. The fair value of each stock-based award is estimated using the Black-Scholes option valuation model and is expensed straight-line over the vesting period.

The Company records stock options issued to nonemployees at fair value, periodically remeasures to reflect the current fair value at each reporting period, and recognizes expense over the related service period. When applicable, these equity instruments are accounted for based on the fair value of the consideration received or the fair value of the equity instrument issued, whichever is more reliably measurable.

Convertible Preferred Stock and Convertible Preferred Stock Warrants

Convertible preferred stock is initially recorded at the proceeds received, net of issuance costs and warrants, where applicable. As described in Note 3, in April 2012, the Company closed the initial public offering of its common stock. Upon closing, all outstanding shares of the Company's convertible preferred stock were converted into 66,255,529 shares of common stock. Also upon closing, the Company's restated certificate of incorporation became effective and authorized 10.0 million shares of $0.01 par value undesignated preferred stock.

The Company accounts for freestanding warrants as liabilities at their fair value. The Company measures the fair value of the convertible preferred stock warrants at the end of each reporting period and records the change in fair value to other income (expense). For the years ended December 31, 2010, 2011, and 2012, the Company recorded other income (expense) related to this re-measurement of $(74,000), $(864,000) and $587,000, respectively. As described in Note 3, in April 2012, the Company closed the initial public offering of its common stock. Upon closing, all outstanding warrants to purchase shares of convertible preferred stock were converted into warrants to purchase shares of common stock and reclassified to stockholders' equity.

Comprehensive Loss

Comprehensive loss is defined as the change in equity of a business enterprise during a period from transactions, and other events and circumstances, from non-owner sources and currently consists of net loss and changes in unrealized gains and losses on available-for-sale securities.

Other Income (Expense)

The Company records gains and losses on the remeasurement of fair value of convertible preferred stock warrants, the recognition of federal and state sponsored tax incentives and other one-time income or expense-related items in other income (expense).

In January 2010, the Massachusetts Life Sciences Center ("MLSC"), an independent agency of the Commonwealth of Massachusetts, awarded the Company $1,500,000 of tax incentives under its Life Sciences Tax Incentive Program. These incentives allowed the Company to monetize approximately $1,350,000 of state research and development tax credits. The Company received this monetization in 2010. In exchange for these incentives, the Company pledged to hire an incremental 50 employees and retain these employees until at least December 31, 2014. Failure to do so could result in the repayment of a portion of these incentives. The Company deferred and is amortizing the benefit of this monetization on a straight-line basis over the five-year performance period, with a cumulative catch-up in the period the pledge is achieved. For the years ended December 31, 2010, 2011 and 2012, the Company recognized $270,000 of benefit in other income in each period.

In October 2010, the Company received grants totaling $2,445,000 under the Federal Qualifying Therapeutic Discovery Projects program as provided for under Section 48D of the Internal Revenue Code, enacted as part of the Patient Protection and Affordable Care Act of 2010. The Company received $1,941,000 during 2010 and $504,000 during the first quarter of 2011 related to these grants. For the year ended December 31, 2010, the Company recognized $2,445,000 as other income related to these grants.

In January 2011, the MLSC awarded the Company an additional $1,347,000 of tax incentives under its Life Sciences Tax Incentive Program, which allowed the Company to monetize approximately $1,212,000 of state research and development tax credits. The Company received this monetization in the second quarter of 2011. In exchange for these incentives, the Company pledged to hire an incremental 50 employees and retain these employees until at least December 31, 2015. Failure to do so could result in the repayment of a portion of these incentives. The Company deferred and is amortizing the benefit of this monetization on a straight-line basis over the five-year performance period, with a cumulative catch-up in the period the pledge is achieved. For the years ended December 31, 2010 and 2011, the Company did not recognize any benefit in other income. For the year ended December 31, 2012, the Company recognized $484,000 of benefit in other income.

Additionally, other income recognized during the year ended December 31, 2011 included the impact of a cash settlement of $1.8 million from a former service provider.

Deferred Financing Costs

The Company capitalizes certain legal, accounting and other fees that are directly associated with in-process debt and equity financings as current assets until such financings occur. In the case of an equity financing, after occurrence, these costs are recorded in equity or mezzanine equity, net of proceeds received. In the case of a debt financing, these costs are recorded as assets and amortized over the term of the debt.

As of December 31, 2011, the Company recorded deferred financing costs of $1,946,000 on the accompanying consolidated balance sheet in contemplation of an initial public offering. As discussed in Note 3, in April 2012, the Company closed the initial public offering of its common stock. Upon closing, $2,748,000 of deferred financing costs were netted against the equity proceeds within stockholders' equity.

Income Taxes

The Company accounts for income taxes under the asset and liability method. Under this method, deferred tax assets and liabilities are recognized for the estimated future tax consequences attributable to differences between financial statement carrying amounts of existing assets and liabilities and their respective tax bases. Deferred tax assets and liabilities are measured using enacted rates in effect for the year in which these temporary differences are expected to be recovered or settled. Valuation allowances are provided if based on the weight of available evidence, it is more likely than not that some or all of the deferred tax assets will not be realized.

The Company provides reserves for potential payments of tax to various tax authorities related to uncertain tax positions and other issues. Reserves are based on a determination of whether and how much of a tax benefit taken by the Company in its tax filing is more likely than not to be realized following resolution of any potential contingencies present related to the tax benefit. Potential interest and penalties associated with such uncertain tax positions are recorded as components of income tax expense. To date, the Company has not taken any uncertain tax positions or recorded any reserves, interest or penalties.

Goodwill and Intangible Assets

Goodwill and indefinite-lived intangible assets, including in-process research and development ("IPR&D"), are evaluated for impairment on an annual basis or more frequently if an indicator of impairment is present. No impairment of goodwill or indefinite-lived intangible assets resulted from the Company's most recent evaluation which occurred in the third quarter of 2012. The Company's next annual impairment evaluation will be made in the third quarter of 2013 unless indicators arise that would require the Company to evaluate at an earlier date. The Company commences amortization of indefinite-lived intangible assets once the assets have reached technological feasibility or are determined to have an alternative future use and amortizes the assets over their estimated future life.

The Company's evaluation of goodwill and IPR&D impairment included a qualitative assessment to determine whether further impairment testing of goodwill and indefinite-lived intangible assets was necessary. It was determined that it was not more likely than not that an impairment existed, and therefore, that further impairment evaluation was not necessary. This determination required management to make significant estimates, judgments and assumptions as to development activities and future commercial potential of IPR&D and to assess the impact of significant events, milestones and changes to expectations and activities that may have occurred since the last impairment evaluation. Specifically, management considered estimated time and cost until the expected commencement of commercial activities, estimates of expected future revenues and cash flows, estimates of probabilities of success of the Company's IPR&D and discount rates. Significant changes to these estimates, judgments and assumptions could materially change the outcome of management's impairment assessment.

The Company commences amortization of indefinite-lived intangible assets, such as IPR&D, once the assets have reached technological feasibility or are determined to have an alternative future use and amortizes the assets over their estimated future life. Amortization of IPR&D has not commenced as of December 31, 2012.

Definite-lived intangible assets, such as core technology, are evaluated for impairment whenever events or circumstances indicate that the carrying value may not be fully recoverable. Definite-lived intangible assets are separate from goodwill and indefinite-lived intangible assets and are deemed to have a definite life. The Company amortizes these assets over their estimated useful life. The Company has not recorded any impairment charges related to definite-lived intangible assets.

Reclassifications

Certain prior year amounts have been reclassified to conform with the current year presentation.

Recent Accounting Pronouncements

In June 2011, the Financial Accounting Standards Board ("FASB") issued an amendment to the accounting guidance for presentation of comprehensive income. Under the amended guidance, a company may present the total of comprehensive income, the components of net income and the components of other comprehensive income either in a single continuous statement of comprehensive income or in two separate but consecutive statements. In either case, a company is required to present each component of net income along with total net income, each component of other comprehensive income along with a total for other comprehensive income and a total amount for comprehensive income. For public companies, the amendment is effective for fiscal years, and interim periods within those years, beginning after December 15, 2011, and shall be applied retrospectively. The Company adopted this amendment on January 1, 2012. Other than a change in presentation, the adoption of this guidance did not have a material impact on the Company's consolidated financial statements.

In September 2011, the FASB amended the authoritative guidance regarding the testing for goodwill impairment. Under the amendments, an entity has the option to first assess qualitative factors to determine whether the existence of events or circumstances leads to a determination that it is more likely than not that the fair value of a reporting unit is less than its carrying amount. If, after assessing the totality of events or circumstances, an entity determines it is not more likely than not that the fair value reporting of a reporting unit is less than the carrying amount, then performing the two-step impairment test is unnecessary. The changes are effective for annual and interim goodwill impairment tests performed for fiscal years beginning after December 15, 2011, however, early adoption is permitted. The Company adopted this authoritative guidance on January 1, 2012 with no impact.

In July 2012, the FASB issued ASU No. 2012-02, Testing Indefinite-Lived Intangible Assets for Impairment ("ASU 2012-02"). ASU 2012-02 is intended to reduce the cost and complexity of testing indefinite-lived intangible assets other than goodwill for impairment. It allows companies to perform a "qualitative" assessment to determine whether further impairment testing of indefinite-lived intangible assets is necessary, similar in approach to the goodwill impairment test. ASU 2012-02 is effective for annual and interim impairment tests performed for fiscal years beginning after September 15, 2012. Early adoption is permitted, and the Company adopted ASU 2012-02 in the third quarter of 2012 upon its annual impairment testing of indefinite-lived intangible assets with no impact.

In February 2013, the FASB issued amendments to the accounting guidance for presentation of comprehensive income to improve the reporting of reclassifications out of accumulated other comprehensive income. The amendments do not change the current requirements for reporting net income or other comprehensive income, but do require an entity to provide information about the amounts reclassified out of accumulated other comprehensive income by component. In addition, an entity is required to present, either on the face of the statement where the net income is presented or in the notes, significant amounts reclassified out of accumulated other comprehensive income by the respective line items of net income but only if the amount reclassified is required under GAAP to be reclassified to net income in its entirety in the same reporting period. For other amounts that are not required under GAAP to be reclassified in their entirety to net income, an entity is required to cross-reference to other disclosures required under GAAP that provide additional detail about these amounts. For public companies, these amendments are effective prospectively for reporting periods beginning after December 15, 2012. Other than a change in presentation, the Company does not believe the adoption of this guidance will have a material impact on our consolidated financial statements.

Initial Public Offering	12 Months Ended
Initial Public Offering	Dec. 31, 2012
Initial Public Offering
Initial Public Offering	3. Initial Public Offering In April 2012, the Company closed the initial public offering of its common stock pursuant to a registration statement on Form S-1, as amended. The Company sold an aggregate of 15,042,459 shares of common stock under the registration statement at a public offering price of $7.00 per share, including 742,459 shares pursuant to the exercise by the underwriters of an over-allotment option. Net proceeds were approximately $98.1 million, after deducting underwriting discounts and commissions and other offering expenses but prior to the payment of dividends on the Company's Series B convertible preferred stock. Upon closing the initial public offering, all outstanding shares of the Company's convertible preferred stock were converted into 66,255,529 shares of common stock, all outstanding warrants to purchase shares of convertible preferred stock were converted into warrants to purchase shares of common stock and approximately $4.3 million of cash dividends became payable to the holders of Series B convertible preferred stock.

Marketable Securities

12 Months Ended

Dec. 31, 2012

Marketable Securities

Marketable Securities

4. Marketable Securities

Available-for-sale securities, all of which have maturities of twelve months or less, as of December 31, 2012 consisted of the following:

	Amortized Cost		Unrealized Gains		Unrealized Losses			Fair Value

	(in thousands)
December 31, 2012:
Certificate of deposit	$	240	$	—	$	—		$	240
Commercial paper		12,479				(14	)		12,465
Corporate debt securities		59,557		3		(27	)		59,533

Total	$	72,276	$	3	$	(41	)	$	72,238

The aggregate fair value of securities held by the Company in an unrealized loss position for less than 12 months as of December 31, 2012 was $51.4 million, representing 18 securities. To determine whether an other-than-temporary impairment exists, the Company performs an analysis to assess whether it intends to sell, or whether it would more likely than not be required to sell, the security before the expected recovery of the amortized cost basis. Where the Company intends to sell a security, or may be required to do so, the security's decline in fair value is deemed to be other-than-temporary and the full amount of the unrealized loss is recognized on the statement of comprehensive loss as an other-than-temporary impairment charge. When this is not the case, the Company performs additional analysis on all securities to evaluate losses associated with the creditworthiness of the security. Credit losses are identified where the Company does not expect to receive cash flows, based on using a single best estimate, sufficient to recover the amortized cost basis of a security and amount of the loss recognized in other income (expense).

Marketable securities in an unrealized loss position as of December 31, 2012 consisted of the following:

	Aggregate Fair Value

	(in thousands)
December 31, 2012:
Commercial paper		12,465	(14	)
Corporate debt securities		38,899	(27	)

	$	51,364	(41	)

The Company does not intend to sell and it is not more likely than not that the Company will be required to sell the above investments before recovery of the amortized cost basis, which may be maturity. The Company determined that there was no material change in the credit risk of the above investments. As a result, the Company determined it did not hold any investments with an other-than-temporary-impairment as of December 31, 2012.

- Definition

The entire disclosure for investments in certain debt and equity securities.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-Glossary Debt Security

-URI http://asc.fasb.org/extlink&oid=6509901

Reference 2: http://www.xbrl.org/2003/role/presentationRef

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-Name Accounting Standards Codification

-Topic 310

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Reference 3: http://www.xbrl.org/2003/role/presentationRef

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Reference 4: http://www.xbrl.org/2003/role/presentationRef

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-URI http://asc.fasb.org/extlink&oid=6511694

Reference 10: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 115

-Paragraph 3, 19, 20, 21, 22, 137

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

+ Details

Net Loss Per Common Share

12 Months Ended

Dec. 31, 2012

Net Loss Per Common Share

Net Loss Per Common Share

5. Net Loss Per Common Share

Basic net loss per share is calculated by dividing the net loss available to common stockholders by the weighted-average number of common shares outstanding during the period, without consideration for common stock equivalents. Diluted net loss per share is computed by dividing the net loss available to common stockholders by the weighted-average number of common share equivalents outstanding for the period determined using the treasury-stock method. For purposes of this calculation, convertible preferred stock, stock options and warrants are considered to be common stock equivalents and are only included in the calculation of diluted net loss per share when their effect is dilutive.

The following table presents the computation of basic and diluted net loss per share available to common stockholders:

	Years ended December 31,

(in thousands, except per share amount)	2010			2011			2012
Net Loss Per Share:
Numerator:
Net loss attributable to Merrimack Pharmaceuticals, Inc.	$	(50,104	)	$	(79,223	)	$	(91,277	)
Plus: Unaccreted dividends on convertible preferred stock		(11,185	)		(7,789	)		(2,107	)

Net loss available to common stockholders—basic and diluted		(61,289	)		(87,012	)		(93,384	)
Denominator:
Weighted-average common shares—basic and diluted		10,994			11,343			72,831

Net loss per share available to common stockholders—basic and diluted	$	(5.57	)	$	(7.67	)	$	(1.28	)

The following common stock equivalents of potentially dilutive securities have been excluded from the computation of diluted weighted average shares outstanding as of December 31, 2010, 2011 and 2012 as the Company recorded a net loss in all periods and, therefore, they would be anti-dilutive:

	Years ended December 31,

(in thousands)	2010		2011		2012
Convertible preferred stock		55,253		66,256		—
Options to purchase common stock		16,214		17,617		18,066
Convertible preferred stock warrants		306		302		—
Common stock warrants		2,937		2,640		2,842

- Definition

The entire disclosure for earnings per share.

+ References+ Details

License and Collaboration Agreements

12 Months Ended

Dec. 31, 2012

License and Collaboration Agreements

License and Collaboration Agreements

6. License and Collaboration Agreements

Sanofi

On September 30, 2009, the Company entered into a license and collaboration agreement with Sanofi for the development and commercialization of a drug candidate being developed by the Company under the name MM-121. The agreement became effective on November 10, 2009 and Sanofi paid the Company a nonrefundable, noncreditable upfront license fee of $60.0 million. During the years ended 2010, 2011 and 2012, the Company received milestone payments of $10.0 million, $10.0 million and $5.0 million, respectively. These milestone payments were associated with dosing the first patients in Phase 2 clinical trials in breast, non-small cell lung and ovarian cancers. The Company is eligible to receive additional future development, regulatory and sales milestone payments as well as future royalty payments depending on the success of MM-121.

Under the agreement, Sanofi is responsible for all MM-121 development and manufacturing costs. The Company has the right, but not the obligation, to co-promote and commercialize MM-121 in the United States and to participate in the development of MM-121 through Phase 2 proof of concept trials. Sanofi reimburses the Company for direct costs incurred in development and compensates the Company for its internal development efforts based on a full time equivalent ("FTE") rate. Also as part of the agreement, the Company was required to manufacture certain quantities of MM-121 and, at Sanofi's and the Company's option, may continue to manufacture additional quantities of MM-121 in the future. Sanofi reimburses the Company for direct costs incurred in manufacturing and compensates the Company for its internal manufacturing efforts based on an FTE rate. The Company satisfied its manufacturing obligations during 2010 and has elected to continue to manufacture quantities of MM-121.

