Merrimack Pharmaceuticals (MACK) 10-K 4 Mar 14 2013 FY Annual report Financial data

Document_and_Entity_Informatio

Document and Entity Information (USD $)	12 Months Ended
	Dec. 31, 2013	Feb. 28, 2014	Jun. 28, 2013
Document And Entity Information [Abstract]	'	'	'
Document Type	'10-K	'	'
Amendment Flag	'false	'	'
Document Period End Date	31-Dec-13	'	'
Document Fiscal Year Focus	'2013	'	'
Document Fiscal Period Focus	'FY	'	'
Entity Registrant Name	'MERRIMACK PHARMACEUTICALS INC	'	'
Entity Central Index Key	'0001274792	'	'
Current Fiscal Year End Date	'--12-31	'	'
Entity Well-known Seasoned Issuer	'No	'	'
Entity Current Reporting Status	'Yes	'	'
Entity Voluntary Filers	'No	'	'
Entity Filer Category	'Accelerated Filer	'	'
Entity Common Stock, Shares Outstanding	'	103,038,416	'
Entity Public Float	'	'	$604,930,711

Consolidated_Balance_Sheets

Consolidated Balance Sheets (USD $)	Dec. 31, 2013	Dec. 31, 2012
In Thousands, unless otherwise specified
Current assets:	'	'
Cash and cash equivalents	$65,086	$37,714
Available-for-sale securities	90,116	72,238
Restricted cash	101	100
Accounts receivable	5,857	9,267
Prepaid expenses and other current assets	5,484	8,982
Total current assets	166,644	128,301
Restricted cash	584	528
Property and equipment, net	13,364	6,297
Other assets	175	1,068
Intangible assets, net	1,845	2,165
In-process research and development	6,200	7,010
Goodwill	3,605	3,605
Total assets	192,417	148,974
Current liabilities:	'	'
Accounts payable, accrued expenses and other	38,814	25,644
Deferred revenues	9,336	9,350
Deferred rent	1,336	1,153
Long-term debt, current portion	8,248	2,373
Total current liabilities	57,734	38,520
Deferred revenues, net of current portion	66,139	71,114
Deferred rent, net of current portion	6,538	6,323
Deferred tax incentives, net of current portion	507	755
Long-term debt, net of current portion	103,427	37,482
Accrued interest	1,200	1,200
Total liabilities	235,545	155,394
Commitments and contingencies (Note 16)	'	'
Non-controlling interest	337	97
Stockholders' deficit:	'	'
Preferred stock, $0.01 par value: 10,000 shares authorized at December 31, 2013 and 2012, respectively; no shares issued or outstanding at December 31, 2013 or 2012	'	'
Common stock, $0.01 par value: 200,000 shares authorized at December 31, 2013 and 2012, respectively, 102,523 and 95,825 issued and outstanding at December 31, 2013 and 2012, respectively	1,025	958
Additional paid-in capital	527,779	434,679
Accumulated other comprehensive loss	-24	-38
Accumulated deficit	-572,245	-442,116
Total stockholders' deficit	-43,465	-6,517
Total liabilities, non-controlling interest and stockholders' deficit	$192,417	$148,974

Consolidated_Balance_Sheets_Pa

Consolidated Balance Sheets (Parenthetical) (USD $)	Dec. 31, 2013	Dec. 31, 2012
Statement Of Financial Position [Abstract]	'	'
Preferred stock, par value	$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	$0.01	$0.01
Common stock, shares authorized	200,000,000	200,000,000
Common stock, shares issued	102,523,000	95,825,000
Common stock, shares outstanding	102,523,000	95,825,000

Consolidated_Statements_of_Com

Consolidated Statements of Comprehensive Loss (USD $)	12 Months Ended
In Thousands, except Per Share data, unless otherwise specified	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Statement Of Income And Comprehensive Income [Abstract]	'	'	'
Collaboration revenues	$47,786	$48,921	$34,215
Operating expenses:	'	'	'
Research and development	147,139	125,858	100,630
General and administrative	21,187	15,805	14,454
Total operating expenses	168,326	141,663	115,084
Loss from operations	-120,540	-92,742	-80,869
Other income and expenses	'	'	'
Interest income	166	184	56
Interest expense	-10,938	-553	-13
Other, net	627	1,357	1,150
Net loss	-130,685	-91,754	-79,676
Less net income (loss) attributable to non-controlling interest	240	-477	-453
Net loss attributable to Merrimack Pharmaceuticals, Inc.	-130,925	-91,277	-79,223
Other comprehensive income (loss):	'	'	'
Unrealized loss on available-for-sale securities	14	-38	'
Other comprehensive income (loss)	14	-38	'
Comprehensive loss	($130,911)	($91,315)	($79,223)
Net loss per share available to common stockholders-basic and diluted	($1.32)	($1.28)	($7.67)
Weighted-average common shares used in computing net loss per share available to common stockholders-basic and diluted	98,919	72,831	11,343