The Company applied revenue recognition guidance to determine whether the performance obligations under this collaboration including the license, the right to future technology, back-up compounds, participation on steering committees, development services and manufacturing services could be accounted for separately or as a single unit of accounting. The Company determined that its development services performance obligation is considered a separate unit of accounting as it is set at the Company's option, has stand-alone value and the FTE rate is considered fair value. Therefore, the Company recognizes cost reimbursements for MM-121 development services as incurred. The Company determined that the license, the right to future technology, back-up compounds, participation on steering committees and manufacturing services performance obligations represented a single unit of accounting. As the Company cannot reasonably estimate its level of effort over the collaboration, the Company recognizes revenue from the upfront payment, milestone payment and manufacturing services payments using the contingency-adjusted performance model over the expected development period, which is currently estimated to be 12 years from the effective date of the agreement. Under this model, when a milestone is earned or manufacturing services are rendered and product is delivered, revenue is immediately recognized on a pro-rata basis in the period the milestone was achieved or product was delivered based on the time elapsed from the effective date of the agreement. Thereafter, the remaining portion is recognized on a straight-line basis over the remaining development period.

During the years ended December 31, 2010, 2011 and 2012, the Company recognized revenue based on the following components of the Sanofi agreement:

	Years ended December 31,

(in thousands)	2010		2011		2012
Upfront payment	$	5,000	$	5,000	$	5,000
Milestone payment		949		2,616		2,975
Development services		13,279		25,053		36,905
Manufacturing services and other		630		1,456		3,307

Total	$	19,858	$	34,125	$	48,187

As of December 31, 2011 and 2012, the Company maintained the following assets and liabilities related to the Sanofi agreement:

	December 31,

(in thousands)	2011		2012
Accounts receivable, billed	$	4,478	$	1,577
Accounts receivable, unbilled		2,925		7,690
Deferred revenues		84,466		79,913

GTC Biotherapeutics, Inc.

In July 2009, the Company entered into a license agreement with GTC Biotherapeutics, Inc. ("GTC") for the development of MM-093 by GTC. As consideration, GTC returned 662,000 shares of the Company's Series C convertible preferred stock to the Company. The Company determined the fair value of the consideration transferred to be $1,469,000. The Company applied revenue recognition guidance to determine that the performance obligations under this agreement, including the license, the right to future technology, and manufacturing support should be accounted for as a single unit of accounting. The consideration received is being recognized on a straight-line basis over the expected performance period, which was originally estimated to be 19 years from the effective date of the agreement.

During the fourth quarter of 2012, GTC notified the Company of their intent to terminate the license agreement in three months in accordance with the terms of the license agreement. The expected development term of the license agreement ended on March 19, 2013 when the Company received GTC's final notice of termination. This change in the estimate of expected development term resulted in $657,000 of additional revenue recognized during the fourth quarter of 2012.

During the years ended December 31, 2010, 2011 and 2012 the Company recognized revenue of $76,000, $76,000 and $733,000, respectively. As of December 31, 2010, 2011 and 2012, the Company had $1,356,000, $1,279,000 and $553,000 of deferred revenue, respectively.

PharmaEngine, Inc.

On May 5, 2011, the Company entered into an assignment, sublicense and collaboration agreement with PharmaEngine, Inc. ("PharmaEngine") under which the Company reacquired rights in Europe and certain countries in Asia to a drug being developed under the name MM-398. In exchange, the Company agreed to pay PharmaEngine a nonrefundable, noncreditable upfront payment of $10.0 million and will be required to make up to an aggregate of $80.0 million in development and regulatory milestone payments and $130.0 million in sales milestone payments upon the achievement of specified development, regulatory and annual net sales milestones. During the first quarter of 2012, the Company paid a milestone of $5.0 million under the collaboration agreement with PharmaEngine in connection with dosing the first patient in a Phase 3 clinical trial of MM-398 in pancreatic cancer. PharmaEngine is also entitled to tiered royalties on net sales of MM-398 in Europe and certain countries in Asia. The Company is responsible for all future development costs of MM-398 except those required specifically for regulatory approval in Taiwan.

During the years ended December 31, 2011 and 2012, the Company recognized research and development expenses of $11.2 million and $6.2 million, respectively, related to the agreement with PharmaEngine. As of December 31, 2011 and 2012, the Company had amounts payable of $280,000 and $345,000, respectively, related to the agreement with PharmaEngine.

Fair Value of Financial Instruments

12 Months Ended

Dec. 31, 2012

Fair Value of Financial Instruments

Fair Value of Financial Instruments

7. Fair Value of Financial Instruments

The carrying amounts of cash and cash equivalents, available-for-sale securities, prepaid expenses, accounts receivable, accounts payable and accrued expenses and other short-term assets and liabilities approximate fair value due to the short-term nature of these instruments. A derivative liability and convertible preferred stock warrants are also carried at fair value.

Fair value is an exit price, representing the amount that would be received from the sale of an asset or paid to transfer a liability in an orderly transaction between market participants. Fair value is determined based on observable and unobservable inputs. Observable inputs reflect readily obtainable data from independent sources, while unobservable inputs reflect certain market assumptions. As a basis for considering such assumptions, GAAP establishes a three-tier value hierarchy, which prioritizes the inputs used to develop the assumptions and for measuring fair value as follows: (Level 1) observable inputs such as quoted prices in active markets for identical assets; (Level 2) inputs other than the quoted prices in active markets that are observable either directly or indirectly; and (Level 3) unobservable inputs in which there is little or no market data, which requires the Company to develop its own assumptions. This hierarchy requires the Company to use observable market data, when available, and to minimize the use of unobservable inputs when determining fair value.

The following tables show assets and liabilities measured at fair value on a recurring basis as of December 31, 2011 and 2012 and the input categories associated with those assets and liabilities:

As of December 31, 2011 (in thousands)	Level 1		Level 2		Level 3

Assets:
Cash equivalents—U.S. treasury securities	$	35,076	$	—	$	—
Liabilities:
Convertible preferred stock warrants		—		—		1,516

As of December 31, 2012 (in thousands)	Level 1		Level 2		Level 3

Assets:
Cash equivalents—money market funds	$	25,668	$	—	$	—
Cash equivalents—certificates of deposit		—		480		—
Cash equivalents—corporate debt securities		—		5,017		—
Investments—certificates of deposit		—		240		—
Investments—commercial paper		—		12,465
Investments—corporate debt securities		—		59,533		—
Liabilities:
Derivative liability		—		—		196

The Company's investment portfolio consists of investments classified as cash equivalents and available-for-sale securities. All highly liquid investments with an original maturity of three months or less when purchased are considered to be cash equivalents. The Company's cash and cash equivalents are invested in U.S. treasury and various corporate debt securities that approximate their face value. All marketable securities with an original maturity when purchased of greater than three months are classified as available-for-sale. Available-for-sale securities are carried at fair value, with the unrealized gains and losses reported in other comprehensive income (loss). The amortized cost of securities in this category is adjusted for amortization of premiums and accretion of discounts to maturity. The fair value of the convertible preferred stock warrants as of December 31, 2011 was determined using the Black-Scholes option valuation model. The fair value of the derivative liability as of December 31, 2012 was determined using a probability-weighted valuation based upon the likelihood of Silver Creek achieving a qualified financing, as described in Note 12.

The following table provides a roll-forward of the fair value of the liabilities categorized as Level 3 instruments, for the year ended December 31, 2012:

(in thousands)	Convertible preferred stock warrants			Derivative Liability

Balance, December 31, 2011	$	1,516		$	—
Unrealized gain included in other income (expense)		(587	)		—
Reclassification to common stock warrants		(929	)		—
Portion of convertible note allocated to derivative		—			196

Balance, December 31, 2012	$	—		$	196

- Definition

The entire disclosure for the fair value of financial instruments (as defined), including financial assets and financial liabilities (collectively, as defined), and the measurements of those instruments as well as disclosures related to the fair value of non-financial assets and liabilities. Such disclosures about the financial instruments, assets, and liabilities would include: (1) the fair value of the required items together with their carrying amounts (as appropriate); (2) for items for which it is not practicable to estimate fair value, disclosure would include: (a) information pertinent to estimating fair value (including, carrying amount, effective interest rate, and maturity, and (b) the reasons why it is not practicable to estimate fair value; (3) significant concentrations of credit risk including: (a) information about the activity, region, or economic characteristics identifying a concentration, (b) the maximum amount of loss the entity is exposed to based on the gross fair value of the related item, (c) policy for requiring collateral or other security and information as to accessing such collateral or security, and (d) the nature and brief description of such collateral or security; (4) quantitative information about market risks and how such risks are managed; (5) for items measured on both a recurring and nonrecurring basis information regarding the inputs used to develop the fair value measurement; and (6) for items presented in the financial statement for which fair value measurement is elected: (a) information necessary to understand the reasons for the election, (b) discussion of the effect of fair value changes on earnings, (c) a description of [similar groups] items for which the election is made and the relation thereof to the balance sheet, the aggregate carrying value of items included in the balance sheet that are not eligible for the election; (7) all other required (as defined) and desired information.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 107

-Paragraph 3, 10, 14, 15

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 2: http://www.xbrl.org/2003/role/presentationRef

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-Paragraph 15C, 15D

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Reference 3: http://www.xbrl.org/2003/role/presentationRef

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Reference 4: http://www.xbrl.org/2003/role/presentationRef

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-Number 133

-Paragraph 44A, 44B

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-Publisher FASB

-Name Accounting Standards Codification

-Topic 825

-SubTopic 10

-Section 50

-Paragraph 10

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-Publisher FASB

-Name Accounting Standards Codification

-Topic 825

-SubTopic 10

-Section 50

-Paragraph 28

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-Publisher FASB

-Name Accounting Standards Codification

-Topic 820

-SubTopic 10

-Section 50

-Paragraph 2

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-Name Accounting Standards Codification

-Topic 825

-SubTopic 10

-Section 50

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-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 107

-Paragraph 15B

-Subparagraph a, b

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-Name Statement of Financial Accounting Standard (FAS)

-Number 159

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-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 107

-Paragraph 15A

-Subparagraph a-d

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+ Details

Consolidated Subsidiaries	12 Months Ended
Consolidated Subsidiaries	Dec. 31, 2012
Consolidated Subsidiaries
Consolidated Subsidiaries	8. Consolidated Subsidiaries Hermes BioSciences, Inc. On October 6, 2009, (the "Acquisition Date"), the Company completed the acquisition of all outstanding shares of Hermes, a privately-held biotechnology company developing lipidic nano-carriers to allow for targeted delivery of small molecule drugs, including chemotherapies, with the goal of improving cancer treatment safety and efficacy. As consideration for the acquisition, the Company issued 4,383,000 shares of common stock with an estimated fair value of $9,292,000 based on an internal valuation prepared by the Company. The acquisition also included a contingent consideration arrangement that required additional shares to be issued by the Company to Hermes' former stockholders based on the occurrence and timing of certain potential future financing events. The range of additional shares that the Company could have been required to issue on the Acquisition Date as contingent consideration was between 0 and 1,100,000 and issuance could have occurred up to 24 months after the Acquisition Date. The estimated fair value of the contingent consideration recognized on the Acquisition Date of $178,000 was determined by performing a probability weighted analysis of the likelihood of occurrence of potential future financing events. That estimate was based on significant inputs not observable in the market, or Level 3 inputs. Key assumptions included management's estimates of the probabilities of such potential future financing events occurring. As of December 31, 2010, 400,000 additional shares could have been issued as contingent consideration. However, the Company determined a zero probability that the contingent consideration would ultimately be paid and recognized a gain of $178,000 for the year ended December 31, 2010. On July 8, 2011, the Company satisfied the contingent consideration triggering event, which reduced the shares that could be issued from 400,000 to zero. Silver Creek Pharmaceuticals, Inc. Silver Creek was incorporated on June 22, 2010 and commenced operations on August 20, 2010. On August 20, 2010, the Company purchased 12,000,000 shares of Silver Creek Series A preferred stock in exchange for technology licenses. On August 20, 2010 and December 17, 2010, Silver Creek issued a total of 4,190,000 shares of Silver Creek Series A preferred stock to other investors in exchange for $4,165,000, net of $25,000 of issuance costs. The Company consolidated Silver Creek on August 20, 2010, as the Company concluded that Silver Creek is a variable interest entity and the Company is the primary beneficiary. The Company has the ability to direct the activities of Silver Creek through its ownership percentage and through the board of director seats controlled by the Company and its related parties and de facto agents. As of December 31, 2011 and 2012, the Company owned 74% of the voting stock of Silver Creek and, as of December 31, 2011 and 2012, the Company recorded a non-controlling interest of $574,000 and $97,000, respectively, as a component of mezzanine equity on the Company's consolidated balance sheets based on the terms of the Silver Creek Series A preferred stock. As of December 31, 2011, the Company consolidated Silver Creek total assets and total liabilities of $2,302,000 and $39,000, respectively. As of December 31, 2012, the Company consolidated Silver Creek total assets and total liabilities of $2,202,000 and $1,763,000, respectively. As of December 31, 2011 and 2012, employees and directors of the Company owned approximately 6% of Silver Creek Series A preferred stock. Merrimack Pharmaceuticals (Bermuda) Ltd. Merrimack Pharmaceuticals (Bermuda) Ltd. was incorporated in Bermuda during 2011, is wholly owned by the Company and holds certain intellectual property rights with respect to MM-398.

Goodwill and Intangible Assets, Net

12 Months Ended

Dec. 31, 2012

Goodwill and Intangible Assets, Net

Goodwill and Intangible Assets, Net

9. Goodwill and Intangible Assets, Net

As part of the acquisition of Hermes, the Company recognized acquired IPR&D of $7,010,000 related to several development programs: an antibody-targeted nanotherapeutic that contains a chemotherapy drug, a nanotherapeutic that contains a chemotherapy drug and other programs in the amounts of $2,800,000, $3,400,000 and $810,000, respectively. The Company also acquired intangible assets of $3,200,000 related to core nano-carrier technology. These values were determined at the time of acquisition by estimating the costs to develop the acquired IPR&D into commercially viable products, estimating the net cash flows from such projects and discounting the net cash flows back to their present values. The probability of success factors and discount rates used for each project considered the uncertainty surrounding the successful development of the acquired IPR&D.

As of December 31, 2011 and 2012, none of the IPR&D projects have reached technological feasibility nor do they have any alternative future use. Therefore, the Company has not commenced amortization of those assets. The core technology asset is being amortized on a straight-line basis over a period of ten years, which is management's best estimate of the useful life of this technology. Accordingly, the full value of the IPR&D recorded at the Acquisition Date remained unchanged as of December 31, 2011 and 2012.

Changes in the carrying value of goodwill, IPR&D and intangible assets for the years ended December 31, 2010, 2011 and 2012 were as follows:

(in thousands)	Intangible assets			IPR&D		Goodwill

Balance, December 31, 2009		3,125			7,010		3,605
Amortization		(320	)		—		—

Balance, December 31, 2010		2,805			7,010		3,605
Amortization		(320	)		—		—

Balance, December 31, 2011	$	2,485		$	7,010	$	3,605
Amortization		(320	)		—		—

Balance, December 31, 2012		2,165			7,010		3,605

Definite-lived intangible assets subject to amortization consist of core technology acquired from Hermes. The Company commenced amortization of these assets as of the Acquisition Date on a straight-line basis over a period of ten years, which is the estimated useful life of this technology. Amortization expense is expected to be as follows for the next five-year period:

Years Ended December 31,	(in thousands)

2013	$	320
2014		320
2015		320
2016		320
2017		320

- Definition

The entire disclosure for the aggregate amount of goodwill and a description of intangible assets, which may include (a) for amortizable intangible assets (also referred to as finite-lived intangible assets), the carrying amount, the amount of any significant residual value, and the weighted-average amortization period, (b) for intangible assets not subject to amortization (also referred to as indefinite-lived intangible assets), the carrying amount, and (c) the amount of research and development assets acquired and written off in the period, including the line item in the income statement in which the amounts written off are aggregated, if not readily apparent from the income statement. Also discloses (a) for amortizable intangibles assets in total and by major class, the gross carrying amount and accumulated amortization, the total amortization expense for the period, and the estimated aggregate amortization expense for each of the five succeeding fiscal years, (b) for intangible assets not subject to amortization the carrying amount in total and by major class, and (c) for goodwill, in total and for each reportable segment, the changes in the carrying amount of goodwill during the period (including the aggregate amount of goodwill acquired, the aggregate amount of impairment losses recognized, and the amount of goodwill included in the gain (loss) on disposal of a reporting unit). If any part of goodwill has not been allocated to a reportable segment, discloses the unallocated amount and the reasons for not allocating. For each impairment loss recognized related to an intangible asset (excluding goodwill), discloses: (a) a description of the impaired intangible asset and the facts and circumstances leading to the impairment, (b) the amount of the impairment loss and the method for determining fair value, (c) the caption in the income statement or the statement of activities in which the impairment loss is aggregated, and (d) the segment in which the impaired intangible asset is reported. For each goodwill impairment loss recognized, discloses: (a) a description of the facts and circumstances leading to the impairment, (b) the amount of the impairment loss and the method of determining the fair value of the associated reporting unit, and (c) if a recognized impairment loss is an estimate not finalized and the reasons why the estimate is not final. May also disclose the nature and amount of any significant adjustments made to a previous estimate of an impairment loss.

+ References+ Details

Property and Equipment, Net

12 Months Ended

Dec. 31, 2012

Property and Equipment, Net

Property and Equipment, Net

10. Property and Equipment, Net

Property and equipment consisted of the following:

	December 31,

(in thousands)	2011			2012
Lab equipment	$	11,757		$	12,616
IT equipment		2,204			2,346
Leasehold improvements		7,698			8,200
Furniture and fixtures		329			330
Construction in process		348			1,774

		22,336			25,266
Less: Accumulated depreciation and amortization		(16,130	)		(18,969	)

	$	6,206		$	6,297

Depreciation expense was $4,059,000, $5,006,000 and $3,510,000 for the years ended December 31, 2010, 2011 and 2012, respectively.

During 2010, the Company sold fully depreciated fixed assets of $26,000, resulting in a gain on disposal. No fixed assets were disposed of or sold during 2011. During 2012, the Company disposed of $671,000 of fully depreciated assets.

In August 2004, the Company entered into an equipment financing agreement with a leasing company. The agreement involved the sale of some of the Company's fixed assets to and the leasing of those assets back from the leasing company. The Company's option to draw further on this lease facility expired during 2008. Property and equipment under capital leases as of December 31, 2011 and 2012 was $4,114,000 and $0, respectively. For the years ended December 31, 2010, 2011 and 2012, depreciation of property and equipment under capital lease totaled $409,000, $26,000 and $0, respectively.

There were no recognized impairment charges related to fixed assets in the years ended December 31, 2010, 2011 or 2012.

- Definition

The entire disclosure for long-lived, physical assets that are used in the normal conduct of business to produce goods and services and not intended for resale. Examples include land, buildings, machinery and equipment, and other types of furniture and equipment including, but not limited to, office equipment, furniture and fixtures, and computer equipment and software. This disclosure may include property plant and equipment accounting policies and methodology, a schedule of property, plant and equipment gross, additions, deletions, transfers and other changes, depreciation, depletion and amortization expense, net, accumulated depreciation, depletion and amortization expense and useful lives, income statement disclosures, assets held for sale and public utility disclosures.

+ References+ Details

Accounts Payables, Accrued Expenses and Other

12 Months Ended

Dec. 31, 2012

Accounts Payables, Accrued Expenses and Other

Accounts Payables, Accrued Expenses and Other

11. Accounts Payables, Accrued Expenses and Other

Accounts payable, accrued expenses and other as of December 31, 2011 and 2012 consisted of the following:

	December 31,

(in thousands)	2011		2012
Accounts payable		4,656		283
Accrued goods and services	$	9,189	$	17,615
Accrued payroll and related benefits		3,666		5,853
Accrued interest		—		306
Accrued dividends payable		—		28
Contractual liability (Note 19)		—		851

Total accounts payable, accrued expenses and other	$	17,511	$	24,936

Debt

12 Months Ended

Dec. 31, 2012

Debt

Debt

12. Debt

Loans Payable

On November 8, 2012, the Company entered into the Loan Agreement with Hercules that provided for an initial term loan advance of $25.0 million, which closed on November 8, 2012, and an additional term loan of $15.0 million, which closed on December 14, 2012. The term loans bear interest at an annual rate equal to the greater of 10.55% and 10.55% plus the prime rate of interest minus 5.25%, but may not exceed 12.55%. Net proceeds from both advances received during the fourth quarter of 2012 were $39.6 million.

The Loan Agreement provides for interest-only payments for twelve months and repayment of the aggregate outstanding principal balance of the loans in monthly installments starting on December 1, 2013 and continuing through May 1, 2016. If the Company receives aggregate gross proceeds of at least $75 million in one or more transactions prior to December 1, 2013, including pursuant to a financing or collaboration, the Company may elect to extend the interest-only period by six months so that the aggregate outstanding principal balance of the loans would be repaid in monthly installments starting on June 1, 2014 and continuing through November 1, 2016. At the Company's option, the Company may elect to prepay all or any part of the outstanding term loans without penalty.

Upon full repayment or maturity of the loans, the Company is required to pay Hercules a fee of $1.2 million, which has been recorded as a discount to the loans and as a long-term liability on the consolidated balance sheets. Additionally, the Company reimbursed Hercules for costs incurred related to the loans of $396,000, which has been reflected as a discount to the carrying value of the loans. The Company is amortizing these loan discounts totaling $1.6 million to interest expense over the term of the loans using the effective interest method. For the year ended December 31, 2012, cash and noncash interest expense related to the Hercules loans payable was $475,000 and $78,000, respectively.

In connection with the Loan Agreement, the Company granted Hercules a security interest in all of the Company's personal property now owned or hereafter acquired, excluding intellectual property but including the proceeds from the sale, if any, of intellectual property, and a negative pledge on intellectual property. The Loan Agreement also contains certain representations, warranties and non-financial covenants of the Company. In addition, the Loan Agreement grants Hercules an option to purchase up to an aggregate of $1.0 million of the Company's equity securities sold to institutional accredited investors in a private financing within one year after the closing of the Loan Agreement upon the same terms and conditions afforded to such investors.

The Loan Agreement defines events of default to include the occurrence of an event that results in a material adverse effect upon the Company's business, operations, properties, assets or condition (financial or otherwise); the Company's ability to perform its obligations when due in accordance with the terms of the Loan Agreement, or upon the ability of Hercules to enforce any of its rights or remedies with respect to such obligations; or the collateral under the Loan Agreement or Hercules' liens on such collateral or the priority of such liens. As of December 31, 2012, there have been no events of default under the Loan Agreement. As of December 31, 2012, the Company has recorded loans payable related to the Loan Agreement of $38.5 million.

Future minimum payments under the loans payable outstanding as of December 31, 2012 are as follows:

Years Ending December 31: (in thousands)

2013	$	5,439
2014		18,138
2015		18,123
2016		8,830

	$	50,530
Less interest		(9,330	)
Less unamortized discount		(2,720	)
Less current portion		(998	)

Loans payable, net of current portion	$	37,482

The carrying value of the loans payable approximates fair value.

Silver Creek Convertible Note

On December 21, 2012, the Company's majority-owned subsidiary Silver Creek entered into a Note Purchase Agreement with certain lenders. The notes issued pursuant to the Note Purchase Agreement bear interest at 6%. The notes mature and convert, along with accrued interest, into Silver Creek Series A preferred stock on December 31, 2013. If at any time prior to maturity Silver Creek enters into a qualifying equity financing, defined as a sale or series of related sales of equity securities prior to the maturity date and resulting in at least $4.0 million of gross proceeds, the notes will automatically convert into the next qualifying equity financing at a 25% discount. The Company determined that this convertible feature met the definition of a derivative and required separate accounting treatment. The derivative was estimated to be valued at $196,000 at December 21, 2012 and December 31, 2012 using a probability-weighted model, and was recorded as derivative liability on the consolidated balance sheets.

	December 31, 2012

	(in thousands)
Total convertible note outstanding	$	1,571
Unamortized discount		(196	)

Net carrying amount of the convertible note		1,375

- Definition

The entire disclosure for information about short-term and long-term debt arrangements, which includes amounts of borrowings under each line of credit, note payable, commercial paper issue, bonds indenture, debenture issue, own-share lending arrangements and any other contractual agreement to repay funds, and about the underlying arrangements, rationale for a classification as long-term, including repayment terms, interest rates, collateral provided, restrictions on use of assets and activities, whether or not in compliance with debt covenants, and other matters important to users of the financial statements, such as the effects of refinancing and noncompliance with debt covenants.

+ References+ Details

Convertible Preferred Stock

12 Months Ended

Dec. 31, 2012

Convertible Preferred Stock

Convertible Preferred Stock

13. Convertible Preferred Stock

At December 31, 2011, each share of the convertible preferred stock was convertible at the option of the holder into common stock of the Company based on a defined conversion ratio, adjustable for certain standard anti-dilution adjustments. Upon the Company's firm commitment underwritten public offering of shares of common stock with a per share offering price equal to or greater than the greater of $4.40 or 250% of the conversion price then in effect for the Series C convertible preferred stock, which results in aggregate gross proceeds to the Company of at least $50 million, then all outstanding shares of convertible preferred stock automatically convert to shares of common stock, with dividends of approximately $4.3 million on the Series B convertible preferred stock to be declared and paid in cash.

	Convertible Preferred Stock

	Carrying Value		Shares Outstanding		Liquidation Preference		Conversion Price
Series B	$	14,046		3,874	$	4.40	$	2.85
Series C		24,459		14,424	$	1.89	$	1.89
Series D		28,267		8,086	$	3.50	$	3.50
Series E		64,531		14,991	$	4.50	$	4.50
Series F		59,973		11,776	$	5.10	$	5.10
Series G		76,949		11,000	$	7.00	$	7.00

Balance at December 31, 2011	$	268,225		64,151

In April 2012, the Company completed an initial public offering of its common stock, upon which all outstanding shares of the Company's convertible preferred stock were converted into 66,255,529 shares of common stock and $4.3 million of dividends on the Company's Series B convertible preferred stock became payable.

Series F Amount	12 Months Ended
Series F Amount	Dec. 31, 2012
Series F Amount
Series F Amount	14. Series F Amount During 2010, management determined that the Company may not have obtained all of the stockholder approvals required with respect to the Restated Articles of Organization that it filed with the Secretary of the Commonwealth of the Commonwealth of Massachusetts (the "Massachusetts Secretary") on November 2, 2007 (the "2007 Restated Articles"). Among other changes, the 2007 Restated Articles were intended to authorize the 11,776,000 shares of Series F convertible preferred stock (the "Series F") that the Company agreed to issue to purchasers in 2007 and 2008. In addition, the Company filed Articles of Amendment to the 2007 Restated Articles with the Massachusetts Secretary on November 5, 2009 (the "2009 Amendment") that the Company believes were ineffective as a result of the failure to obtain the requisite stockholder approvals for the 2007 Restated Articles. As a result, the Series F was not legally issued convertible preferred stock, but rather an unsettled obligation to issue Series F. In order to properly authorize and issue the Series F, in July and August 2010, the board of directors and stockholders of the Company, respectively, approved new Restated Articles of Organization (the "2010 Restated Articles") that provided for the amendments contemplated by the 2007 Restated Articles and the 2009 Amendment. In order to provide the purchasers with shares of Series F having the economic benefit of the accruing dividends to which they would have been entitled had the Series F been properly authorized and issued as originally intended, the 2010 Restated Articles authorized the Series F in sub-series, with each sub-series corresponding to a closing date in 2007 or 2008. The preferences, limitations and relative rights of the shares of each sub-series of Series F authorized by the 2010 Restated Articles are the same as to the preferences, limitations and relative rights of the shares of Series F intended to be authorized by the 2007 Restated Articles and the 2009 Amendment. The 2010 Restated Articles were filed with the Massachusetts Secretary of State on October 6, 2010. Following the filing of the 2010 Restated Articles, the Company entered into an Exchange Agreement with each individual and entity that originally agreed to purchase shares of Series F in 2007 or 2008. Pursuant to the Exchange Agreements, the Company agreed to exchange the rights to receive the shares of Series F that it had agreed to issue in 2007 and 2008 for the same number of shares of the applicable sub-series of Series F authorized by the 2010 Restated Articles. Such exchanges were completed on October 6, 2010. The Company recorded imputed noncash interest expense related to Series F for financial reporting purposes of $3,673,000 for the year ended December 31, 2010 due to the delayed delivery of Series F. Upon completion of the exchanges of Series F on October 6, 2010, the Company issued 11,776,000 shares of Series F. The Series F amount was relieved and the initial investment of $5.10 per share was recorded as convertible preferred stock and the accrued noncash interest expense of $12,974,000 was recorded as additional paid-in capital during the fourth quarter of 2010.

Stock Warrants

12 Months Ended

Dec. 31, 2012

Stock Warrants

Stock Warrants

15. Stock Warrants

The following is a description of the common and convertible preferred stock warrant activity of the Company:

(in thousands, except per share amounts)			Weighted average exercise price				Weighted average exercise price

Balance—December 31, 2009	2,937			2.35	317		$	3.42
Exercised	—				(11	)	$	1.89

Balance—December 31, 2010	2,937			2.93	306		$	3.48
Expired	(1	)		2.47	—			—
Exercised	(296	)		2.46	(4	)	$	1.89

Balance—December 31, 2011	2,640		$	2.98	302		$	3.50
Conversion	302		$	3.50	(302	)	$	3.50
Expired	—			—	—			—
Exercised	(100	)	$	2.63	—			—

Balance—December 31, 2012	2,842		$	3.05	—			—

During 2012, warrants to purchase 100,000 shares of common stock were cashless exercised and 71,000 shares of common stock were issued.

During 2010, 2,596,000 warrants held by a related party stockholder were modified to extend the expiration dates by 4 years and increase the exercise prices from $2.12 and $2.47 to $3.00 per share. The modification was valued using a Black-Scholes option valuation model and the Company accounted for the $1,803,000 of incremental value within additional paid-in capital.

Common Stock

12 Months Ended

Dec. 31, 2012

Common Stock

Common Stock

16. Common Stock

During 2010, the Company changed the par value of its common stock from no par to $0.01 par and recognized a $17,547,000 reduction to common stock and a corresponding increase to additional paid-in capital. During the first quarter of 2012, the Company amended its certificate of incorporation to increase the number of authorized shares of common stock to 200.0 million shares of $0.01 par value common stock. As of December 31, 2011 and 2012, the Company had 138.5 million shares and 200.0 million shares, respectively, of $0.01 par value common stock authorized. There were 11,834,000 and 95,825,000 shares of common stock issued and outstanding as of December 31, 2011 and 2012, respectively. The shares reserved for future issuance as of December 31, 2011 and 2012 consisted of the following:

(in thousands)

Conversion of Series B, Series C, Series D, Series E, Series F and Series G convertible preferred stock	66,256	—
Convertible preferred stock warrants	302	—
Common stock warrants	2,640	2,842
Options to purchase common stock	17,617	18,066

	86,815	20,908

Stock-Based Compensation

12 Months Ended

Dec. 31, 2012

Stock-Based Compensation

Stock-Based Compensation

17. Stock-Based Compensation

Prior to 2008, the Company granted equity awards to employees, officers and consultants under the 1999 Stock Option Plan (as amended, the "1999 Plan"). In 2008, the Company adopted the 2008 Stock Incentive Plan (as amended, the "2008 Plan") for employees, officers, directors, consultants and advisors and decided that no additional shares of common stock would be issued under the 1999 Plan. As of December 31, 2011, there were 830,000 shares of common stock available to be issued under the 2008 Plan. The 2011 Stock Incentive Plan (the "2011 Plan") became effective upon closing of the Company's initial public offering in April 2012. Upon effectiveness of the 2011 Plan, no further awards were available to be issued under the 2008 Plan. The 2011 Plan is administered by the Board of Directors of the Company and permits the Company to grant incentive and non-qualified stock options, stock appreciation rights, restricted stock, restricted stock units and other stock-based awards. The 2011 Plan increased the total number of shares of common stock available to be issued by 3.5 million, and additional awards become available for grant by reason of the forfeiture, cancellation, expiration or termination of existing awards. As of December 31, 2012, there were 1.3 million shares of common stock available to be issued under the 2011 Plan.

During the years ended December 31, 2010, 2011 and 2012, the Company issued options to purchase 2.9 million, 2.3 million and 3.3 million shares of common stock, respectively. These options generally vest over a three-year period for employees. Prior to the closing of the Company's initial public offering in April 2012, options previously granted to directors had vested immediately. After the closing of the Company's initial public offering in April 2012, options granted to directors vest over a one-year period. During the years ended December 31, 2010, 2011 and 2012, the Company also issued options to purchase less than 0.1 million shares of common stock to non-employees in each period. The assumptions used to estimate the fair value of options granted to non-employees at the date of grant were materially consistent with those used for employee and director grants.

The Company recognized stock-based compensation expense as follows:

	Years ended December 31,

(in thousands)	2010		2011		2012
Employee awards:
Research and development	$	2,787	$	3,597	$	4,234
General and administrative		1,706		2,875		2,510

Stock-based compensation for employee awards		4,493		6,472		6,744
Stock-based compensation for nonemployee awards		58		480		145

Total stock-based compensation	$	4,551	$	6,952	$	6,889

The fair value of options granted in 2010, 2011 and 2012 were estimated at the date of grant using the following assumptions:

	Years ended December 31,

	2010	2011	2012
Risk-free interest rate	1.7 - 2.8%	1.3 - 2.5%	0.7 - 1.1%
Expected dividend yield	0%	0%	0%
Expected term	5 - 5.9 years	5 - 5.9 years	5 - 5.9 years
Expected volatility	73 - 77%	71 - 73%	66 - 72%

The Company uses the simplified method to calculate the expected term, as it does not have sufficient historical exercise data to provide a reasonable basis upon which to estimate expected term. The computation of expected volatility is based on the historical volatility of comparable companies from a representative peer group selected based on industry and market capitalization. The risk-free interest rate is based on a treasury instrument whose term is consistent with the expected life of the stock options. Management estimates expected forfeitures based on historical experience and recognizes compensation costs only for those equity awards expected to vest.

The following table summarizes stock option activity:

			Weighted Average Exercise Price			Aggregate Intrinsic Value

Outstanding at December 31, 2011	17,617		$	2.56
Granted	3,251		$	7.45
Exercised	(2,622	)	$	2.06
Cancelled	(180	)	$	4.34

Outstanding at December 31, 2012	18,066		$	3.50	6.54	$	51,486
Vested and expected to vest at December 31, 2012	17,741		$	3.43	6.49	$	51,434
Exercisable at December 31, 2012	13,616		$	2.52	5.70	$	49,155

The aggregate intrinsic value was calculated as the difference between the exercise price of the stock options and the fair value of the underlying common stock as of the respective balance sheet date. The aggregate intrinsic value of options exercised in 2010, 2011 and 2012 was $145,000, $1,392,000 and $13,721,000, respectively.