Consolidated_Statements_of_Con

Consolidated Statements of Convertible Preferred Stock, Non-Controlling Interest and Stockholders' Deficit (USD $)	Total	Temporary Equity, Parent [Member]	Temporary Equity, Parent [Member]	Temporary Equity, Parent [Member]	Non-controlling Interest [Member]	Non-controlling Interest [Member]	Common Stock [Member]	Additional Paid-in Capital [Member]	Accumulated Other Comprehensive Loss [Member]	Accumulated Deficit [Member]
In Thousands, except Share data		Series B-G Convertible Preferred Stock [Member]	Series G Preferred Stock [Member]	Series C Preferred Stock [Member]		Series B-G Convertible Preferred Stock [Member]
Balance at Dec. 31, 2010	($219,964)	$191,257	'	'	$1,027	'	$111	$51,541	'	($271,616)
Balance (in shares) at Dec. 31, 2010	'	53,148,000	'	'	'	'	'	'	'	'
Balance (in shares) at Dec. 31, 2010	'	'	'	'	'	'	11,073,000	'	'	'
Loss attributable to non-controlling interest	453	'	'	'	-453	'	'	'	'	453
Issuance of Series C stock as a result of warrant exercises	'	'	'	19	'	'	'	'	'	'
Exercise of stock options and common stock warrants	1,745	'	'	'	'	'	7	1,738	'	'
Issuance of Series C stock as a result of warrant exercises (in shares)	'	'	'	3,000	'	'	'	'	'	'
Issuance of Series G stock	'	'	76,949	'	'	'	'	'	'	'
Exercise of stock options and common stock warrants (shares)	'	'	'	'	'	'	761,000	'	'	'
Issuance of Series G stock (in shares)	'	'	11,000,000	'	'	'	'	'	'	'
Stock-based compensation	6,952	'	'	'	'	'	'	6,952	'	'
Net loss	-79,676	'	'	'	'	'	'	'	'	-79,676
Balance at Dec. 31, 2011	-290,490	268,225	'	'	574	'	118	60,231	'	-350,839
Balance (in shares) at Dec. 31, 2011	'	'	'	'	'	'	11,834,000	'	'	'
Balance (in shares) at Dec. 31, 2011	'	64,151,000	'	'	'	'	'	'	'	'
Loss attributable to non-controlling interest	477	'	'	'	-477	'	'	'	'	477
Exercise of stock options and common stock warrants	5,427	'	'	'	'	'	27	5,400	'	'
Exercise of stock options and common stock warrants (shares)	'	'	'	'	'	'	2,693,000	'	'	'
Stock-based compensation	6,889	'	'	'	'	'	'	6,889	'	'
Conversion of convertible preferred stock into common stock	268,225	-268,225	'	'	'	'	663	267,562	'	'
Conversion of convertible preferred stock into common stock (in shares)	'	-64,151,000	'	'	'	'	66,256,000	'	'	'
Initial public offering, net of issuance costs	98,081	'	'	'	'	'	150	97,931	'	'
Initial public offering, net of issuance costs (shares)	'	'	'	'	'	'	15,042,000	'	'	'
Series B dividends declared	-4,263	'	'	'	'	'	'	-4,263	'	'
Conversion of convertible preferred stock warrants to common stock warrants	929	'	'	'	'	'	'	929	'	'
Other comprehensive (loss) income	-38	'	'	'	'	'	'	'	-38	'
Net loss	-91,754	'	'	'	'	'	'	'	'	-91,754
Balance at Dec. 31, 2012	-6,517	'	'	'	97	'	958	434,679	-38	-442,116
Balance (in shares) at Dec. 31, 2012	'	0	'	'	'	'	'	'	'	'
Balance (in shares) at Dec. 31, 2012	'	'	'	'	'	'	95,825,000	'	'	'
Issuance of common stock in a public offering, net of issuance costs (shares)	'	'	'	'	'	'	'	'	'	'
Conversion of Silver Creek convertible notes payable	'	'	'	'	796	'	'	'	'	'
Loss attributable to non-controlling interest	556	'	'	'	-556	'	'	'	'	556
Exercise of stock options and common stock warrants	2,036	'	'	'	'	'	9	2,027	'	'
Exercise of stock options and common stock warrants (shares)	'	'	'	'	'	'	948,000	'	'	'
Stock-based compensation	10,733	'	'	'	'	'	'	10,733	'	'
Conversion of convertible preferred stock into common stock	1,807	'	'	'	'	'	'	1,807	'	'
Initial public offering, net of issuance costs	26,716	'	'	'	'	'	58	26,658	'	'
Initial public offering, net of issuance costs (shares)	'	'	'	'	'	'	5,750,000	'	'	'
Conversion option of convertible senior notes	51,875	'	'	'	'	'	'	51,875	'	'
Other comprehensive (loss) income	14	'	'	'	'	'	'	'	14	'
Net loss	-130,685	'	'	'	'	'	'	'	'	-130,685
Balance at Dec. 31, 2013	($43,465)	$0	'	'	$337	'	$1,025	$527,779	($24)	($572,245)
Balance (in shares) at Dec. 31, 2013	'	'	'	'	'	'	102,523,000	'	'	'

Consolidated_Statements_of_Cas

Consolidated Statements of Cash Flows (USD $)	12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Cash flows from operating activities	'	'	'
Net loss	($130,685)	($91,754)	($79,676)
Adjustments to reconcile net loss to net cash (used in) provided by operating activities	'	'	'
Non-cash interest expense	4,548	78	'
Depreciation and amortization	2,589	3,664	4,596
Stock-based compensation	10,733	6,889	6,952
Other non-cash items	579	-587	864
Changes in operating assets and liabilities:	'	'	'
Purchased premiums and interest on available-for-sale securities	-1,809	-2,354	'
Accounts receivable	3,410	-1,841	-3,681
Prepaid expenses and other current assets	4,037	-2,477	-3,933
Accounts payable, accrued expenses and other	13,304	6,985	8,815
Deferred revenues	-4,989	-5,281	11,963
Deferred rent and tax incentives	2,076	7,892	1,264
Other assets and liabilities, net	1,032	-1,030	19
Net cash used in operating activities	-95,175	-79,816	-52,817
Cash flows from investing activities	'	'	'
Purchase of available-for-sale securities	-112,923	-115,665	'
Proceeds from sales and maturities of available-for-sale securities	95,100	43,880	'
Purchase of property and equipment	-9,857	-3,189	-3,754
Assignment of restricted cash	-57	-628	'
Release of restricted cash	'	381	'
Other investing activities, net	-2	'	7
Net cash used in investing activities	-27,739	-75,221	-3,747
Cash flows from financing activities	'	'	'
Proceeds from public offerings, net of offering costs	26,716	100,025	'
Proceeds from issuance of convertible preferred stock, net of offering costs	'	'	76,949
Proceeds from exercise of common stock and warrants	2,036	5,427	1,745
Principal payment on capital lease obligations	'	-48	-443
Proceeds from issuance of debt, net of issuance costs	121,537	41,128	'
Payments of dividends on Series B convertible preferred stock	-3	-4,235	'
Deferred financing costs	'	'	-1,946
Net cash provided by financing activities	150,286	142,297	76,305
Net increase (decrease) in cash and cash equivalents	27,372	-12,740	19,741
Cash and cash equivalents, beginning of period	37,714	50,454	30,713
Cash and cash equivalents, end of period	65,086	37,714	50,454
Noncash financing and investing activities	'	'	'
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	35	196	'
Value of conversion feature of convertible senior notes, classified in Stockholders' Deficit	51,876	'	'
Changes in property and equipment in accounts payable and accrued expenses	'	412	'
Disposals of fully depreciated assets	210	671	0
Reclassification of deferred financing costs to stockholders' deficit	278	2,748	'
Dividends on Series B convertible preferred stock declared but not paid	'	28	'
Conversion of Silver Creek convertible notes	2,603	'	'
Supplemental disclosure of cash flows	'	'	'
Cash paid for interest	$3,915	$169	$13