As of December 31, 2012, there was $15,924,000 of total unrecognized compensation cost related to nonvested stock awards. As of December 31, 2012, the Company expects to recognize those costs over weighted average periods of approximately 2.1 years.

- Definition

The entire disclosure for compensation-related costs for equity-based compensation, which may include disclosure of policies, compensation plan details, allocation of equity compensation, incentive distributions, equity-based arrangements to obtain goods and services, deferred compensation arrangements, employee stock ownership plan details and employee stock purchase plan details.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 718

-SubTopic 10

-Section 50

-Paragraph 1

-URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5047-113901

Reference 2: http://www.xbrl.org/2003/role/presentationRef

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-Name Accounting Standards Codification

-Topic 505

-SubTopic 50

-Section 50

-Paragraph 1

-URI http://asc.fasb.org/extlink&oid=6406099&loc=d3e25284-112666

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-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 123R

-Paragraph 64, 65, A240

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-Name Accounting Standards Codification

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-Paragraph 1

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-Publisher SEC

-Name Staff Accounting Bulletin (SAB)

-Number Topic 14

Reference 8: http://www.xbrl.org/2003/role/presentationRef

-Publisher AICPA

-Name Statement of Position (SOP)

-Number 93-6

-Paragraph 53

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

+ Details

Income Taxes

12 Months Ended

Dec. 31, 2012

Income Taxes

Income Taxes

18. Income Taxes

As a result of losses incurred, the Company did not provide for any income taxes in the years ended December 31, 2010, 2011 and 2012. A reconciliation of the Company's effective tax rate to the statutory federal income tax rate is as follows:

	Years ended December 31,

	2010			2011			2012
Federal statutory rate		35.0	%		35.0	%		35.0	%
State taxes, net of Federal benefit		4.6			4.2			4.5
Permanent differences		(2.6	)		(0.4	)		(0.2	)
Stock compensation		(2.9	)		(1.2	)		(0.3	)
Change in valuation allowance		(39.2	)		(36.3	)		(35.5	)
Tax credits		5.1			3.9			1.1
Foreign rate differentials		—			(4.4	)		(4.1	)
Other		—			(0.8	)		(0.5	)

		—	%		—	%		—	%

Temporary differences that give rise to significant net deferred tax assets as of December 31, 2011 and 2012 are as follows:

(in thousands)	2011			2012

Deferred tax assets
Net operating losses	$	40,633		$	77,806
Capitalized research and development expenses		47,640			40,083
Credit carryforwards		13,380			14,398
Depreciation		2,337			2,931
Deferred compensation		4,450			5,068
Deferred revenues		26,462			29,936
Accrued expenses		676			1,184
Other		922			1,934

Total gross deferred tax asset		136,500			173,340
Intangible assets		(3,817	)		(3,689	)
Valuation allowance		(132,683	)		(169,651	)

Net deferred taxes	$	—		$	—

At December 31, 2012, the Company had net operating loss carryforwards for federal and state income tax purposes of $210.9 million and $155.5 million, respectively. Included in the federal and state net operating loss carryforwards is approximately $10.3 million of deduction related to the exercise of stock options. This amount represents an excess tax benefit, which will be realized when it results in reduction of cash taxes in accordance with Accounting Standards Codification 718. This excess tax benefit will be directly credited to additional paid-in capital when it is realized. The Company's existing federal and state net operating loss carryforwards have begun to expire and will continue to expire through 2032. The Company also has available research and development credits for federal and state income tax purposes of approximately $11.1 million and $4.8 million, respectively. The federal and state research and development credits will begin to expire in 2022 and 2024, respectively. As of December 31, 2012, the Company also had available investment tax credits for state income tax purposes of $0.4 million, which have begun to expire and will continue to expire through 2013. The Company's management has evaluated the positive and negative evidence bearing upon the realizability of its deferred tax assets, which are comprised principally of net operating loss carryforwards, deferred revenue and capitalized research and development expenses. Under the applicable accounting standards, management has considered the Company's history of losses and concluded that it is more likely than not that the Company will not recognize the benefits of federal and state deferred tax assets. Accordingly, the Company has established a full valuation allowance against the deferred tax assets.

Utilization of the net operating loss and research and development credit carryforwards may be subject to a substantial annual limitation under Section 382 of the Internal Revenue Code of 1986, as amended (the "Internal Revenue Code"), due to ownership change limitations that have occurred previously or that could occur in the future. These ownership changes may limit the amount of net operating loss and research and development credit carryforwards that can be utilized annually to offset future taxable income and tax. The Company has not currently completed an evaluation of ownership changes through December 31, 2012 to assess whether utilization of the Company's net operating loss or research and development credit carryforwards would be subject to an annual limitation under Section 382 of the Internal Revenue Code. To the extent an ownership change occurs in the future, the net operating loss and credit carryforwards may be subject to limitation.

The Company concluded that there are no significant uncertain tax positions requiring recognition in the consolidated financial statements. The Company's evaluation was performed for the tax years ended December 31, 2009 through 2012, the tax years which remain subject to examination by major tax jurisdictions as of December 31, 2012. However, to the extent the Company utilizes net operating losses from years prior to 2009, the statute remains open to the extent of the net operating losses utilized. The Company annually files a federal income tax return and a state income tax return in Massachusetts. The Company's policy is to recognize interest and penalties for uncertain tax positions as a component of income tax expense. The Company has not recognized any interest and penalties historically through December 31, 2012.

The change in the valuation allowance against the deferred tax assets in the years ended December 31, 2010, 2011 and 2012 was as follows:

(in thousands)	Balance at beginning of period				Balance at end of period

December 31, 2010	$	81,420	22,461	—	$	103,881
December 31, 2011	$	103,881	28,802	—	$	132,683
December 31, 2012	$	132,683	36,968	—	$	169,651

- Definition

The entire disclosure for income taxes. Disclosures may include net deferred tax liability or asset recognized in an enterprise's statement of financial position, net change during the year in the total valuation allowance, approximate tax effect of each type of temporary difference and carryforward that gives rise to a significant portion of deferred tax liabilities and deferred tax assets, utilization of a tax carryback, and tax uncertainties information.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 740

-SubTopic 10

-Section 50

-Paragraph 15

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-Name Accounting Standards Codification

-Topic 235

-SubTopic 10

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-Paragraph 1

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-URI http://asc.fasb.org/extlink&oid=6881521&loc=d3e23780-122690

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-Name Regulation S-X (SX)

-Number 210

-Section 08

-Paragraph h

-Article 4

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-Name Statement of Financial Accounting Standard (FAS)

-Number 109

-Paragraph 136, 172

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 8: http://www.xbrl.org/2003/role/presentationRef

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-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

+ Details

Commitments and Contingencies

12 Months Ended

Dec. 31, 2012

Commitments and Contingencies

Commitments and Contingencies

19. Commitments and Contingencies

Operating Leases

The Company leases its office, laboratory and manufacturing space under noncancelable operating leases. Total rent expense under these operating leases was $2,846,000, $3,235,000 and $4,317,000 for the years ended December 31, 2010, 2011 and 2012, respectively.

During March 2012, the Company entered into a facility lease amendment to further expand its office, laboratory and manufacturing space. The amendment leased additional space for a seven-year term effective March 2012. The aggregate additional rent due over the seven-year term of the lease amendment is approximately $2.7 million. As part of this amendment, the landlord agreed to reimburse the Company for a portion of tenant improvements made to the facility, up to a total of $0.5 million.

During August 2012, the Company entered into an Indenture of Lease (the "Amended Lease"), which amended and restated its facility lease, including all previous amendments. Under the Amended Lease, the Company retained its existing office, laboratory and manufacturing space at its existing facility and agreed to occupy approximately 23,000 square feet of additional space, for a total of 109,000 square feet (the "Leased Space"), all of which is leased until June 30, 2019. The aggregate minimum lease payments due over the seven-year term of the Amended Lease are approximately $31.5 million. As part of the Amended Lease, the landlord agreed to reimburse the Company for a portion of tenant improvements made to the facility, up to approximately $6.6 million, with approximately $4.6 million reimbursable in 2012 and $1.0 million reimbursable in each of 2013 and 2014. As a result, the Company recorded amounts receivable from the landlord of $5.6 million in prepaid expenses and other current assets and $1.0 million in other non-current assets, with a corresponding and offsetting entry recorded to deferred rent. As of December 31, 2012, the Company has received $0.6 million of these tenant improvement reimbursements. Tenant improvements recorded in deferred rent are amortized over the term of the lease as reductions to rent expense. The Amended Lease expires on June 30, 2019. The Company retains an option to renew the Amended Lease with respect to all of the Leased Space for an additional period of either one or five years.

Future minimum lease payments under noncancelable operating leases at December 31, 2012 are as follows:

Years ended December 31,		(in thousands)

2013			4,319
2014			4,439
2015			4,601
2016			4,735
2017			4,838
2018 and thereafter			7,427

Contingencies

Contractual Matter

The Company manufactures MM-121 under a license and collaboration agreement with Sanofi. Under this agreement, Sanofi reimburses the Company for direct costs incurred in manufacturing. During 2009 and 2010, the Company utilized a third party contractor to perform fill-finish manufacturing services. This third party contractor experienced U.S. Food and Drug Administration ("FDA") inspection issues with its quality control process that resulted in a formal warning letter from the FDA. Following a review by Sanofi and the Company, some MM-121 was pulled from clinical trial sites and replaced with MM-121 that was filled by a different contractor. Sanofi had requested that the Company assume financial responsibility for the MM-121 material that was pulled from clinical trial sites. The Company and Sanofi have since agreed that, beginning in April 2012 and throughout 2013, the Company will reimburse Sanofi approximately $1.2 million of previously billed amounts. The Company's revenue recognition model for manufacturing services performed under the license and collaboration agreement with Sanofi is to recognize these services over the period of performance, which is currently estimated to be 12 years from the effective date of the agreement. Removal of these previously billed amounts from the revenue recognition model and establishing this contractual liability resulted in an earnings reduction of $0.2 million for the year ended December 31, 2012. The Company has accrued $0.9 million related to this contractual matter as of December 31, 2012.

- Definition

The entire disclosure for commitments and contingencies.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name FASB Interpretation (FIN)

-Number 14

-Paragraph 3

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 2: http://www.xbrl.org/2003/role/presentationRef

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-Name Statement of Financial Accounting Standard (FAS)

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+ Details

Related Party Transactions	12 Months Ended
Related Party Transactions	Dec. 31, 2012
Related Party Transactions
Related Party Transactions	20. Related Party Transactions In connection with the initial public offering of the Company's common stock, Sanofi, a collaborator, purchased 5,217,391 shares of the Company's common stock in April 2012. In June 2012, the Company entered into a Right of Review Agreement (the "Agreement") with Sanofi pursuant to which, if the Company determines to enter into negotiations with a third party regarding any license, option, collaboration, joint venture or similar transaction involving any therapeutic or companion diagnostic product candidate in the Company's pipeline (an "Opportunity"), the Company will notify Sanofi of such Opportunity. Following such notice, Sanofi will have a specified period of time to determine whether to exercise an additional right to exclusively negotiate an agreement with the Company with respect to such Opportunity for a specified period of time. In addition, in specified circumstances, if the Company subsequently proposes to enter into any third party agreement, the Company must first offer the same terms and conditions to Sanofi. The Agreement terminates on April 1, 2017. In December 2012, Silver Creek entered into a $1.6 million convertible note payable, of which $0.3 million was with directors, officers, scientific advisory board members and related parties of the Company.

- Definition

The entire disclosure for related party transactions. Examples of related party transactions include transactions between (a) a parent company and its subsidiary; (b) subsidiaries of a common parent; (c) and entity and its principal owners; and (d) affiliates.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

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-Name Regulation S-X (SX)

-Number 210

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-Name Accounting Standards Codification

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+ Details

Selected Quarterly Financial Data (Unaudited)

12 Months Ended

Dec. 31, 2012

Selected Quarterly Financial Data (Unaudited)

Selected Quarterly Financial Data (Unaudited)

22. Selected Quarterly Financial Data (Unaudited)

The following table contains quarterly financial information for 2011 and 2012. The Company believes that the following information reflects all normal recurring adjustments necessary for a fair statement of the information for the periods presented. The operating results for any quarter are not necessarily indicative of results for any future period.

	First Quarter			Second Quarter			Third Quarter			Fourth Quarter

	(in thousands, except per share data)
2011
Collaboration revenues	$	6,461		$	6,595		$	8,582		$	12,577
Total operating expenses		21,102			36,019			27,219			30,744
Net loss		(13,535	)		(29,196	)		(18,724	)		(18,221	)
Net loss attributable to Merrimack Pharmaceuticals, Inc.		(13,457	)		(29,051	)		(18,599	)		(18,116	)
Net loss per share available to common stockholders—basic and diluted	$	(1.34	)	$	(2.76	)	$	(1.81	)	$	(1.76	)

	First Quarter			Second Quarter			Third Quarter			Fourth Quarter

	(in thousands, except per share data)
2012
Collaboration revenues	$	11,344		$	12,063		$	11,323		$	14,191
Total operating expenses		35,379			32,368			35,197			38,719
Net loss		(23,402	)		(20,139	)		(23,320	)		(24,893	)
Net loss attributable to Merrimack Pharmaceuticals, Inc.		(23,284	)		(20,026	)		(23,199	)		(24,768	)
Net loss per share available to common stockholders—basic and diluted	$	(2.14	)	$	(0.22	)	$	(0.25	)	$	(0.26	)

- Definition

The entire disclosure for the quarterly financial data in the annual financial statements. The disclosure may include a tabular presentation of financial information for each fiscal quarter for the current and previous year, including revenues, gross profit, income or loss before extraordinary items and earnings per share data. It also includes an indication if the information in the note is unaudited, comments on the aggregate effect of year-end adjustments, and an explanation of matters or transactions that affect comparability or are pertinent to an understanding of the information furnished.

+ References+ Details

Subsequent Events	12 Months Ended
Subsequent Events	Dec. 31, 2012
Subsequent Events
Subsequent Events	23. Subsequent Events In January 2013, the Company received notice of award of $0.5 million of tax incentives from the MLSC, which will allow the Company to monetize approximately $0.4 million of state research and development tax credits. In exchange for these incentives, the Company pledged to hire an incremental 20 employees and to maintain the additional headcount through at least December 31, 2017. Failure to do so could result in the Company being required to repay some or all of these incentives. In February 2013, the Company registered 3,353,882 additional shares of common stock related to the 2011 Plan.

Summary of Significant Accounting Policies (Policies)

12 Months Ended

Dec. 31, 2012

Summary of Significant Accounting Policies

Basis of presentation and consolidation

Basis of Presentation and Consolidation

Use of Estimates

Use of Estimates

Segment and Geographic Information

Segment and Geographic Information

Cash, Cash Equivalents and Restricted Cash

Cash, Cash Equivalents and Restricted Cash

Available-for-Sale Securities

Available-for-Sale Securities

Concentration of Credit Risk

Concentration of Credit Risk

Property and Equipment

Property and Equipment

Asset classification		Estimated useful life (in years)

Lab equipment		3 - 7
IT equipment		3 - 7
Leaseholds improvements		Lesser of useful life or lease term
Furniture and fixtures		3 - 7

Non-controlling Interest

Non-controlling Interest

The Company's financial statement activity related to Silver Creek during these periods was as follows:

(in thousands)	Non-controlling Interest

Balance at December 31, 2009	$	—
Acquisition of Silver Creek preferred stock		1,082
Net loss attributable to Silver Creek		(55	)

Balance at December 31, 2010	$	1,027
Net loss attributable to Silver Creek		(453	)

Balance at December 31, 2011	$	574
Net loss attributable to Silver Creek		(477	)

Balance at December 31, 2012	$	97

Derivative Liability

Derivative Liability

Revenue Recognition

Revenue Recognition

Royalty revenue will be recognized upon the sale of the related products provided the Company has no remaining performance obligations under the arrangement.

Research and Development Expenses

Research and Development Expenses

Stock-Based Compensation

Stock-Based Compensation

Convertible Preferred Stock and Convertible Preferred Stock Warrants

Convertible Preferred Stock and Convertible Preferred Stock Warrants

Comprehensive Loss

Comprehensive Loss

Other Income (Expense)

Other Income (Expense)

Additionally, other income recognized during the year ended December 31, 2011 included the impact of a cash settlement of $1.8 million from a former service provider.

Deferred Financing Costs

Deferred Financing Costs

Income Taxes

Income Taxes

Goodwill and Intangible Assets

Goodwill and Intangible Assets

Reclassifications

Reclassifications

Certain prior year amounts have been reclassified to conform with the current year presentation.

Recent Accounting Pronouncements

Recent Accounting Pronouncements

Summary of Significant Accounting Policies (Tables)

12 Months Ended

Dec. 31, 2012

Summary of Significant Accounting Policies

Schedule of estimated useful lives of property and equipment including leasehold improvements

Asset classification		Estimated useful life (in years)

Lab equipment		3 - 7
IT equipment		3 - 7
Leaseholds improvements		Lesser of useful life or lease term
Furniture and fixtures		3 - 7

Schedule of entity's financial statement activity related to Silver Creek during these periods

(in thousands)	Non-controlling Interest

Balance at December 31, 2009	$	—
Acquisition of Silver Creek preferred stock		1,082
Net loss attributable to Silver Creek		(55	)

Balance at December 31, 2010	$	1,027
Net loss attributable to Silver Creek		(453	)

Balance at December 31, 2011	$	574
Net loss attributable to Silver Creek		(477	)

Balance at December 31, 2012	$	97

- Definition

The tabular disclosure of the effects of any changes in a parent's ownership interest in a subsidiary on the equity attributable to the parent if the ownership interests in a subsidiary changes during the period. The changes represented by this element did not result in the deconsolidation of the subsidiary.

+ References+ Details

- Definition

Tabular disclosure of the useful life and salvage value of long-lived, physical assets that are used in the normal conduct of business to produce goods and services and not intended for resale. Examples include land, buildings, machinery and equipment, and other types of furniture and equipment including, but not limited to, office equipment, furniture and fixtures, and computer equipment and software.

+ References+ Details

Marketable Securities (Tables)

12 Months Ended

Dec. 31, 2012

Marketable Securities

Schedule of available-for-sale securities, all of which have maturities of twelve months or less

	Amortized Cost		Unrealized Gains		Unrealized Losses			Fair Value

	(in thousands)
December 31, 2012:
Certificate of deposit	$	240	$	—	$	—		$	240
Commercial paper		12,479				(14	)		12,465
Corporate debt securities		59,557		3		(27	)		59,533

Total	$	72,276	$	3	$	(41	)	$	72,238

Schedule of marketable securities in an unrealized loss position

	Aggregate Fair Value

	(in thousands)
December 31, 2012:
Commercial paper		12,465	(14	)
Corporate debt securities		38,899	(27	)

	$	51,364	(41	)

Net Loss Per Common Share (Tables)

12 Months Ended

Dec. 31, 2012

Net Loss Per Common Share

Schedule of computation of basic and diluted net loss per share available to common stockholders

	Years ended December 31,

(in thousands, except per share amount)	2010			2011			2012
Net Loss Per Share:
Numerator:
Net loss attributable to Merrimack Pharmaceuticals, Inc.	$	(50,104	)	$	(79,223	)	$	(91,277	)
Plus: Unaccreted dividends on convertible preferred stock		(11,185	)		(7,789	)		(2,107	)

Net loss available to common stockholders—basic and diluted		(61,289	)		(87,012	)		(93,384	)
Denominator:
Weighted-average common shares—basic and diluted		10,994			11,343			72,831

Net loss per share available to common stockholders—basic and diluted	$	(5.57	)	$	(7.67	)	$	(1.28	)

Schedule of potentially dilutive securities excluded from computation of diluted weighted average shares

	Years ended December 31,

(in thousands)	2010		2011		2012
Convertible preferred stock		55,253		66,256		—
Options to purchase common stock		16,214		17,617		18,066
Convertible preferred stock warrants		306		302		—
Common stock warrants		2,937		2,640		2,842

License and Collaboration Agreements (Tables)

12 Months Ended

Dec. 31, 2012

License and Collaboration Agreements

Schedule of revenue recognized and assets and liabilities under collaborative arrangements

	Years ended December 31,
(in thousands)	2010		2011		2012
Upfront payment	$	5,000	$	5,000	$	5,000
Milestone payment		949		2,616		2,975
Development services		13,279		25,053		36,905
Manufacturing services and other		630		1,456		3,307

Total	$	19,858	$	34,125	$	48,187

	December 31,

(in thousands)	2011		2012
Accounts receivable, billed	$	4,478	$	1,577
Accounts receivable, unbilled		2,925		7,690
Deferred revenues		84,466		79,913

Fair Value of Financial Instruments (Tables)

12 Months Ended

Dec. 31, 2012

Fair Value of Financial Instruments

Schedule of assets and liabilities measured at fair value on a recurring basis

As of December 31, 2011 (in thousands)	Level 1		Level 2		Level 3

Assets:
Cash equivalents—U.S. treasury securities	$	35,076	$	—	$	—
Liabilities:
Convertible preferred stock warrants		—		—		1,516

As of December 31, 2012 (in thousands)	Level 1		Level 2		Level 3

Assets:
Cash equivalents—money market funds	$	25,668	$	—	$	—
Cash equivalents—certificates of deposit		—		480		—
Cash equivalents—corporate debt securities		—		5,017		—
Investments—certificates of deposit		—		240		—
Investments—commercial paper		—		12,465
Investments—corporate debt securities		—		59,533		—
Liabilities:
Derivative liability		—		—		196

Schedule of fair value of the liabilities categorized as Level 3 instruments

(in thousands)	Convertible preferred stock warrants			Derivative Liability

Balance, December 31, 2011	$	1,516		$	—
Unrealized gain included in other income (expense)		(587	)		—
Reclassification to common stock warrants		(929	)		—
Portion of convertible note allocated to derivative		—			196

Balance, December 31, 2012	$	—		$	196

- Definition

Tabular disclosure of the fair value measurement of liabilities using significant unobservable inputs (Level 3), a reconciliation of the beginning and ending balances, separately presenting changes attributable to the following: (1) total gains or losses for the period (realized and unrealized), segregating those gains or losses included in earnings (or changes in net assets), and gains or losses recognized in other comprehensive income (loss) and a description of where those gains or losses included in earnings (or changes in net assets) are reported in the statement of income (or activities); (2) purchases, sales, issues, and settlements (each type disclosed separately); and (3) transfers in and transfers out of Level 3 (for example, transfers due to changes in the observability of significant inputs) by class of liability.

+ References+ Details

Goodwill and Intangible Assets, Net (Tables)

12 Months Ended

Dec. 31, 2012

Goodwill and Intangible Assets, Net

Schedule of changes in the carrying value of goodwill, IPR&D and intangible assets

(in thousands)	Intangible assets			IPR&D		Goodwill

Balance, December 31, 2009		3,125			7,010		3,605
Amortization		(320	)		—		—

Balance, December 31, 2010		2,805			7,010		3,605
Amortization		(320	)		—		—

Balance, December 31, 2011	$	2,485		$	7,010	$	3,605
Amortization		(320	)		—		—

Balance, December 31, 2012		2,165			7,010		3,605

Schedule of expected amortization expense for intangible assets for the next five-year period

Years Ended December 31,	(in thousands)

2013	$	320
2014		320
2015		320
2016		320
2017		320

Accounts Payables, Accrued Expenses and Other (Tables)

12 Months Ended

Dec. 31, 2012

Accounts Payables, Accrued Expenses and Other

Schedule of accounts payable, accrued expenses and other

	December 31,

(in thousands)	2011		2012
Accounts payable		4,656		283
Accrued goods and services	$	9,189	$	17,615
Accrued payroll and related benefits		3,666		5,853
Accrued interest		—		306
Accrued dividends payable		—		28
Contractual liability (Note 19)		—		851

Total accounts payable, accrued expenses and other	$	17,511	$	24,936

Debt (Tables)

12 Months Ended

Dec. 31, 2012

Convertible notes

Debt

Schedule of debt

	December 31, 2012

	(in thousands)
Total convertible note outstanding	$	1,571
Unamortized discount		(196	)

Net carrying amount of the convertible note		1,375

Debt

Debt

Schedule of future minimum payments under the loans payable

Years Ending December 31: (in thousands)

2013	$	5,439
2014		18,138
2015		18,123
2016		8,830

	$	50,530
Less interest		(9,330	)
Less unamortized discount		(2,720	)
Less current portion		(998	)

Loans payable, net of current portion	$	37,482

Convertible preferred stock (Tables)

12 Months Ended

Dec. 31, 2012

Convertible Preferred Stock

Schedule of convertible preferred stock

	Convertible Preferred Stock
	Carrying Value		Shares Outstanding		Liquidation Preference		Conversion Price
Series B	$	14,046		3,874	$	4.40	$	2.85
Series C		24,459		14,424	$	1.89	$	1.89
Series D		28,267		8,086	$	3.50	$	3.50
Series E		64,531		14,991	$	4.50	$	4.50
Series F		59,973		11,776	$	5.10	$	5.10
Series G		76,949		11,000	$	7.00	$	7.00

Balance at December 31, 2011	$	268,225		64,151

- Definition

Tabular disclosure of an entity's stock, including par or stated value per share, number and dollar amount of share subscriptions, shares authorized, shares issued, shares outstanding, number and dollar amount of shares held in an employee trust, dividend per share, total dividends, share conversion features, par value plus additional paid in capital, the value of treasury stock and other information necessary to a fair presentation, and EPS information. Stock by class includes common, convertible, and preferred stocks which are not redeemable or redeemable solely at the option of the issuer. Includes preferred stock with redemption features that are solely within the control of the issuer and mandatorily redeemable stock if redemption is required to occur only upon liquidation or termination of the reporting entity. If more than one issue is outstanding, state the title of each issue and the corresponding dollar amount; dollar amount of any shares subscribed but unissued and the deduction of subscriptions receivable there from; number of shares authorized, issued, and outstanding.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.5-02.28,29)

-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 480

-SubTopic 10

-Section S99

-Paragraph 1

-URI http://asc.fasb.org/extlink&oid=14643745&loc=d3e177068-122764

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 150

-Paragraph 9

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 4: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 505

-SubTopic 10

-Section 50

-Paragraph 8

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-Name Accounting Standards Codification

-Topic 235

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.4-08.(d),(e))

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-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 02

-Paragraph 29

-Article 5

Reference 7: http://www.xbrl.org/2003/role/presentationRef

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-Name Accounting Standards Codification

-Topic 505

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-Topic 505

-SubTopic 10

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-Paragraph 4

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-Topic 505

-SubTopic 10

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-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 02

-Paragraph 30

-Article 5

Reference 11: http://www.xbrl.org/2003/role/presentationRef

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-Name Accounting Standards Codification

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-Paragraph 2

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-Name Accounting Standards Codification

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-Publisher FASB

-Name Accounting Standards Codification

-Topic 505

-SubTopic 10

-Section 50

-Paragraph 7

-URI http://asc.fasb.org/extlink&oid=6928386&loc=d3e21521-112644

+ Details

Stock Warrants (Tables)

12 Months Ended

Dec. 31, 2012

Stock Warrants

Schedule of common and convertible preferred stock warrant activity of the Company

(in thousands, except per share amounts)			Weighted average exercise price				Weighted average exercise price

Balance—December 31, 2009	2,937			2.35	317		$	3.42
Exercised	—				(11	)	$	1.89

Balance—December 31, 2010	2,937			2.93	306		$	3.48
Expired	(1	)		2.47	—			—
Exercised	(296	)		2.46	(4	)	$	1.89

Balance—December 31, 2011	2,640		$	2.98	302		$	3.50
Conversion	302		$	3.50	(302	)	$	3.50
Expired	—			—	—			—
Exercised	(100	)	$	2.63	—			—

Balance—December 31, 2012	2,842		$	3.05	—			—

Stock-Based Compensation (Tables)

12 Months Ended

Dec. 31, 2012

Stock-Based Compensation

Schedule of recognized stock-based compensation expense

	Years ended December 31,

(in thousands)	2010		2011		2012
Employee awards:
Research and development	$	2,787	$	3,597	$	4,234
General and administrative		1,706		2,875		2,510

Stock-based compensation for employee awards		4,493		6,472		6,744
Stock-based compensation for nonemployee awards		58		480		145

Total stock-based compensation	$	4,551	$	6,952	$	6,889

Schedule of assumptions used to calculate fair value of options granted

	Years ended December 31,

	2010	2011	2012
Risk-free interest rate	1.7 - 2.8%	1.3 - 2.5%	0.7 - 1.1%
Expected dividend yield	0%	0%	0%
Expected term	5 - 5.9 years	5 - 5.9 years	5 - 5.9 years
Expected volatility	73 - 77%	71 - 73%	66 - 72%

Summary of stock option activity

			Weighted Average Exercise Price			Aggregate Intrinsic Value

Outstanding at December 31, 2011	17,617		$	2.56
Granted	3,251		$	7.45
Exercised	(2,622	)	$	2.06
Cancelled	(180	)	$	4.34

Outstanding at December 31, 2012	18,066		$	3.50	6.54	$	51,486
Vested and expected to vest at December 31, 2012	17,741		$	3.43	6.49	$	51,434
Exercisable at December 31, 2012	13,616		$	2.52	5.70	$	49,155

Income Taxes (Tables)

12 Months Ended

Dec. 31, 2012

Income Taxes

Schedule of reconciliation of the Company's effective tax rate to the statutory federal income tax rate

	Years ended December 31,

	2010			2011			2012
Federal statutory rate		35.0	%		35.0	%		35.0	%
State taxes, net of Federal benefit		4.6			4.2			4.5
Permanent differences		(2.6	)		(0.4	)		(0.2	)
Stock compensation		(2.9	)		(1.2	)		(0.3	)
Change in valuation allowance		(39.2	)		(36.3	)		(35.5	)
Tax credits		5.1			3.9			1.1
Foreign rate differentials		—			(4.4	)		(4.1	)
Other		—			(0.8	)		(0.5	)

		—	%		—	%		—	%

Schedule of temporary differences that give rise to significant net deferred tax assets

(in thousands)	2011			2012

Deferred tax assets
Net operating losses	$	40,633		$	77,806
Capitalized research and development expenses		47,640			40,083
Credit carryforwards		13,380			14,398
Depreciation		2,337			2,931
Deferred compensation		4,450			5,068
Deferred revenues		26,462			29,936
Accrued expenses		676			1,184
Other		922			1,934

Total gross deferred tax asset		136,500			173,340
Intangible assets		(3,817	)		(3,689	)
Valuation allowance		(132,683	)		(169,651	)

Net deferred taxes	$	—		$	—

Schedule of changes in the valuation allowance

(in thousands)	Balance at beginning of period				Balance at end of period

December 31, 2010	$	81,420	22,461	—	$	103,881
December 31, 2011	$	103,881	28,802	—	$	132,683
December 31, 2012	$	132,683	36,968	—	$	169,651

- Definition

Tabular disclosure of valuation allowances to reduce deferred tax assets to net realizable value, including identification of the deferred tax asset more likely than not will not be fully realized and the corresponding amount of the valuation allowance.

+ References+ Details

Selected Quarterly Financial Data (Unaudited) (Tables)

12 Months Ended

Dec. 31, 2012

Selected Quarterly Financial Data (Unaudited)

Schedule of quarterly financial information

	First Quarter			Second Quarter			Third Quarter			Fourth Quarter

	(in thousands, except per share data)
2011
Collaboration revenues	$	6,461		$	6,595		$	8,582		$	12,577
Total operating expenses		21,102			36,019			27,219			30,744
Net loss		(13,535	)		(29,196	)		(18,724	)		(18,221	)
Net loss attributable to Merrimack Pharmaceuticals, Inc.		(13,457	)		(29,051	)		(18,599	)		(18,116	)
Net loss per share available to common stockholders—basic and diluted	$	(1.34	)	$	(2.76	)	$	(1.81	)	$	(1.76	)

	First Quarter			Second Quarter			Third Quarter			Fourth Quarter

	(in thousands, except per share data)
2012
Collaboration revenues	$	11,344		$	12,063		$	11,323		$	14,191
Total operating expenses		35,379			32,368			35,197			38,719
Net loss		(23,402	)		(20,139	)		(23,320	)		(24,893	)
Net loss attributable to Merrimack Pharmaceuticals, Inc.		(23,284	)		(20,026	)		(23,199	)		(24,768	)
Net loss per share available to common stockholders—basic and diluted	$	(2.14	)	$	(0.22	)	$	(0.25	)	$	(0.26	)

Nature of the Business (Details) (USD $) In Millions, except Share data, unless otherwise specified	12 Months Ended	1 Months Ended
	Dec. 31, 2012 item	Apr. 30, 2012 Initial public offering Common stock
Nature of the Business
Number of targeted therapeutic oncology candidates	6
Public Offering
Aggregate shares of common stock sold in connection with initial public offering (in shares)		15,042,459
Public offering price (in dollars per share)		$ 7.00
Common stock issued to underwriters under an over-allotment option (in shares)		742,459
Net proceeds from initial public offering		$ 98.1

- Definition

Number of new stock issued during the period.

+ References+ Details

Nature of the Business (Details 2) (Loan Agreement with Hercules, USD $) In Millions, unless otherwise specified	0 Months Ended	3 Months Ended
	Nov. 08, 2012	Dec. 31, 2012
Loan Agreement with Hercules
Nature of the business
Initial term loan advance	$ 25.0
Additional term loan which closed on December 14, 2012	15.0
Net proceeds from loan		$ 39.6

- Definition

The cash inflow from amounts received from issuance of long-term debt that is wholly or partially secured by collateral. Excludes proceeds from tax exempt secured debt.

+ References+ Details

Summary of Significant Accounting Policies (Details) (USD $)	12 Months Ended
	Dec. 31, 2012 item	Dec. 31, 2011
Segment and Geographic Information
Number of operating segments	1
Number of geographic segments	1
Cash, Cash Equivalents and Restricted Cash
Restricted cash	$ 628,000	$ 381,000
Silver Creek
Principles of consolidation
Ownership interest (as a percent)	74.00%	74.00%

- Definition

The carrying amounts of cash and cash equivalent items which are restricted as to withdrawal or usage. Restrictions may include legally restricted deposits held as compensating balances against borrowing arrangements, contracts entered into with others, or entity statements of intention with regard to particular deposits; however, time deposits and short-term certificates of deposit are not generally included in legally restricted deposits. Excludes compensating balance arrangements that are not agreements which legally restrict the use of cash amounts shown on the balance sheet. This element is for unclassified presentations; for classified presentations there is a separate and distinct element.