Nature_of_the_Business

Nature of the Business	12 Months Ended
	Dec. 31, 2013
Accounting Policies [Abstract]	'
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 novel 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 has also recently entered into an agreement to utilize its manufacturing expertise to develop, manufacture and exclusively supply bulk drug to a third party, who will in turn process the drug into a finished product and commercialize it globally. The Company’s discovery and development efforts are 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, its ability to secure additional capital to fund operations, success of clinical trials, 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.
	As of December 31, 2013, the Company had unrestricted cash and cash equivalents and available-for-sale securities of $155.2 million. The Company expects that its existing unrestricted cash and cash equivalents and available-for-sale securities as of December 31, 2013, anticipated interest income and funding under its license and collaboration agreement with Sanofi related to MM-121 will enable the Company to fund operations into 2015. In the event that the Company obtains favorable results from the Phase 3 clinical trial of MM-398, the Company expects that anticipated additional expenses in 2014 related to the commercialization of MM-398 will be offset by cash received from potential collaboration opportunities.
	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_Account

Summary of Significant Accounting Policies	12 Months Ended
	Dec. 31, 2013
Accounting Policies [Abstract]	'
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 2009, and Merrimack Pharmaceuticals (Bermuda) Ltd., which was incorporated during 2011. The Company also consolidates its 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 most significant estimates in these consolidated financial statements include revenue recognition, including the estimated percentage of billable expenses in any particular budget period, periods of meaningful use of licensed products, estimates used in accounting for revenue separability and recognition, useful lives with respect to long-lived assets and intangibles, accounting for stock-based compensation, convertible preferred stock warrants, contingencies, intangible assets, goodwill, in-process research and development, derivative liability, valuation of convertible debt, tax valuation reserves and accrued expenses, including clinical research costs. 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 typically 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, 2013 and 2012, the Company recorded restricted cash of $685,000 and $628,000, respectively, which were primarily related to the Company’s facility lease.
	Available-for-Sale Securities
	The Company classifies marketable securities with a remaining maturity when purchased of greater than three months as available-for-sale. Available-for-sale 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. 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’ deficit 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. There were no realized gains or losses recognized on the sale or maturity of available-for-sale securities during the years ended December 31, 2013, 2012 or 2011.
	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 each of the years ended December 31, 2013 and 2012, Sanofi represented greater than 98% of collaboration revenues. As of December 31, 2013 and 2012, Sanofi represented greater than 98% and 99% of accounts receivable, respectively.
	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		7-Mar
	IT equipment		7-Mar
	Leaseholds improvements		Lesser of useful life or lease term
	Furniture and fixtures		7-Mar
	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. 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, 2013 and 2012, respectively, as a component of mezzanine equity.
	Derivative Liability
	In December 2012, the Company’s majority owned subsidiary, Silver Creek, entered into a Note Purchase Agreement pursuant to which it issued convertible notes to various lenders in aggregate principal amounts of $1.6 million in December 2012, $0.3 million in February 2013 and $0.6 million in December 2013. Upon issuance, the Company determined that the underlying convertible notes represented share-settled debt and the potential conversion of the convertible notes into Silver Creek’s next qualifying series of preferred stock at a discount met the definition of a derivative, which was classified as a liability and remeasured each reporting period. Prior to conversion on December 31, 2013, the derivative was remeasured with changes in fair value recognized in earnings. The aggregate principal of $2.5 million and accrued interest of $0.1 million converted into shares of Silver Creek Series A preferred stock upon maturity on December 31, 2013. The Company estimated the value of the derivative liability issued in connection with the convertible notes payable at zero upon full conversion as of December 31, 2013 and $196,000 as of December 31, 2012.
	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 or profit-sharing 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 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, separates and allocates consideration in a multiple element arrangement according to the relative selling price of each deliverable. 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 entered into a collaboration agreement with Watson Laboratories, Inc. (“Actavis”) in November 2013, which was evaluated under the accounting guidance on revenue recognition for multiple element arrangements. See Note 5, “License and Collaboration Agreements,” for additional information.
	The Company’s license and collaboration agreements executed prior to January 1, 2011 continue to be accounted for under previously issued revenue recognition guidance for multiple element arrangements and milestone revenue recognition. 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 counterparty 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.
	The Company did not materially modify any of its previously-existing multiple element arrangements during the years ended December 31, 2013 and 2012.
	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 non-employees 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.
	Comprehensive Income (Loss)
	Comprehensive income (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 and Expense
	The Company records gains and losses on the remeasurement of fair value of convertible preferred stock warrants and other derivative liabilities, the federal and state sponsored tax incentives and other one-time income or expense-related items in other income.
	In January 2010, the Massachusetts Life Sciences Center (“MLSC”), an independent agency of the Commonwealth of Massachusetts, awarded the Company $1.5 million of tax incentives under its Life Sciences Tax Incentive Program. These incentives allowed the Company to monetize approximately $1.4 million of state research and development tax credits. The Company received this payment 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 some or all of these incentives. The Company deferred and is amortizing the benefit of this incentive 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, 2013, 2012 and 2011, the Company recognized $0.3 million of benefit in other income in each period.
	In January 2011, the MLSC awarded the Company an additional $1.3 million of tax incentives under its Life Sciences Tax Incentive Program, which allowed the Company to monetize approximately $1.2 million of state research and development tax credits. The Company received this payment 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 some or all of these incentives. The Company deferred and is amortizing the benefit of this incentive 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, 2013 and 2012, the Company recognized $0.2 million and $0.5 million, respectively, of benefit in other income. For the year ended December 31, 2011, the Company did not recognize any benefit in other income.
	In January 2013, the MLSC awarded the Company an additional $0.5 million of tax incentives under its Life Science Tax Incentive Program, which allows the Company to monetize approximately $0.4 million of state research and development tax credits. The Company received this payment in the fourth quarter of 2013. 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. The Company has deferred and will amortize the benefit of this incentive on a straight-line basis over the five-year performance period, commencing with a cumulative catch-up when the pledge is achieved.
	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.
	In April 2012, the Company closed the initial public offering of its common stock. Upon closing, $2.7 million of deferred financing costs were netted against the equity proceeds within stockholders’ deficit.
	In July 2013, the Company closed a follow-on underwritten public offering of additional shares of common stock and issued convertible senior notes in concurrent public offerings. Upon closing, $0.6 million of aggregate deferred financing costs were netted against the proceeds of these offerings.
	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 resulted from the Company’s most recent evaluation, which occurred in the third quarter of 2013. The Company’s next annual impairment evaluation will be made in the third quarter of 2014 unless indicators arise that would require the Company to evaluate at an earlier date.
	When performing an evaluation of goodwill impairment, the Company has the option to first assess qualitative factors to determine whether it is necessary to perform the quantitative two-step impairment test. If the Company elects this option and finds, as a result of the qualitative assessment, that it is more likely than not that the fair value of the reporting unit is less than its carrying amount, the quantitative two-step impairment test is required; otherwise, no further testing is required. This requires the Company to assess the impact of significant events, milestones and changes to expectations and activities that may have occurred since the last impairment evaluation. Significant changes to these estimates, judgments and assumptions could materially change the outcome of the impairment assessment. Alternatively, the Company may elect to not first assess qualitative factors and immediately perform the quantitative two-step impairment test. If such an election occurs, in the first step, the fair value of the Company’s reporting unit is compared to the carrying value. If the carrying value of the net assets assigned to the reporting unit exceeds the fair value of the reporting unit, then the second step of the impairment test is performed in order to determine the implied fair value of the reporting unit’s goodwill. If the carrying value of the reporting unit’s goodwill exceeds the implied fair value, then the Company would record an impairment loss equal to the difference. As described above, the Company operates in one operating segment, which is considered the only reporting unit.
	The Company’s evaluation of IPR&D impairment included a qualitative assessment to determine whether further impairment testing of indefinite-lived intangible assets was necessary. For all but one IPR&D asset, it was determined that it was not more likely than not that an impairment existed as of August 31, 2013 and, therefore, impairment evaluations were not performed. In this one case, because it was determined that it was more likely than not that an impairment of one IPR&D asset existed as of August 31, 2013, an impairment evaluation was performed. These determinations and the evaluation 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, estimates of expected intellectual property protection and discount rates. Significant changes to these estimates, judgments and assumptions could materially change the outcome of management’s impairment assessment. The impairment evaluation resulted in the Company recognizing a $0.8 million impairment charge related to an early-stage preclinical program, which was charged to research and development expense. See Note 8, “Goodwill and Intangible Assets, Net,” for additional information.
	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 lives. Amortization of remaining IPR&D has not commenced as of December 31, 2013.
	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 lives. The Company has not recorded any impairment charges related to definite-lived intangible assets.
Subsequent Events
The Company has evaluated all events or transactions that occurred after December 31, 2013 up through the date the Company issued these financial statements. Other than the items discussed at Note 20, “Subsequent Events,” the Company did not have any other material recognizable or unrecognizable subsequent events.
Recent Accounting Pronouncements
In February 2013, the Financial Accounting Standards Board (“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 adoption of this guidance did not have a material impact on the Company’s consolidated financial statements.
In July 2013, the FASB issued guidance to address the diversity in practice related to the financial statement presentation of unrecognized tax benefits as either a reduction of a deferred tax asset or a liability when a net operating loss carryforward, a similar tax loss or a tax credit carryforward exists. This guidance is effective prospectively for fiscal years, and interim periods within those years, beginning after December 15, 2013. The adoption of this guidance is not expected to have a material impact on the Company’s consolidated financial statements.