+ References+ Details

Summary of Significant Accounting Policies (Details 3)	12 Months Ended
Summary of Significant Accounting Policies (Details 3)	Dec. 31, 2012	Dec. 31, 2011
Concentration of Credit Risk
Number of issues with limited investment as per investment policy	1
Collaboration revenues \| Sales risk \| Sanofi \| Minimum
Concentration of Credit Risk
Concentration risk (as a percent)	98.00%	99.00%
Accounts receivable \| Credit risk \| Sanofi \| Minimum
Concentration of Credit Risk
Concentration risk (as a percent)	99.00%	99.00%

Summary of Significant Accounting Policies (Details 5) (Silver Creek, USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Silver Creek
Consolidated subsidiaries
Balance at the beginning of the period	$ 574	$ 1,027
Acquisition of Silver Creek preferred stock			1,082
Net loss attributable to Silver Creek	(477)	(453)	(55)
Balance at the end of the period	$ 97	$ 574	$ 1,027

Summary of Significant Accounting Policies (Details 6) (USD $)	Dec. 31, 2012	Dec. 21, 2012 Silver Creek Convertible Note Silver Creek
Summary of significant accounting policies
Derivative liability	$ 196,000	$ 196,000

- Definition

Fair value as of the balance sheet date of the embedded derivative or group of embedded derivatives classified as a liability.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 133

-Paragraph 12-16, 44A, 44B

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 133

-Paragraph 44A

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 133

-Paragraph 44A, 44B

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 4: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 133

-Paragraph 44B

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 5: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 815

-SubTopic 10

-Section 50

-Paragraph 4B

-Subparagraph (c)

-URI http://asc.fasb.org/extlink&oid=7476318&loc=SL5624163-113959

+ Details

Summary of Significant Accounting Policies (Details 7) (USD $)	12 Months Ended				1 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Apr. 30, 2012	Apr. 30, 2012 Initial public offering Common stock
Public Offering
Convertible preferred stock converted into common stock (in shares)					66,255,529
Convertible Preferred Stock and Convertible Preferred Stock Warrants
Number of preferred stock authorized (in shares)	10,000,000	10,000,000		10,000,000
Par value of preferred stock ( in dollars per share )	$ 0.01	$ 0.01		$ 0.01
Change in fair value of convertible preferred stock warrants	$ 587,000	$ (864,000)	$ (74,000)

- Definition

The number of new shares issued in the conversion of stock in a noncash (or part noncash) transaction. Noncash is defined as transactions during a period that do not result in cash receipts or cash payments in the period. "Part noncash" refers to that portion of the transaction not resulting in cash receipts or cash payments in the period.

+ References+ Details

- Definition

Face amount or stated value per share of nonredeemable preferred stock (or preferred stock redeemable solely at the option of the issuer); generally not indicative of the fair market value per share.

+ References+ Details

Summary of Significant Accounting Policies (Details 8) (USD $)	1 Months Ended	12 Months Ended			1 Months Ended	12 Months Ended			1 Months Ended	12 Months Ended	1 Months Ended	3 Months Ended	12 Months Ended
	Apr. 30, 2012	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Jan. 31, 2010 Life Sciences Tax Incentive Program 2010 item	Dec. 31, 2012 Life Sciences Tax Incentive Program 2010	Dec. 31, 2011 Life Sciences Tax Incentive Program 2010	Dec. 31, 2010 Life Sciences Tax Incentive Program 2010	Jan. 31, 2011 Life Sciences Tax Incentive Program 2011 item	Dec. 31, 2012 Life Sciences Tax Incentive Program 2011	Oct. 31, 2010 Federal Qualifying Therapeutic Discovery Projects program	Mar. 31, 2011 Federal Qualifying Therapeutic Discovery Projects program	Dec. 31, 2010 Federal Qualifying Therapeutic Discovery Projects program
Summary of significant accounting policies
Tax incentive awarded					$ 1,500,000				$ 1,347,000
State research and development tax credits monetized					1,350,000				1,212,000
Number of incremental employees pledged to be hired					50				50
Performance period					5 years				5 years
Amount of benefit recognized						270,000	270,000	270,000		484,000
Grants awarded											2,445,000
Grants received												504,000	1,941,000
Other income		(1,357,000)	(1,150,000)	(2,669,000)									(2,445,000)
Other Income (Expense)
Settlement from former service provider			1,800,000
Deferred Financing Costs
Deferred financing costs		32,000	1,946,000
Deferred financing costs netted against the equity proceeds within stockholders' equity	$ 2,748,000	$ 2,748,000		$ 25,000

Initial Public Offering (Details) (Initial public offering, USD $) In Millions, except Share data, unless otherwise specified	1 Months Ended
	Apr. 30, 2012
Public Offering
Cash dividends payable to Series B convertible preferred stock holders	$ 4.3
Common stock
Public Offering
Aggregate shares of common stock sold in connection with initial public offering (in shares)	15,042,459
Public offering price (in dollars per share)	$ 7.00
Common stock issued to underwriters under an over-allotment option (in shares)	742,459
Net proceeds from initial public offering	$ 98.1
Convertible preferred stock converted into common stock (in shares)	66,255,529

- Definition

+ References+ Details

- Definition

Equity impact of preferred stock cash dividends declared by an entity during the period. This element includes paid and unpaid dividends declared during the period.

+ References+ Details

- Definition

Number of new stock issued during the period.

+ References+ Details

Marketable Securities (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2012 item
Marketable Securities
Amortized Cost	$ 72,276
Unrealized Gains	3
Unrealized Losses	(41)
Fair Value	72,238
Number of securities in unrealized loss position for less than 12 months	18
Aggregate Fair Value	51,364
Unrealized Losses	(41)
Certificate of deposit
Marketable Securities
Amortized Cost	240
Fair Value	240
Commercial paper
Marketable Securities
Amortized Cost	12,479
Unrealized Losses	(14)
Fair Value	12,465
Aggregate Fair Value	12,465
Unrealized Losses	(14)
Corporate debt securities
Marketable Securities
Amortized Cost	59,557
Unrealized Gains	3
Unrealized Losses	(27)
Fair Value	59,533
Aggregate Fair Value	38,899
Unrealized Losses	$ (27)

- Definition

This item represents the aggregate fair value of investments in debt and equity securities categorized neither as held-to-maturity nor trading securities that have been in a continuous unrealized loss position for less than twelve months.

+ References+ Details

- Definition

Amount of available-for-sale debt securities at cost, net of adjustments, which include, but are not limited to, accretion, amortization, collection of cash, previous other-than-temporary impairments (OTTI) recognized in earnings (less any cumulative-effect adjustments, as defined) and fair value hedge accounting adjustments.

+ References+ Details

- Definition

This element represents the portion of the balance sheet assertion valued at fair value by the entity whether such amount is presented as a separate caption or as a parenthetical disclosure. Additionally, this element may be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements. The element may be used in both the balance sheet and disclosure in the same submission. This item represents Available-for-sale Securities which consist of all investments in certain debt and equity securities neither classified as trading or held-to-maturity securities. A debt security represents a creditor relationship with an enterprise. Debt securities include, among other items, US Treasury securities, US government securities, municipal securities, corporate bonds, convertible debt, commercial paper, and all securitized debt instruments. An equity security represents an ownership interest in an enterprise or the right to acquire or dispose of an ownership interest in an enterprise at fixed or determinable prices. Equity securities include, among other things, common stock, certain preferred stock, warrant rights, call options, and put options, but do not include convertible debt. An entity may opt to provide the reader with additional narrative text to better understand the nature of investments in debt and equity securities which are categorized as Available-for-sale.

+ References+ Details

- Definition

This item represents the gross unrealized gains for securities, at a point in time, which are categorized neither as held-to-maturity nor trading securities.

+ References+ Details

- Definition

Disclosure regarding factors used to determine that the impairment of securities not categorized as either held-to-maturity securities or trading securities (hence equal to available for sale securities) where cost exceeds fair value is not an other than temporary impairment (OTTI). This item contains disclosure of the number of investment positions in the available-for-sale investments determined to be temporarily impaired.

+ References+ Details

Net Loss Per Common Share (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Numerator - basic:
Net loss attributable to Merrimack Pharmaceuticals, Inc.	$ (24,768)	$ (23,199)	$ (20,026)	$ (23,284)	$ (18,116)	$ (18,599)	$ (29,051)	$ (13,457)	$ (91,277)	$ (79,223)	$ (50,104)
Plus: Unaccreted dividends on convertible preferred stock									(2,107)	(7,789)	(11,185)
Net loss available to common stockholders - basic									(93,384)	(87,012)	(61,289)
Numerator - diluted:
Net loss attributable to Merrimack Pharmaceuticals, Inc.	(24,768)	(23,199)	(20,026)	(23,284)	(18,116)	(18,599)	(29,051)	(13,457)	(91,277)	(79,223)	(50,104)
Plus: Unaccreted dividends on convertible preferred stock									(2,107)	(7,789)	(11,185)
Net loss available to common stockholders - diluted									$ (93,384)	$ (87,012)	$ (61,289)
Denominator:
Weighted-average common shares-basic and diluted									72,831	11,343	10,994
Net loss per share available to common stock holders-basic and diluted (in dollar per share)	$ (0.26)	$ (0.25)	$ (0.22)	$ (2.14)	$ (1.76)	$ (1.81)	$ (2.76)	$ (1.34)	$ (1.28)	$ (7.67)	$ (5.57)
Convertible preferred stock
Potentially dilutive securities excluded from computation of diluted weighted average shares
Potentially dilutive securities (in shares)										66,256	55,253
Options to purchase common stock
Potentially dilutive securities excluded from computation of diluted weighted average shares
Potentially dilutive securities (in shares)									18,066	17,617	16,214
Convertible preferred stock warrants
Potentially dilutive securities excluded from computation of diluted weighted average shares
Potentially dilutive securities (in shares)										302	306
Common stock warrants
Potentially dilutive securities excluded from computation of diluted weighted average shares
Potentially dilutive securities (in shares)									2,842	2,640	2,937

License and Collaboration Agreements (Details) (USD $)	3 Months Ended								12 Months Ended						1 Months Ended	12 Months Ended			1 Months Ended	3 Months Ended	12 Months Ended			1 Months Ended	12 Months Ended		1 Months Ended
License and Collaboration Agreements (Details) (USD $)	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2012 Sanofi Phase 2 clinical trials in breast cancer and non-small cell lung cancer and ovarian cancer	Dec. 31, 2011 Sanofi Phase 2 clinical trials in breast cancer and non-small cell lung cancer and ovarian cancer	Dec. 31, 2010 Sanofi Phase 2 clinical trials in breast cancer and non-small cell lung cancer and ovarian cancer	Nov. 30, 2009 License and Collaboration Agreements Sanofi	Dec. 31, 2012 License and Collaboration Agreements Sanofi	Dec. 31, 2011 License and Collaboration Agreements Sanofi	Dec. 31, 2010 License and Collaboration Agreements Sanofi	Jul. 31, 2009 License and Collaboration Agreements GTC	Dec. 31, 2012 License and Collaboration Agreements GTC	Dec. 31, 2012 License and Collaboration Agreements GTC	Dec. 31, 2011 License and Collaboration Agreements GTC	Dec. 31, 2010 License and Collaboration Agreements GTC	Jul. 31, 2009 License and Collaboration Agreements GTC Series C convertible preferred stock	Dec. 31, 2012 License and Collaboration Agreements PharmaEngine	Dec. 31, 2011 License and Collaboration Agreements PharmaEngine	May 31, 2011 License and Collaboration Agreements PharmaEngine Development and regulatory milestone	May 31, 2011 License and Collaboration Agreements PharmaEngine Sales milestone
License and collaboration agreements
Upfront license fee received															$ 60,000,000
Milestone payments received												5,000,000	10,000,000	10,000,000
Expected development period from the effective date of agreement															12 years				19 years
License agreement termination period																				3 months
Additional revenue recongnized due to a change in estimate																				657,000
Recognized revenue under collaboration agreements
Upfront payment																5,000,000	5,000,000	5,000,000
Milestone payment																2,975,000	2,616,000	949,000
Development services																36,905,000	25,053,000	13,279,000
Manufacturing services and other																3,307,000	1,456,000	630,000
Total																48,187,000	34,125,000	19,858,000
Assets and liabilities related to collaboration agreement
Accounts receivable, billed																1,577,000	4,478,000
Accounts receivable, unbilled																7,690,000	2,925,000
Deferred revenues																79,913,000	84,466,000			553,000	553,000	1,279,000	1,356,000
Collaborative arrangement additional information
Shares received as consideration under the license agreement																								662,000
Fair value of shares received as consideration under the license agreement																								1,469,000
Revenue recognized	14,191,000	11,323,000	12,063,000	11,344,000	12,577,000	8,582,000	6,595,000	6,461,000	48,921,000	34,215,000	20,305,000										733,000	76,000	76,000
Maximum milestone payment obligation																											80,000,000	130,000,000
Research and development expenses									125,858,000	100,630,000	58,278,000														6,200,000	11,200,000
Amounts payable related to the agreement																									$ 345,000	$ 280,000

Fair Value of Financial Instruments (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Assets
Investments	$ 72,238
Liabilities
Convertible preferred stock warrants (in shares)		1,516
Corporate debt securities
Assets
Investments	59,533
Certificate of deposit
Assets
Investments	240
Commercial paper
Assets
Investments	12,465
Recurring basis \| Level 1
Assets
Cash equivalents - money market funds	25,668
Recurring basis \| Level 1 \| U.S. treasury securities
Assets
Cash equivalents		35,076
Recurring basis \| Level 2 \| Corporate debt securities
Assets
Cash equivalents	5,017
Investments	59,533
Recurring basis \| Level 2 \| Certificate of deposit
Assets
Cash equivalents	480
Investments	240
Recurring basis \| Level 2 \| Commercial paper
Assets
Investments	12,465
Recurring basis \| Level 3
Liabilities
Convertible preferred stock warrants (in shares)		1,516
Derivative liability	$ 196

- Definition

+ References+ Details

- Definition

Amount of short-term, highly liquid investments that are both readily convertible to known amounts of cash and so near their maturity that they present insignificant risk of changes in value because of changes in interest rates. Excludes cash and cash equivalents within disposal group and discontinued operation.

+ References+ Details

- Definition

Derivative instrument obligations meeting the definition of a liability which the Company is a party to as of the balance sheet date.

+ References+ Details

- Definition

Investment in short-term money-market instruments (such as commercial paper, banker's acceptances, repurchase agreements, government securities, certificates of deposit, and so forth) which are highly liquid (that is, readily convertible to known amounts of cash) and so near their maturity that they present an insignificant risk of changes in value because of changes in interest rates. Generally, only investments with original maturities of three months or less qualify as cash equivalents by definition. Original maturity means an original maturity to the entity holding the investment. For example, both a three-month US Treasury bill and a three-year Treasury note purchased three months from maturity qualify as cash equivalents. However, a Treasury note purchased three-years ago does not become a cash equivalent when its remaining maturity is three months.

+ References+ Details

Fair Value of Financial Instruments (Details 2) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012
Convertible preferred stock warrants
Roll-forward of the fair value of the convertible preferred stock warrants categorized as Level 3 instruments
Balance at the beginning of the period	$ 1,516
Unrealized gain included in other income (expense)	(587)
Reclassification to common stock warrants	(929)
Derivative liability
Roll-forward of the fair value of the convertible preferred stock warrants categorized as Level 3 instruments
Portion of convertible note allocated to derivative	196
Balance at the end of the period	$ 196

Consolidated Subsidiaries (Details) (Hermes, USD $)	0 Months Ended	12 Months Ended
Consolidated Subsidiaries (Details) (Hermes, USD $)	Oct. 06, 2009	Dec. 31, 2010
Consolidated subsidiaries
Shares of common stock issued	4,383,000
Common shares of Merrimack Pharmaceuticals, Inc.	$ 9,292,000
Number of additional shares that could have been required to be issued as contingent consideration		400,000
Contingent consideration	178,000
Gain recognized on settlement of contingent consideration		$ 178,000
Minimum
Consolidated subsidiaries
Number of additional shares that could have been required to be issued as contingent consideration	0
Maximum
Consolidated subsidiaries
Number of additional shares that could have been required to be issued as contingent consideration	1,100,000
Period for issuance of additional shares as contingent consideration	24 months

- Definition

Fair value, as of the acquisition date, of potential payments under the contingent consideration arrangement including cash and shares.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 141

-Paragraph 51

-Subparagraph f

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 141R

-Paragraph 65

-Subparagraph a, b

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 141R

-Paragraph 68

-Subparagraph g(1)

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 4: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 141R

-Paragraph 29, 30, 41, 42, 64

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 5: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 805

-SubTopic 30

-Section 25

-Paragraph 5

-URI http://asc.fasb.org/extlink&oid=6911189&loc=d3e6405-128476

Reference 6: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 805

-SubTopic 20

-Section 35

-Paragraph 4

-URI http://asc.fasb.org/extlink&oid=6910684&loc=d3e4570-128470

Reference 7: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 805

-SubTopic 30

-Section 25

-Paragraph 7

-URI http://asc.fasb.org/extlink&oid=6911189&loc=d3e6411-128476

Reference 8: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 805

-SubTopic 20

-Section 30

-Paragraph 18

-URI http://asc.fasb.org/extlink&oid=18499824&loc=d3e4237-128469

Reference 9: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 805

-SubTopic 20

-Section 30

-Paragraph 19

-URI http://asc.fasb.org/extlink&oid=18499824&loc=d3e4243-128469

Reference 10: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 805

-SubTopic 20

-Section 40

-Paragraph 3

-URI http://asc.fasb.org/extlink&oid=6910732&loc=d3e4805-128471

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-Publisher FASB

-Name Accounting Standards Codification

-Topic 805

-SubTopic 30

-Section 25

-Paragraph 6

-URI http://asc.fasb.org/extlink&oid=6911189&loc=d3e6408-128476

Reference 12: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 805

-SubTopic 20

-Section 25

-Paragraph 28

-URI http://asc.fasb.org/extlink&oid=6946367&loc=d3e3642-128468

Reference 13: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 805

-SubTopic 20

-Section 25

-Paragraph 27

-URI http://asc.fasb.org/extlink&oid=6946367&loc=d3e3629-128468

Reference 14: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 805

-SubTopic 30

-Section 50

-Paragraph 1

-Subparagraph (b)(3),(c)(1)

-URI http://asc.fasb.org/extlink&oid=7488404&loc=d3e6927-128479

+ Details

- Definition

The acquisition-date fair value of the equity interests of the acquirer, including the number of instruments or interests issued or issuable in consideration for the business combination.