Marketable_Securities

Marketable Securities	12 Months Ended
	Dec. 31, 2013
Investments Debt And Equity Securities [Abstract]	'
Marketable Securities	'
	3. Marketable Securities
	Available-for-sale securities, all of which have maturities of twelve months or less, as of December 31, 2013 consisted of the following:
		Amortized		Unrealized			Unrealized			Fair
		Cost		Gains			Losses			Value
		(in thousands)
	December 31, 2013:
	Commercial paper	$	49,685	$	—		$	(5	)	$	49,680
	Corporate debt securities		40,455		—			(19	)		40,436
	Total	$	90,140	$	—		$	(24	)	$	90,116
	The aggregate fair value of securities held by the Company in an unrealized loss position for less than 12 months as of December 31, 2013 was $77.1 million, representing 21 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 with unrealized losses 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).
	Available-for-sale securities in an unrealized loss position as of December 31, 2013 consisted of the following:
		Aggregate		Unrealized
		Fair Value		Losses
		(in thousands)
	December 31, 2013:
	Commercial paper	$	36,683	$	(5	)
	Corporate debt securities		40,436		(19	)
		$	77,119	$	(24	)
	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 their amortized cost bases, 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, 2013.

Net_Loss_Per_Common_Share

Net Loss Per Common Share	12 Months Ended
	Dec. 31, 2013
Earnings Per Share [Abstract]	'
Net Loss Per Common Share	'
	4. 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)	2013			2012			2011
	Net Loss Per Share:
	Numerator:
	Net loss attributable to Merrimack Pharmaceuticals, Inc.	$	(130,925	)	$	(91,277	)	$	(79,223	)
	Plus: Unaccreted dividends on convertible preferred stock		—			(2,107	)		(7,789	)
	Net loss available to common stockholders—basic and diluted		(130,925	)		(93,384	)		(87,012	)
	Denominator:
	Weighted-average common shares—basic and diluted		98,919			72,831			11,343
	Net loss per share available to common stockholders—basic and diluted	$	(1.32	)	$	(1.28	)	$	(7.67	)
	As discussed in Note 11, “Borrowings,” in July 2013, the Company issued $125.0 million aggregate principal amount of 4.50% convertible senior notes due 2020 (the “Notes”) in an underwritten public offering. Upon any conversion of the Notes while the Company has indebtedness outstanding under the Loan and Security Agreement (the “Loan Agreement”) with Hercules Technology Growth Capital, Inc. (“Hercules”), the Notes will be settled in shares of the Company’s common stock. Following the repayment and satisfaction in full of the Company’s obligations to Hercules under the Loan Agreement, upon any conversion of the Notes, the Notes may be settled, at the Company’s election, in cash, shares of the Company’s common stock or a combination of cash and shares of the Company’s common stock. For purposes of calculating the maximum dilutive impact, it is presumed that the conversion premium will be settled in common stock, inclusive of a contractual make-whole provision resulting from a fundamental change, and the resulting potential common shares included in diluted earnings per share if the effect is more dilutive. The stock options, warrants and conversion premium on the Notes are excluded from the calculation of diluted loss per share because the net loss for the years ended December 31, 2013, 2012 and 2011 causes such securities to be anti-dilutive. The potential dilutive effect of these securities is shown in the chart below:
		Years ended December 31,
	(in thousands)	2013			2012			2011
	Common stock warrants		2,777			2,842			2,640
	Convertible preferred stock		—			—			66,256
	Options to purchase common stock		20,107			18,066			17,617
	Convertible preferred stock warrants		—			—			302
	Conversion of the Notes		25,000			—			—

License_and_Collaboration_Agre

License and Collaboration Agreements	12 Months Ended
	Dec. 31, 2013
Organization Consolidation And Presentation Of Financial Statements [Abstract]	'
License and Collaboration Agreements	'
	5. 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. Over the life of the agreement, the Company has received total milestone payments of $25.0 million. These milestone payments were $10.0 million associated with dosing the first patient in a Phase 2 clinical trial in breast cancer in 2010, $10.0 million associated with dosing the first patient in a Phase 2 clinical trial in non-small cell lung cancer in 2011 and $5.0 million associated with dosing the first patient in a Phase 2 clinical trial in ovarian cancer in 2012. 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. 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. The Company has satisfied its manufacturing obligations under the agreement as of December 31, 2013. Sanofi reimburses the Company for direct costs incurred in both development and manufacturing and compensates the Company for its internal development efforts based on a full time equivalent (“FTE”) rate.
	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 within the period they are incurred and billable. Billable expenses are defined during each specified budget period. For the year ended December 31, 2013, this specified budget period is the 12-month annual period ended December 31, 2013. In the event that total development services expense incurred and expected to be incurred, during any particular budget period, exceed the total contractually allowed billable amount for development services during the same period, the Company recognizes only a percentage of the development services incurred as revenue during that period. This percentage is calculated as total development services expense incurred during the specified budget period divided by the sum of total development services expense incurred plus estimated development services expense to be incurred during the specified period, multiplied by the total contractually allowed billable amount for development services during the specified period, less development services revenue previously recognized within the specified period. 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, 2013, 2012 and 2011, the Company recognized revenue based on the following components of the Sanofi agreement:
		Years ended December 31,
	(in thousands)	2013		2012		2011
	Upfront payment	$	5,000	$	5,000	$	5,000
	Milestone payment		2,083		2,975		2,616
	Development services		36,283		36,905		25,053
	Manufacturing services and other		3,867		3,307		1,456
	Total	$	47,233		48,187	$	34,125
	The Company performs development services for which revenue is recognized under the Sanofi agreement in accordance with the specified budget period. During the year and specified budget period ended December 31, 2013, the Company performed $10.1 million of development services in excess of recognized revenue. During the years ended December 31, 2012 and 2011, development services approximated recognized revenue.
	As of December 31, 2013 and 2012, the Company maintained the following assets and liabilities related to the Sanofi agreement:
		December 31,
	(in thousands)	2013		2012
	Accounts receivable, billed	$	2,357	$	1,577
	Accounts receivable, unbilled		3,417		7,690
	Deferred revenues		73,392		79,913
	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 pay 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, 2013, 2012 and 2011, the Company recognized research and development expenses of $1.5 million, $6.2 million and $11.2 million, respectively, related to the agreement with PharmaEngine. These amounts include a $5.0 million milestone payment expensed in the year ended December 31, 2012 and a $10.0 million upfront payment expensed in the year ended December 31, 2011. As of December 31, 2013 and 2012, the Company had amounts accrued and payable of $0.6 million and $0.3 million, respectively, related to the agreement with PharmaEngine.
	Actavis
	In November 2013, the Company and Actavis entered into a development, license and supply agreement pursuant to which the Company will develop, manufacture and exclusively supply the bulk form of doxorubicin HCl liposome injection (the “Initial Product”) to Actavis. Under the agreement, Actavis is responsible for all costs related to finished product processing and global commercialization. Pursuant to the agreement, additional products may be developed for Actavis in the future. The Company is eligible to receive up to $15.5 million, including $2.0 million upfront, which was received in December 2013, and the remainder in development funding and development, regulatory and commercial milestone payments related to the Initial Product. The Company will also receive a double digit share of net profits on global sales of the Initial Product and any additional products. The Company will manufacture and supply the Initial Product to Actavis in bulk form at an agreed upon unit price.
	The agreement will expire with respect to each product ten years after Actavis’ first sale of such product, unless terminated earlier, and will automatically renew for additional two year periods thereafter unless either party provides notice of non-renewal. Either party may terminate the agreement in the event of an uncured material breach or bankruptcy filing by the other party. Actavis may also terminate the agreement for convenience in specified circumstances upon 90 days’ prior written notice.
	The Company applied revenue recognition guidance to determine whether the performance obligations under this collaboration, including the license, participation on steering committees, development services, and manufacturing and supply services could be accounted for separately or as a single unit of accounting. The Company determined that these obligations represent a single unit of accounting and will recognize revenue as product is supplied to Actavis. Therefore, the Company has deferred total billed and billable milestones and development expenses of $2.1 million as of December 31, 2013.
	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. On March 19, 2013, GTC terminated the license agreement. As a result, the Company recognized the remaining $0.6 million of deferred revenue related to this license agreement during the first quarter of 2013.

Fair_Value_of_Financial_Instru

Fair Value of Financial Instruments	12 Months Ended
	Dec. 31, 2013
Fair Value Disclosures [Abstract]	'
Fair Value of Financial Instruments	'
	6. Fair Value of Financial Instruments
	The carrying value of financial instruments, including cash and cash equivalents, restricted cash, available-for-sale securities, prepaid expenses, accounts receivable, accounts payable and accrued expenses, and other short-term assets and liabilities approximate their respective fair values due to the short-term maturities of these instruments and debts. The derivative liability is 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.
	Recurring Fair Value Measurement
	The following tables show assets and liabilities measured at fair value on a recurring basis as of December 31, 2013 and 2012 and the input categories associated with those assets and liabilities:
	As of December 31, 2013	Level 1			Level 2			Level 3
	(in thousands)
	Assets:
	Cash equivalents—money market funds	$	47,740		$	—		$	—
	Cash equivalents—commercial paper		—			13,998			—
	Investments—commercial paper		—			49,680			—
	Investments—corporate debt securities		—			40,436			—
	As of December 31, 2012	Level 1			Level 2			Level 3
	(in thousands)
	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 derivative liability as of December 31, 2012 and throughout the year ended December 31, 2013 was determined using a probability-weighted valuation model based on the likelihood of Silver Creek achieving a qualified financing. The significant unobservable input used in the fair value measurement of the Company’s derivative liability was the probability of Silver Creek’s successful achievement of aggregate financings of at least $4.0 million of gross proceeds prior to December 31, 2013. As discussed in Note 11, “Borrowings,” Silver Creek’s derivative liability was remeasured upon the maturation and conversion of the underlying note payable on December 31, 2013 to a zero percent probability of achieving the aggregate financings and converting at a discount, with the change in fair value recognized in other income.
	The following table provides a roll-forward of the fair value of the liabilities categorized as Level 3 instruments, for the years ended December 31, 2013 and 2012:
	(in thousands)	Convertible			Derivative Liability
		preferred
		stock warrants
	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
	Portion of convertible note allocated to derivative		—			35
	Remeasurement of derivative liability upon conversion of underlying note		—			(231	)
	Balance, December 31, 2013	$	—		$	—
	Non-Recurring Fair Value Measurements
	Certain assets, including IPR&D, may be measured at fair value on a non-recurring basis in periods subsequent to initial recognition. During the third quarter of 2013, the Company made a decision to deprioritize and delay efforts to further develop an early-stage preclinical program. As a result of this decision, in connection with the Company’s annual impairment test performed in the third quarter of 2013, the fair value estimate for the IPR&D asset related to the early-stage preclinical program incorporated the assumptions of significantly lower estimated cash flows from future revenues and a delay in when those cash flows would occur. The fair value was derived from assumptions that are representative of those a market participant would use in estimating fair value. The impairment analysis resulted in the Company recognizing a $0.8 million impairment charge related to the early-stage preclinical program, which was charged to research and development expense.
	The following table provides quantitative information associated with the fair value measurement of the Company’s non-recurring Level 3 inputs:
		Fair Value as of			Valuation		Unobservable Input	Percentage
		August 31, 2013			Technique
		(in thousands)
	IPR&D asset	$	—		Income approach—Probability weighted discounted cash flow analysis		Discount rate		25.7	%
	Other Fair Value Measurements
	The estimated fair value and carrying value of the $125.0 million aggregate principal amount of the Notes was $127.5 million and $125.0 million, respectively, as of December 31, 2013. The Company estimated the fair value of the Notes by using a quoted market rate in an inactive market, which is classified as a Level 2 input.
	The estimated fair value and carrying value of the Hercules loans payable was $39.5 million and $40.3 million, respectively, as of December 31, 2013. The Company estimated the fair value of the loans payable by using publically available information related to Hercules’ portfolio of debt investments based on unobservable inputs, which is classified as a Level 3 input.