+ References+ Details

Consolidated Subsidiaries (Details 2) (USD $)	12 Months Ended					0 Months Ended	4 Months Ended
Consolidated Subsidiaries (Details 2) (USD $)	Dec. 31, 2011	Dec. 31, 2012	Dec. 31, 2012 Silver Creek	Dec. 31, 2011 Silver Creek	Dec. 31, 2010 Silver Creek	Aug. 20, 2010 Silver Creek Series A Preferred Stock	Dec. 17, 2010 Silver Creek Series A Preferred Stock	Dec. 31, 2012 Silver Creek Series A Preferred Stock	Dec. 31, 2011 Silver Creek Series A Preferred Stock
Consolidated subsidiaries
Additional shares purchased by parent company related to initial purchase						12,000,000
Shares issued to other investors							4,190,000
Proceeds from shares issued to other investors						$ 4,165,000
Issuance cost for shares issued to other investors	1,946,000					25,000
Ownership interest (as a percent)			74.00%	74.00%
Non-controlling interest	574,000	97,000	97,000	574,000	1,027,000
Total assets of consolidated subsidiaries	85,299,000	148,974,000	2,202,000	2,302,000
Total liabilities of consolidated subsidiaries	$ 106,990,000	$ 155,394,000	$ 1,763,000	$ 39,000
Shares owned by employees and directors of the parent company (as a percent)								6.00%	6.00%

Goodwill and Intangible Assets, Net (Details) (Hermes, USD $)	Oct. 06, 2009
Goodwill and intangible assets, net
Acquired IPR&D recognized in a business combination	$ 7,010,000
Core nano-carrier technology
Goodwill and intangible assets, net
Acquired intangible assets	3,200,000
Antibody-targeted nanotherapeutic program with chemotherapy drug
Goodwill and intangible assets, net
Acquired IPR&D recognized in a business combination	2,800,000
Nanotherapeutic program with chemotherapy drug
Goodwill and intangible assets, net
Acquired IPR&D recognized in a business combination	3,400,000
Other in-process research and development programs
Goodwill and intangible assets, net
Acquired IPR&D recognized in a business combination	$ 810,000

Goodwill and Intangible Assets, Net (Details 2) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Intangible assets
Balance at the beginning of the period	$ 2,485	$ 2,805	$ 3,125
Amortization	(320)	(320)	(320)
Balance at the end of the period	2,165	2,485	2,805
Amortization period of core technology acquired from Hermes	10 years
IPR & D
In-process research and development	7,010	7,010	7,010	7,010
Goodwill
Goodwill	3,605	3,605	3,605	3,605
Expected amortization expenses for the next five-year period:
2013	320
2014	320
2015	320
2016	320
2017	$ 320

Property and Equipment, Net (Details) (USD $)	12 Months Ended
Property and Equipment, Net (Details) (USD $)	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Property and equipment, net
Property and equipment, gross	$ 25,266,000	$ 22,336,000
Less: Accumulated depreciation and amortization	(18,969,000)	(16,130,000)
Property and equipment, net	6,297,000	6,206,000
Depreciation	3,510,000	5,006,000	4,059,000
Gross amount of fully depreciated fixed assets disposed			26,000
Gross amount of fully depreciated fixed assets disposed	671,000
Recognized impairment charges	0	0	0
Lab equipment
Property and equipment, net
Property and equipment, gross	12,616,000	11,757,000
IT equipment
Property and equipment, net
Property and equipment, gross	2,346,000	2,204,000
Leasehold improvements
Property and equipment, net
Property and equipment, gross	8,200,000	7,698,000
Furniture and fixtures
Property and equipment, net
Property and equipment, gross	330,000	329,000
Construction in process
Property and equipment, net
Property and equipment, gross	1,774,000	348,000
Property and equipment under capital leases
Property and equipment, net
Property and equipment, net	0	4,114,000
Depreciation	$ 0	$ 26,000	$ 409,000

- Definition

The cumulative amount of depreciation, depletion and amortization (related to property, plant and equipment, but not including land) that has been recognized in the income statement.

+ References+ Details

- Definition

The charge against earnings resulting from the aggregate write down of all assets from their carrying value to their fair value.

+ References+ Details

- Definition

The amount of expense recognized in the current period that reflects the allocation of the cost of tangible assets over the assets' useful lives. Includes production and non-production related depreciation.

+ References+ Details

- Definition

Gross amount of long-lived physical assets used in the normal conduct of business and not intended for resale. Examples include, but are not limited to, land, buildings, machinery and equipment, office equipment, furniture and fixtures, and computer equipment.

+ References+ Details

- Definition

Amount, net of accumulated depreciation, depletion and amortization, of long-lived physical assets used in the normal conduct of business and not intended for resale. Examples include, but are not limited to, land, buildings, machinery and equipment, office equipment, furniture and fixtures, and computer equipment.

+ References+ Details

Accounts Payables, Accrued Expenses and Other (Details) (USD $)	Dec. 31, 2012	Dec. 31, 2011
Accounts payable, accrued expenses and other
Accounts payable	$ 283,000	$ 4,656,000
Accrued goods and services	17,615,000	9,189,000
Accrued payroll and related benefits	5,853,000	3,666,000
Accrued interest	306,000
Accrued dividends payable	28,000
Total accounts payable, accrued expenses, and other	24,936,000	17,511,000
License and Collaboration Agreements \| Sanofi
Accounts payable, accrued expenses and other
Contractual liability	$ 851,000

- Definition

Carrying value as of the balance sheet date of dividends declared but unpaid on equity securities issued by the entity and outstanding. Used to reflect the current portion of the liabilities (due within one year or within the normal operating cycle if longer).

+ References+ Details

- Definition

Carrying value as of the balance sheet date of [accrued] interest payable on all forms of debt, including trade payables, that has been incurred and is unpaid. Used to reflect the current portion of the liabilities (due within one year or within the normal operating cycle if longer).

+ References+ Details

- Definition

Aggregate carrying amount of current liabilities (due within one year or within the normal operating cycle if longer) not separately disclosed in the balance sheet. Includes costs that are statutory in nature, are incurred on contractual obligations, or accumulate over time and for which invoices have not yet been received or will not be rendered and of liabilities not separately disclosed.

+ References+ Details

Debt (Details) (USD $)			0 Months Ended	12 Months Ended	0 Months Ended					0 Months Ended
Debt (Details) (USD $)	Dec. 31, 2012	Dec. 31, 2012 Debt	Nov. 08, 2012 Loan Agreement with Hercules	Dec. 31, 2012 Loan Agreement with Hercules	Nov. 08, 2012 Loan Agreement with Hercules Minimum	Nov. 08, 2012 Loan Agreement with Hercules First alternative interest rate	Nov. 08, 2012 Loan Agreement with Hercules Second alternative interest rate	Dec. 31, 2012 Convertible notes Silver Creek	Dec. 21, 2012 Convertible notes Silver Creek	Dec. 21, 2012 Convertible notes Silver Creek Minimum
Debt
Initial term loan advance			$ 25,000,000
Additional term loan			15,000,000
Minimum stated percentage rate (as a percent)						10.55%
Stated percentage used as basis for calculation of alternative interest rate							10.55%
Description of reference rate that is added to stated percentage rate in calculation of variable rate							prime rate
Percentage points subtracted from alternative interest rate							5.25%
Maximum interest rate on term loan (as a percent)							12.55%
Period of interest-only payment			12 months
Amount of gross proceeds from qualifying equity financing in which holders would automatically convert into the next qualifying equity financing at a 25% dicount										4,000,000
Amount of gross proceeds in one or more transactions prior to December 1, 2013 that the Company may elect to extend the interest-only period by six months					75,000,000
Extension period for interest only payments			6 months
Discount on loan recorded as a long-term liability to be paid upon full repayment or maturity of the loans			1,200,000
Discount on loans related to costs reimbursed by the entity			396,000
Cash interest expense				475,000
Non cash interest expense				78,000
Value of the Company's equity securities available for purchase by the lender			1,000,000
Period to purchase equity securities sold to institutional accredited investors under the Loan Agreement			1 year
Future minimum payments under the loans payable
2013		5,439,000
2014		18,138,000
2015		18,123,000
2016		8,830,000
Loan payable				50,530,000
Less interest				(9,330,000)
Total convertible note outstanding								1,571,000
Less unamortized discount			(1,600,000)	(2,720,000)				(196,000)
Less current portion				(998,000)
Loans payable, net of current portion				37,482,000
Loans payable								1,375,000
Interest rate (as a percent)									6.00%
Discount on automatic conversion into the next qualifying equity financing (as a percent)									25.00%
Derivative liability	$ 196,000								$ 196,000

Convertible Preferred Stock (Details) (Initial public offering, USD $) In Millions, except Per Share data, unless otherwise specified	12 Months Ended
	Dec. 31, 2011
Common stock
Convertible preferred stock
Aggregate gross proceeds from an initial public offering that would trigger automatic conversion of preferred shares to common shares	$ 50
Common stock \| Minimum
Convertible preferred stock
Minimum offering price per share in an initial public offering to trigger automatic conversion of preferred shares to common shares	$ 4.40
Minimum offering price per share in an initial public offering expressed as a percentage of the Series C conversion price to trigger automatic conversion of preferred shares to common shares	250.00%
Series B Preferred Stock
Convertible preferred stock
Dividend to be declared and paid in cash on automatic conversion of preferred shares	$ 4.3

Convertible Preferred Stock (Details 2) (USD $) In Thousands, except Share data, unless otherwise specified	Dec. 31, 2011
Convertible preferred stock
Carrying value	$ 268,225
Shares Outstanding	64,151
Series B Preferred Stock
Convertible preferred stock
Carrying value	14,046
Shares Outstanding	3,874
Liquidation preference (in dollars per share)	$ 4.40
Conversion price (in dollars per share)	$ 2.85
Series C Preferred Stock
Convertible preferred stock
Carrying value	24,459
Shares Outstanding	14,424
Liquidation preference (in dollars per share)	$ 1.89
Conversion price (in dollars per share)	$ 1.89
Series D Preferred Stock
Convertible preferred stock
Carrying value	28,267
Shares Outstanding	8,086
Liquidation preference (in dollars per share)	$ 3.50
Conversion price (in dollars per share)	$ 3.50
Series E Preferred Stock
Convertible preferred stock
Carrying value	64,531
Shares Outstanding	14,991
Liquidation preference (in dollars per share)	$ 4.50
Conversion price (in dollars per share)	$ 4.50
Series F Preferred Stock
Convertible preferred stock
Carrying value	59,973
Shares Outstanding	11,776
Liquidation preference (in dollars per share)	$ 5.10
Conversion price (in dollars per share)	$ 5.10
Series G Preferred Stock
Convertible preferred stock
Carrying value	$ 76,949
Shares Outstanding	11,000
Liquidation preference (in dollars per share)	$ 7.00
Conversion price (in dollars per share)	$ 7.00

- Definition

The per share liquidation preference (or restrictions) of stock classified as temporary equity that has a preference in involuntary liquidation considerably in excess of the par or stated value of the shares. The liquidation preference is the difference between the preference in liquidation and the par or stated values of the share. Temporary equity is a security with redemption features that are outside the control of the issuer, is not classified as an asset or liability in conformity with GAAP, and is not mandatorily redeemable. Includes any type of security that is redeemable at a fixed or determinable price or on a fixed or determinable date or dates, is redeemable at the option of the holder, or has conditions for redemption which are not solely within the control of the issuer. If convertible, the issuer does not control the actions or events necessary to issue the maximum number of shares that could be required to be delivered under the conversion option if the holder exercises the option to convert the stock to another class of equity. If the security is a warrant or a rights issue, the warrant or rights issue is considered to be temporary equity if the issuer cannot demonstrate that it would be able to deliver upon the exercise of the option by the holder in all cases. Includes stock with put option held by ESOP and stock redeemable by holder only in the event of a change in control of the issuer.

+ References+ Details

- Definition

Amount to be paid per share that is classified as temporary equity by entity upon redemption. Temporary equity is a security with redemption features that are outside the control of the issuer, is not classified as an asset or liability in conformity with GAAP, and is not mandatorily redeemable. Includes any type of security that is redeemable at a fixed or determinable price or on a fixed or determinable date or dates, is redeemable at the option of the holder, or has conditions for redemption which are not solely within the control of the issuer. If convertible, the issuer does not control the actions or events necessary to issue the maximum number of shares that could be required to be delivered under the conversion option if the holder exercises the option to convert the stock to another class of equity. If the security is a warrant or a rights issue, the warrant or rights issue is considered to be temporary equity if the issuer cannot demonstrate that it would be able to deliver upon the exercise of the option by the holder in all cases. Includes stock with put option held by ESOP and stock redeemable by holder only in the event of a change in control of the issuer.

+ References+ Details

- Definition

The number of securities classified as temporary equity that have been sold (or granted) to the entity's shareholders. Securities issued include securities outstanding and securities held in treasury. Temporary equity is a security with redemption features that are outside the control of the issuer, is not classified as an asset or liability in conformity with GAAP, and is not mandatorily redeemable. Includes any type of security that is redeemable at a fixed or determinable price or on a fixed or determinable date or dates, is redeemable at the option of the holder, or has conditions for redemption which are not solely within the control of the issuer. If convertible, the issuer does not control the actions or events necessary to issue the maximum number of shares that could be required to be delivered under the conversion option if the holder exercises the option to convert the stock to another class of equity. If the security is a warrant or a rights issue, the warrant or rights issue is considered to be temporary equity if the issuer cannot demonstrate that it would be able to deliver upon the exercise of the option by the holder in all cases. Includes stock with put option held by ESOP and stock redeemable by holder only in the event of a change in control of the issuer.

+ References+ Details

Convertible Preferred Stock (Details 3) (USD $) In Thousands, except Share data, unless otherwise specified			1 Months Ended
	Dec. 31, 2012	Apr. 30, 2012 Series B Preferred Stock	Apr. 30, 2012 Initial public offering Common stock
Convertible preferred stock
Convertible preferred stock converted into common stock (in shares)			66,255,529
Accrued dividends on preferred stock paid in cash	$ 28	$ 4,300

- Definition

+ References+ Details

Series F Amount (Details) (USD $)	0 Months Ended		3 Months Ended	12 Months Ended
Series F Amount (Details) (USD $)	Oct. 06, 2010	Nov. 02, 2007	Dec. 31, 2010	Dec. 31, 2010
Convertible preferred stock
Accrued noncash interest expense recorded as additional paid-in capital				$ 12,974,000
Series F preferred stock
Convertible preferred stock
Shares intended to be authorized under the 2007 Restated Articles		11,776,000
Imputed noncash interest expense				3,673,000
Number of shares issued	11,776,000
Initial investment recorded as convertible preferred stock (in dollars per share)	$ 5.10
Accrued noncash interest expense recorded as additional paid-in capital			$ 12,974,000

Stock Warrants (Details) (USD $)	3 Months Ended	12 Months Ended			12 Months Ended
Stock Warrants (Details) (USD $)	Sep. 30, 2010 Common stock warrant	Dec. 31, 2012 Common stock warrant	Dec. 31, 2011 Common stock warrant	Dec. 31, 2009 Common stock warrant	Dec. 31, 2012 Convertible preferred stock warrants	Dec. 31, 2011 Convertible preferred stock warrants	Dec. 31, 2010 Convertible preferred stock warrants
Warrants for the purchase of stock
Balance at the beginning of the period (in shares)		2,640,000	2,937,000	2,937,000	302,000	306,000	317,000
Conversion (in shares)		302,000			302,000
Expired (in shares)			(1,000)
Exercised (in shares)		(100,000)	(296,000)			(4,000)	(11,000)
Balance at the end of the period (in shares)		2,842,000	2,640,000	2,937,000		302,000	306,000
Weighted average exercise price
Balance at the beginning of the period (in dollars per share)		$ 2.98	$ 2.93	$ 2.35	$ 3.50	$ 3.48	$ 3.42
Conversion (in dollars per share)		$ 3.50			$ 3.50
Expired (in dollars per share)			$ 2.47
Exercised (in dollars per share)		$ 2.63	$ 2.46			$ 1.89	$ 1.89
Balance at the end of the period (in dollars per share)		$ 3.05	$ 2.98	$ 2.35		$ 3.50	$ 3.48
Additional disclosure
Number of warrants cashless exercised (in shares)		100,000
Number of common stock issued as a result of cashless exercise of warrants (in shares)		71,000
Number of warrants held by related party in which expiration and exercise price was modified (in shares)	2,596,000
Extended period of extension	4 years
Exercise price before modification, one (in dollars per share)	$ 2.12
Exercise price before modification, two (in dollars per share)	$ 2.47
Exercise price of warrants held by related party (in dollars per share)	$ 3.00
Incremental value of warrants accounted within additional paid-in capital	$ 1,803,000

Common Stock (Details) (USD $)	12 Months Ended
Common Stock (Details) (USD $)	Dec. 31, 2010	Dec. 31, 2012	Mar. 31, 2012	Dec. 31, 2011
Common Stock
Common stock, par value (in dollars per share)	$ 0.01	$ 0.01	$ 0.01	$ 0.01
Common stock
Common stock, authorized shares		200,000,000	200,000,000	138,500,000
Common stock, issued shares		95,825,000		11,834,000
Common stock, outstanding shares		95,825,000		11,834,000
Shares reserved for future issuance
Conversion of Series B, Series C, Series D, Series E, Series F and Series G convertible preferred stock				66,256,000
Convertible preferred stock warrants				302,000
Common stock warrants (in shares)		2,842,000		2,640,000
Option to purchase common stock		18,066,000		17,617,000
Shares reserved for future issuance		20,908,000		86,815,000
Common stock
Common stock
Change in par value	$ (17,547,000)
Common stock, authorized shares		200,000,000	200,000,000	200,000,000
Additional paid-in capital
Common stock
Change in par value	$ 17,547,000

Stock-Based Compensation (Details)	12 Months Ended
Stock-Based Compensation (Details)	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Stock-based compensation
Stock options issued (in shares)	3,251,000
2008 Plan
Stock-based compensation
Common stock available for issuance (in shares)		830,000
2011 Plan
Stock-based compensation
Common stock available for issuance (in shares)	1,300,000
Additional common stock available for issuance (in shares)	3,500,000
2011 Plan \| Director
Stock-based compensation
Vesting period	1 year
2008 Plan and 2011 Plan
Stock-based compensation
Stock options issued (in shares)	3,300,000	2,300,000	2,900,000
2008 Plan and 2011 Plan \| Employee
Stock-based compensation
Vesting period	3 years
2008 Plan and 2011 Plan \| Non-employee \| Maximum
Stock-based compensation
Stock options issued (in shares)	100,000	100,000	100,000

Stock-Based Compensation (Details 2) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Allocation of recognized period cost
Stock-based compensation expense for employee awards	$ 6,744	$ 6,472	$ 4,493
Stock-based compensation expense for non-employee awards	145	480	58
Total stock-based compensation expense	6,889	6,952	4,551
Research and development
Allocation of recognized period cost
Stock-based compensation expense for employee awards	4,234	3,597	2,787
General and administrative
Allocation of recognized period cost
Stock-based compensation expense for employee awards	$ 2,510	$ 2,875	$ 1,706

- Definition

Represents the expense recognized during the period arising from equity-based compensation arrangements (for example, shares of stock, unit, stock options or other equity instruments) with employees, directors and certain consultants qualifying for treatment as employees.