Consolidated_Subsidiaries

Consolidated Subsidiaries	12 Months Ended
	Dec. 31, 2013
Text Block [Abstract]	'
Consolidated Subsidiaries	'
	7. 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.
	Silver Creek Pharmaceuticals, Inc.
	On August 20, 2010, the Company acquired a controlling interest in Silver Creek. In December 2012, as described in Note 11, “Borrowings,” Silver Creek entered into a Note Purchase Agreement pursuant to which it issued convertible notes to various lenders in aggregate principal amounts of $1.6 million in December 2012, $0.3 million in February 2013 and $0.6 million in December 2013. As of December 31, 2013, Silver Creek’s outstanding borrowings and related accrued interest of $2.6 million converted to shares of Silver Creek Series A preferred stock at the Series A preferred stock value of $1.00 per share. As a result of changes to the ownership composition of Silver Creek, the non-controlling interest increased by $0.8 million. As of December 31, 2013 and 2012, the Company owned 64% and 74% of the voting stock of Silver Creek, respectively, and recorded a non-controlling interest of $337,000 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, 2013, the Company consolidated Silver Creek’s total assets and total liabilities of $1.0 million and $0.1 million, respectively. As of December 31, 2012, the Company consolidated Silver Creek’s total assets and total liabilities of $2.2 million and $1.8 million, respectively.
	As of December 31, 2013 and 2012, employees and directors of the Company owned approximately 7% and 6% of the outstanding shares of Silver Creek Series A preferred stock, respectively.
	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

Goodwill and Intangible Assets, Net	12 Months Ended
	Dec. 31, 2013
Goodwill And Intangible Assets Disclosure [Abstract]	'
Goodwill and Intangible Assets, Net	'
	8. Goodwill and Intangible Assets, Net
	As part of the acquisition of Hermes, the Company recognized acquired IPR&D of $7.0 million related to several development programs: an antibody-targeted nanotherapeutic that contains a chemotherapy drug, a nanotherapeutic that contains a chemotherapy drug and other early-stage preclinical programs in the amounts of $2.8 million, $3.4 million and $0.8 million, respectively. The Company also acquired intangible assets of $3.2 million 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, 2013 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 full value of the antibody-targeted nanotherapeutic that contains a chemotherapy drug and the nanotherapeutic that contains a chemotherapy drug recorded at the Acquisition Date remained unchanged as of December 31, 2013 and 2012. 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. The deprioritization and delay of the other early-stage preclinical programs during the year ended December 31, 2013 resulted in an impairment charge of $0.8 million during the third quarter of 2013.
	Changes in the carrying value of goodwill, IPR&D and intangible assets for the years ended December 31, 2013, 2012 and 2011 were as follows:
	(in thousands)	Intangible			IPR&D			Goodwill
		assets
	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
	Amortization		(320	)		—			—
	Impairment		—			(810	)		—
	Balance, December 31, 2013	$	1,845		$	6,200		$	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)
	2014	$	320
	2015		320
	2016		320
	2017		320
	2018		320

Property_and_Equipment_Net

Property and Equipment, Net	12 Months Ended
	Dec. 31, 2013
Property Plant And Equipment [Abstract]	'
Property and Equipment, Net	'
	9. Property and Equipment, Net
	Property and equipment consisted of the following:
		December 31,
	(in thousands)	2013			2012
	Lab equipment	$	13,714		$	12,616
	IT equipment		2,701			2,346
	Leasehold improvements		10,523			8,200
	Furniture and fixtures		340			330
	Construction in process		7,635			1,774
			34,913			25,266
	Less: Accumulated depreciation and amortization		(21,549	)		(18,969	)
		$	13,364		$	6,297
	Depreciation expense was $2.8 million, $3.5 million and $5.0 million for the years ended December 31, 2013, 2012 and 2011, respectively. Capitalized interest costs were immaterial for the years ended December 31, 2013 and 2012. There were no capitalized interest costs for the year ended December 31, 2011.
	During the years ended December 31, 2013 and 2012, the Company disposed of $0.2 million and $0.7 million of fully depreciated assets. No fixed assets were disposed of or sold during the year ended December 31, 2011.
	There were no recognized impairment charges related to fixed assets in the years ended December 31, 2013, 2012 or 2011.

Accounts_Payables_Accrued_Expe

Accounts Payables, Accrued Expenses and Other	12 Months Ended
	Dec. 31, 2013
Payables And Accruals [Abstract]	'
Accounts Payables, Accrued Expenses and Other	'
	10. Accounts Payables, Accrued Expenses and Other
	Accounts payable, accrued expenses and other as of December 31, 2013 and 2012 consisted of the following:
		December 31,
	(in thousands)	2013		2012
	Accounts payable	$	1,889	$	283
	Accrued goods and services		26,031		17,615
	Accrued payroll and related benefits		7,255		5,853
	Accrued interest		2,926		306
	Accrued dividends payable		25		28
	Deferred tax incentives		688		512
	Derivative liability		—		196
	Other contractual liability		—		851
	Total accounts payable, accrued expenses and other	$	38,814	$	25,644

Borrowings

Borrowings	12 Months Ended
	Dec. 31, 2013
Debt Disclosure [Abstract]	'
Borrowings	'
	11. Borrowings
	Future minimum payments under indebtedness agreements outstanding as of December 31, 2013 are as follows:
	Years Ending December 31:		4.50% Convertible			Loan
	(in thousands)		Senior Notes			Agreement
	2014		$	5,594		$	12,353
	2015			5,625			18,130
	2016			5,625			17,916
	2017			5,625			—
	2018 and thereafter			141,875			—
			$	164,344		$	48,399
	Less interest			(39,344	)		(7,199	)
	Less unamortized discount			(52,422	)		(2,103	)
	Less current portion			—			(8,248	)
	Loans payable, net of current portion		$	72,578		$	30,849
	4.50% Convertible Senior Notes
	In July 2013, the Company issued $125.0 million aggregate principal amount of Notes in an underwritten public offering. The Company issued the Notes under an indenture, dated as of July 17, 2013 (the “Base Indenture”) between the Company and Wells Fargo Bank, National Association, as trustee (the “Trustee”), as supplemented by the supplemental indenture, dated as of July 17, 2013, between the Company and the Trustee (together with the Base Indenture, the “Indenture”). As a result of the Notes offering, the Company received net proceeds of approximately $120.6 million, after deducting underwriting discounts and commissions and offering expenses payable by the Company.
	The Notes bear interest at a rate of 4.50% per year, payable semiannually in arrears on January 15 and July 15 of each year, beginning on January 15, 2014. The Notes are general unsecured senior obligations of the Company and rank (i) senior in right of payment to any of the Company’s indebtedness that is expressly subordinated in right of payment to the Notes, (ii) equal in right of payment to any of the Company’s unsecured indebtedness that is not so subordinated, (iii) effectively junior in right of payment to any of the Company’s secured indebtedness to the extent of the value of the assets securing such indebtedness, and (iv) structurally junior to all indebtedness and other liabilities (including trade payables) of the Company’s subsidiaries.
	The Notes will mature on July 15, 2020 (the “Maturity Date”), unless earlier repurchased by the Company or converted at the option of holders. Holders may convert their Notes at their option at any time prior to the close of business on the business day immediately preceding April 15, 2020 only under the following circumstances:
		•		during any calendar quarter commencing after September 30, 2013 (and only during such calendar quarter), if the last reported sale price of the Company’s common stock for at least 20 trading days (whether or not consecutive) during a period of 30 consecutive trading days ending on the last trading day of the immediately preceding calendar quarter is greater than or equal to 130% of the conversion price on each applicable trading day;
		•		during the five business day period after any five consecutive trading day period (the “measurement period”) in which the trading price (as defined in the Notes) per $1,000 principal amount of Notes for each trading day of the measurement period was less than 98% of the product of the last reported sale price of the Company’s common stock and the conversion rate on each such trading day; or
		•		upon the occurrence of specified corporate events set forth in the Indenture.
	On or after April 15, 2020 until the close of business on the business day immediately preceding the Maturity Date, holders may convert their Notes at any time, regardless of the foregoing circumstances. Upon any conversion of Notes that occurs while the Company’s indebtedness to Hercules under the Loan Agreement remains outstanding, the Notes will be settled in shares of the Company’s common stock. Following the repayment and satisfaction in full of the Company’s obligations to Hercules under the Loan Agreement, upon any conversion of the Notes, the Notes may be settled, at the Company’s election, in cash, shares of the Company’s common stock or a combination of cash and shares of the Company’s common stock.
	The initial conversion rate of the Notes is 160 shares of the Company’s common stock per $1,000 principal amount of Notes, which is equivalent to an initial conversion price of $6.25 per share of common stock. The initial conversion price represents a premium of 25% over the public offering price per share of $5.00 in the Company’s concurrent underwritten public offering of common stock, as described in Note 13, “Common Stock.” The conversion rate will be subject to adjustment in some events, but will not be adjusted for any accrued and unpaid interest. In addition, following certain corporate events that occur prior to the Maturity Date, the Company will increase the conversion rate for a holder who elects to convert its Notes in connection with such a corporate event in certain circumstances.
	Upon the occurrence of a fundamental change (as defined in the Indenture) involving the Company, holders of the Notes may require the Company to repurchase all or a portion of their Notes for cash at a price equal to 100% of the principal amount of the Notes to be purchased, plus accrued and unpaid interest to, but excluding, the fundamental change repurchase date.
	The Indenture contains customary terms and covenants and events of default with respect to the Notes. If an event of default (as defined in the Indenture) occurs and is continuing, the Trustee by written notice to the Company, or the holders of at least 25% in aggregate principal amount of the Notes then outstanding by written notice to the Company and the Trustee, may, and the Trustee at the request of such holders shall, declare 100% of the principal of and accrued and unpaid interest on the Notes to be due and payable. In the case of an event of default arising out of certain events of bankruptcy, insolvency or reorganization involving the Company or a significant subsidiary (as set forth in the Indenture), 100% of the principal of and accrued and unpaid interest on the Notes will automatically become due and payable.
	The Company has separately accounted for the liability and equity components of the Notes by bifurcating gross proceeds between the indebtedness, or liability component, and the embedded conversion option, or equity component. This bifurcation was done by estimating an effective interest rate as of the date of issuance for similar notes which do not contain an embedded conversion option. This effective interest rate was estimated to be 15% and was used to compute the initial fair value of the indebtedness of $71.2 million. The gross proceeds received from the issuance of the Notes less the initial amount allocated to the indebtedness resulted in a $53.8 million allocation to the embedded conversion option. The embedded conversion option was recorded in stockholders’ deficit and as debt discount, to be subsequently amortized as interest expense over the term of the Notes. Underwriting discounts and commissions and offering expenses totaled $4.4 million and were allocated to the indebtedness and the embedded conversion option based on their relative values. As a result, $2.5 million attributable to the indebtedness was recorded as debt discount, to be subsequently amortized as interest expense over the term of the Notes, and $1.9 million attributable to the embedded conversion option was netted with the embedded conversion option in stockholders’ deficit.
	For the year ended December 31, 2013, interest expense related to the outstanding principal balance of the Notes was $6.2 million.
	Loan Agreement
	In November 2012, the Company entered into the Loan Agreement with Hercules pursuant to which the Company received loans in the aggregate principal amount of $40.0 million in 2012. In July 2013, in connection with the Notes offering, the Company and Hercules entered into an amendment, consent and waiver to the Loan Agreement that permitted the Notes offering and the issuance of the Notes. 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. In the event the Company receives aggregate gross proceeds of at least $75.0 million in one or more transactions prior to December 1, 2013, the Company has the option to elect to extend the interest-only period by six months so that the aggregate outstanding principal balance of the loans issued pursuant to the Loan Agreement 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. As described above and in Note 13, “Common Stock,” in July 2013, the Company sold an aggregate of 5,750,000 shares of its common stock to the public at a price to the public of $5.00 per share and issued $125.0 million aggregate principal amount of Notes in concurrent underwritten public offerings, and as a result of these offerings, the Company received aggregate net proceeds in excess of $75.0 million. On October 27, 2013, the Company notified Hercules of its election to extend the interest-only period as permitted under the Loan Agreement.
	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, 2013, there have been no events of default under the Loan Agreement.
	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, 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 years ended December 31, 2013 and 2012, interest expense related to the Hercules loans payable were $4.9 million and $0.5 million, respectively.
	Convertible Notes—Silver Creek
	In December 2012, as described in Note 2, “Summary of Significant Accounting Policies–Derivative Liability,” the Company’s majority owned subsidiary, Silver Creek, entered into a Note Purchase Agreement pursuant to which it issued convertible notes to various lenders in aggregate principal amounts of $1.6 million in December 2012, $0.3 million in February 2013 and $0.6 million in December 2013. The notes issued pursuant to the Note Purchase Agreement bore interest at 6% per annum. Upon issuance, these convertible notes contained a feature wherein 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 would 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 $0.2 million for the year ended December 31, 2012 using a probability-weighted model as described in Note 6, “Fair Value of Financial Instruments—Recurring Fair Value Measurement,” and was recorded as derivative liability on the consolidated balance sheets. For the year ended December 31, 2013, the derivative was remeasured upon conversion with the gain in remeasurement recognized in other income. The notes matured and converted, along with accrued interest of $0.1 million, into shares of Silver Creek Series A preferred stock on December 31, 2013. Upon conversion, the Company’s ownership percentage of Silver Creek outstanding preferred stock decreased from 74% to 64%, and a $0.8 million increase to non-controlling interest was recognized.