+ References+ Details

Stock-Based Compensation (Details 3) (Stock options)	12 Months Ended
Stock-Based Compensation (Details 3) (Stock options)	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Weighted average assumptions used for determining fair value of options
Expected dividend yield (as a percent)	0.00%	0.00%	0.00%
Minimum
Weighted average assumptions used for determining fair value of options
Risk-free interest rate (as a percent)	0.70%	1.30%	1.70%
Expected term	5 years	5 years	5 years
Expected volatility (as a percent)	66.00%	71.00%	73.00%
Maximum
Weighted average assumptions used for determining fair value of options
Risk-free interest rate (as a percent)	1.10%	2.50%	2.80%
Expected term	5 years 10 months 24 days	5 years 10 months 24 days	5 years 10 months 24 days
Expected volatility (as a percent)	72.00%	73.00%	77.00%

- Definition

The estimated measure of the percentage by which a share price is expected to fluctuate during a period. Volatility also may be defined as a probability-weighted measure of the dispersion of returns about the mean. The volatility of a share price is the standard deviation of the continuously compounded rates of return on the share over a specified period. That is the same as the standard deviation of the differences in the natural logarithms of the stock prices plus dividends, if any, over the period.

+ References+ Details

Stock-Based Compensation (Details 4) (USD $) Share data in Thousands, except Per Share data, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Shares
Outstanding at the beginning of the period (in shares)	17,617
Granted (in shares)	3,251
Exercised (in shares)	(2,622)
Cancelled (in shares)	(180)
Outstanding at the end of the period (in shares)	18,066	17,617
Number of shares vested and expected to vest	17,741
Number of stock option exercisable	13,616
Weighted Average Exercise Price
Outstanding at the beginning of the period (in dollars per share)	$ 2.56
Grants (in dollars per share)	$ 7.45
Exercises or releases (in dollars per share)	$ 2.06
Cancelled (in dollars per share)	$ 4.34
Outstanding at the end of the period (in dollars per share)	$ 3.50	$ 2.56
Weighted average exercise price of shares vested and expected to vest	$ 3.43
Weighted average exercise price for exercisable option	$ 2.52
Weighted Average Remaining Contractual Term
Outstanding at the end of the period	6 years 6 months 14 days
Vested and expected to vest	6 years 5 months 26 days
Exercisable	5 years 8 months 12 days
Aggregate Intrinsic Value
Outstanding at the end of the period (in dollars)	$ 51,486,000
Options exercisable (in dollars)	49,155,000
Shares vested and expected to vest (in dollars)	51,434,000
Additional disclosures
Aggregate intrinsic value of options exercised	13,721,000	1,392,000	145,000
Unrecognized compensation cost related to nonvested stock awards	$ 15,924,000
Weighted average period over which unrecognized compensation cost is expected to be recognized	2 years 1 month 6 days

Income Taxes (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Reconciliation of the Company's effective tax rate to the statutory federal income tax rate
Federal statutory rate (as a percent)	35.00%	35.00%	35.00%
State taxes, net of Federal benefit (as a percent)	4.50%	4.20%	4.60%
Permanent differences (as a percent)	(0.20%)	(0.40%)	(2.60%)
Stock compensation (as a percent)	(0.30%)	(1.20%)	(2.90%)
Change in valuation allowance (as a percent)	(35.50%)	(36.30%)	(39.20%)
Tax credits (as a percent)	1.10%	3.90%	5.10%
Foreign rate differentials (as a percent)	(4.10%)	(4.40%)
Other (as a percent)	(0.50%)	(0.80%)
Deferred tax assets
Net operating losses	$ 77,806	$ 40,633
Capitalized research and development expenses	40,083	47,640
Credit carryforwards	14,398	13,380
Depreciation	2,931	2,337
Deferred compensation	5,068	4,450
Deferred revenue	29,936	26,462
Accrued expenses	1,184	676
Others	1,934	922
Total gross deferred tax asset	173,340	136,500
Intangible assets	(3,689)	(3,817)
Valuation allowance	$ (169,651)	$ (132,683)

Income Taxes (Details 3) (USD $)	12 Months Ended
Income Taxes (Details 3) (USD $)	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Income Taxes
Federal and state net operating loss carryforwards relate to deductions for stock option compensation	$ 10,300,000
Change in the valuation allowance against the deferred tax assets
Balance at beginning of period	132,683,000	103,881,000	81,420,000
Additions	36,968,000	28,802,000	22,461,000
Balance at end of period	169,651,000	132,683,000	103,881,000
Research and development \| Federal
Tax credit carryforward
Amount of tax credit carryforward	11,100,000
Research and development \| State
Tax credit carryforward
Amount of tax credit carryforward	4,800,000
Investment \| State
Tax credit carryforward
Amount of tax credit carryforward	$ 400,000

- Definition

The amount of the tax credit carryforward, before tax effects, available to reduce future taxable income under enacted tax laws.

+ References+ Details

- Definition

The amount of the valuation allowance recorded as of the balance sheet date pertaining to the specified deferred tax asset for which an assessment was made that it is more likely than not that all or a portion of such deferred tax asset will not be realized through related deductions on future tax returns.

+ References+ Details

Commitments and Contingencies (Details) (USD $)	1 Months Ended		12 Months Ended			1 Months Ended		3 Months Ended
Commitments and Contingencies (Details) (USD $)	Aug. 31, 2012 item	Mar. 31, 2012	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Nov. 30, 2009 License and Collaboration Agreements Sanofi	Dec. 31, 2012 License and Collaboration Agreements Sanofi	Mar. 31, 2012 Quality control issues related to fill-finish manufacturing services License and Collaboration Agreements Sanofi	Dec. 31, 2012 Quality control issues related to fill-finish manufacturing services License and Collaboration Agreements Sanofi
Operating Leases
Total rent expense			$ 4,317,000	$ 3,235,000	$ 2,846,000
Lease term of additional space		7 years
Aggregate rent due over amended term of lease	31,500,000	2,700,000
Maximum reimbursement for tenant improvements agreed by landlord	6,600,000	500,000
Additional space that the entity will occupy under the amended lease agreement	23,000
Aggregate space that the entity will occupy under the amended lease agreement	109,000
Reimbursement for tenant improvements agreed by landlord in 2012	4,600,000
Reimbursement for tenant improvements agreed by landlord in 2013	1,000,000
Reimbursement for tenant improvements agreed by landlord in 2014	1,000,000
Reimbursement for tenant improvements agreed by landlord recorded in prepaid expenses and other current assets with offsetting entry to deferred rent	5,600,000
Reimbursement for tenant improvements agreed by landlord recorded in other assets with offsetting entry to deferred rent	1,000,000
Tenant improvement reimbursements received			600,000
Additional period of renewal of amended lease with respect to entire leased space, option one	1 year
Additional period of renewal of amended lease with respect to entire leased space, option two	5 years
Future minimum lease payments under noncancelable operating leases
2013			4,319,000
2014			4,439,000
2015			4,601,000
2016			4,735,000
2017			4,838,000
2018 and thereafter			7,427,000
Contractual Matter
Previously billed amount the Company will reimburse									1,200,000
Expected development period from the effective date of agreement						12 years
Reduction in earnings due to reimbursement of previously billed amounts								200,000
Amount accrued by company							$ 851,000

Related Party Transactions (Details) (USD $) In Millions, except Share data, unless otherwise specified	1 Months Ended
	Dec. 31, 2012 Silver Creek Convertible notes	Apr. 30, 2012 Sanofi	Dec. 31, 2012 Directors, officers, scientific advisory board members and related parties Convertible notes
Related Party Transactions
Shares of common stock purchased by related party in connection with initial public offering		5,217,391
Debt issued	$ 1.6		$ 0.3

Selected Quarterly Financial Data (Unaudited) (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Selected Quarterly Financial Data (Unaudited)
Collaboration revenues	$ 14,191	$ 11,323	$ 12,063	$ 11,344	$ 12,577	$ 8,582	$ 6,595	$ 6,461	$ 48,921	$ 34,215	$ 20,305
Total operating expenses	38,719	35,197	32,368	35,379	30,744	27,219	36,019	21,102	141,663	115,084	69,481
Net loss	(24,893)	(23,320)	(20,139)	(23,402)	(18,221)	(18,724)	(29,196)	(13,535)	(91,754)	(79,676)	(50,159)
Net loss attributable to Merrimack Pharmaceuticals, Inc.	$ (24,768)	$ (23,199)	$ (20,026)	$ (23,284)	$ (18,116)	$ (18,599)	$ (29,051)	$ (13,457)	$ (91,277)	$ (79,223)	$ (50,104)
Net loss per share available to common stockholders-basic and diluted (in dollars per share)	$ (0.26)	$ (0.25)	$ (0.22)	$ (2.14)	$ (1.76)	$ (1.81)	$ (2.76)	$ (1.34)	$ (1.28)	$ (7.67)	$ (5.57)

- Definition

The portion of profit or loss for the period, net of income taxes, which is attributable to the parent.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 130

-Paragraph 10, 15

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 03

-Paragraph 19

-Article 5

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 04

-Paragraph 20

-Article 9

Reference 4: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 95

-Paragraph 28, 29, 30

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 5: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 944

-SubTopic 225

-Section S99

-Paragraph 1

-Subparagraph (SX 210.7-04.22)

-URI http://asc.fasb.org/extlink&oid=6879464&loc=d3e573970-122913

Reference 6: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 220

-SubTopic 10

-Section 45

-Paragraph 6

-URI http://asc.fasb.org/extlink&oid=20435746&loc=d3e565-108580

Reference 7: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 230

-SubTopic 10

-Section 45

-Paragraph 28

-URI http://asc.fasb.org/extlink&oid=6943989&loc=d3e3602-108585

Reference 8: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Glossary Other Comprehensive Income

-URI http://asc.fasb.org/extlink&oid=6519514

Reference 9: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Glossary Net Income

-URI http://asc.fasb.org/extlink&oid=6518256

Reference 10: http://www.xbrl.org/2003/role/presentationRef

-Publisher AICPA

-Name Accounting Research Bulletin (ARB)

-Number 51

-Paragraph 38

-Subparagraph a

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 11: http://www.xbrl.org/2003/role/presentationRef

-Publisher AICPA

-Name Accounting Research Bulletin (ARB)

-Number 51

-Paragraph A7

-Appendix A

Reference 12: http://www.xbrl.org/2003/role/presentationRef

-Publisher AICPA

-Name Accounting Research Bulletin (ARB)

-Number 51

-Paragraph 38

-Subparagraph d

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 13: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 260

-SubTopic 10

-Section 50

-Paragraph 1

-URI http://asc.fasb.org/extlink&oid=6371337&loc=d3e3550-109257

Reference 14: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 944

-SubTopic 225

-Section S99

-Paragraph 1

-Subparagraph (SX 210.7-04.19)

-URI http://asc.fasb.org/extlink&oid=6879464&loc=d3e573970-122913

Reference 15: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Emerging Issues Task Force (EITF)

-Number 87-21

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 16: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 225

-SubTopic 10

-Section S99

-Paragraph 2

-Subparagraph (SX 210.5-03.18)

-URI http://asc.fasb.org/extlink&oid=6880815&loc=d3e20235-122688

+ Details

- Definition

The consolidated profit or loss for the period, net of income taxes, including the portion attributable to the noncontrolling interest.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 810

-SubTopic 10

-Section 55

-Paragraph 4K

-URI http://asc.fasb.org/extlink&oid=18733213&loc=SL4591552-111686

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 810

-SubTopic 10

-Section 45

-Paragraph 19

-URI http://asc.fasb.org/extlink&oid=7656940&loc=SL4569616-111683

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher AICPA

-Name Accounting Research Bulletin (ARB)

-Number 51

-Paragraph 29

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 4: http://www.xbrl.org/2003/role/presentationRef

-Publisher AICPA

-Name Accounting Research Bulletin (ARB)

-Number 51

-Paragraph 38

-Subparagraph a

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 5: http://www.xbrl.org/2003/role/presentationRef

-Publisher AICPA

-Name Accounting Research Bulletin (ARB)

-Number 51

-Paragraph 38

-Subparagraph c(1)

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-Publisher FASB

-Name Accounting Standards Codification

-Topic 810

-SubTopic 10

-Section 55

-Paragraph 4J

-URI http://asc.fasb.org/extlink&oid=18733213&loc=SL4591551-111686

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-Publisher AICPA

-Name Accounting Research Bulletin (ARB)

-Number 51

-Paragraph A1, A4, A5

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-Publisher FASB

-Name Accounting Standards Codification

-Topic 810

-SubTopic 10

-Section 50

-Paragraph 1A

-Subparagraph (a),(c)

-URI http://asc.fasb.org/extlink&oid=18733093&loc=SL4573702-111684

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-Publisher AICPA

-Name Accounting Research Bulletin (ARB)

-Number 51

-Paragraph 5

-Subparagraph b

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

+ Details

Subsequent Events (Details) (USD $)	12 Months Ended	1 Months Ended		12 Months Ended
Subsequent Events (Details) (USD $)	Dec. 31, 2012 2011 Plan	Feb. 28, 2013 Subsequent events 2011 Plan	Jan. 31, 2013 Subsequent events Life Sciences Tax Incentive Program 2013	Dec. 31, 2012 Subsequent events Life Sciences Tax Incentive Program 2013 item
Subsequent event
Tax incentive awarded			$ 500,000
State research and development tax credits monetized			$ 400,000
Number of incremental employees pledged to be hired				20
Additional shares of common stock registered (in shares)	3,500,000	3,353,882

Retirement Plan	12 Months Ended
Retirement Plan	Dec. 31, 2012
Retirement Plan
Retirement Plan	21. Retirement Plan On May 31, 2002, the Company established a 401(k) defined contribution savings plan for its employees who meet certain service period and age requirements. Contributions are permitted up to the maximum allowed under the Internal Revenue Code of each covered employee's salary. The savings plan permits the Company to contribute at its discretion. For the years ended December 31, 2010, 2011 and 2012, the Company made contributions of $380,000, $487,000 and $581,000, respectively, to the plan.

Income Taxes (Details 2) (USD $) In Millions, unless otherwise specified	Dec. 31, 2012
Federal
Operating loss carryforwards
NOL	$ 210.9
State
Operating loss carryforwards
NOL	$ 155.5

Summary of Significant Accounting Policies (Details 4)	12 Months Ended
Summary of Significant Accounting Policies (Details 4)	Dec. 31, 2012
Lab equipment \| Minimum
Property and Equipment
Estimated useful life	3 years
Lab equipment \| Maximum
Property and Equipment
Estimated useful life	7 years
IT equipment \| Minimum
Property and Equipment
Estimated useful life	3 years
IT equipment \| Maximum
Property and Equipment
Estimated useful life	7 years
Furniture and fixtures \| Minimum
Property and Equipment
Estimated useful life	3 years
Furniture and fixtures \| Maximum
Property and Equipment
Estimated useful life	7 years

Retirement Plan (Details) (USD $)	12 Months Ended
Retirement Plan (Details) (USD $)	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Retirement Plan
Company's contributions to 401(k) defined contribution savings plan	$ 581,000	$ 487,000	$ 380,000