Stock_Warrants

Stock Warrants	12 Months Ended
	Dec. 31, 2013
Text Block [Abstract]	'
Stock Warrants	'
	12. 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)	Warrants			Weighted		Warrants			Weighted
		for the			Average		for the			Average
		Purchase of			Exercise		Purchase of			Exercise
		Common			Price		Convertible			Price
		Stock					Preferred
							Stock
	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		$	3.5
	Conversion		302		$	3.5		(302	)	$	3.5
	Exercised		(100	)	$	2.63		—			—
	Balance—December 31, 2012		2,842		$	3.05		—			—
	Exercised		(65	)	$	2.82
	Balance—December 31, 2013		2,777		$	3.05		—			—
	During the year ended December 31, 2012, warrants to purchase approximately 100,000 shares of common stock were cashless exercised and 71,000 shares of common stock were issued. During the year ended December 31, 2013, warrants to purchase approximately 65,000 shares of common stock were cashless exercised and 35,000 shares of common stock were issued.

Common_Stock

Common Stock	12 Months Ended
	Dec. 31, 2013
Equity [Abstract]	'
Common Stock	'
	13. Common Stock
	In July 2013, the Company sold an aggregate of 5,750,000 shares of its common stock at a price to the public of $5.00 per share in an underwritten public offering and received net proceeds of approximately $26.7 million, after deducting underwriting discounts and commissions and offering expenses payable by the Company.
	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, 2013 and 2012, the Company had 200.0 million shares of $0.01 par value common stock authorized. There were approximately 102,523,000 and 95,825,000 shares of common stock issued and outstanding as of December 31, 2013 and 2012, respectively. The shares reserved for future issuance as of December 31, 2013 and 2012 consisted of the following:
	(in thousands)	December 31, 2013		December 31, 2012
	Common stock warrants		2,777		2,842
	Options to purchase common stock		20,107		18,066
	Conversion of the Notes		25,000		—

StockBased_Compensation

Stock-Based Compensation	12 Months Ended
	Dec. 31, 2013
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]	'
Stock-Based Compensation	'
	14. 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. 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-qualifiedstock 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, for a total of 4.3 million shares. Additional shares also become available for grant by reason of the forfeiture, cancellation, expiration or termination of existing awards. In February 2013, the Company registered 3.4 million additional shares of common stock related to the 2011 Plan. As of December 31, 2013, there were 1.7 million shares of common stock available to be issued under the 2011 Plan.
	During the years ended December 31, 2013, 2012 and 2011, the Company issued options to purchase 3.3 million, 3.3 million and 2.3 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, 2013, 2012 and 2011, 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)	2013			2012			2011
	Employee awards:
	Research and development	$	5,954		$	4,234		$	3,597
	General and administrative		4,808			2,510			2,875
	Stock-based compensation for employee awards		10,762			6,744			6,472
	Stock-based compensation for non-employee awards		(29	)		145			480
	Total stock-based compensation	$	10,733		$	6,889		$	6,952
	The stock-based compensation for non-employee awards recognized during the year ended December 31, 2013 was negative due to the change in fair value of the options granted during previous periods.
	The fair value of employee options granted during the years ended December 31, 2013, 2012 and 2011 was estimated at the date of grant using the following assumptions:
		Years ended December 31,
		2013			2012			2011
	Risk-free interest rate		0.1 – 1.9	%		0.7 – 1.1	%		1.3 –2.5	%
	Expected dividend yield		0	%		0	%		0	%
	Expected term		5.3 – 5.9 years			5 – 5.9 years			5 – 5.9 years
	Expected volatility		67 – 70	%		66 – 72	%		71 – 73	%
	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:
		Shares			Weighted Average			Weighted Average			Aggregate
					Exercise Price			Remaining			Intrinsic Value
								Contractual Term
	Outstanding at December 31, 2012		18,066		$	3.5			6.54		$	51,486
	Granted		3,286		$	6.04
	Exercised		(913	)	$	2.23
	Cancelled		(332	)	$	6.02
	Outstanding at December 31, 2013		20,107		$	3.93			6.11		$	38,348
	Vested and expected to vest at December 31, 2013		19,829		$	3.89			6.07		$	38,307
	Exercisable at December 31, 2013		15,672		$	3.19			5.33		$	37,861
	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. The aggregate intrinsic value of options exercised in 2013, 2012 and 2011 was $2.7 million, $13.7 million and $1.4 million, respectively.
	As of December 31, 2013, there was $15.9 million of total unrecognized compensation cost related to nonvested employee stock awards. As of December 31, 2013, the Company expects to recognize those costs over a weighted average period of approximately 1.8 years.

Income_Taxes

Income Taxes	12 Months Ended
	Dec. 31, 2013
Income Tax Disclosure [Abstract]	'
Income Taxes	'
	15. Income Taxes
	As a result of losses incurred, the Company did not provide for any income taxes in the years ended December 31, 2013, 2012 and 2011. A reconciliation of the Company’s effective tax rate to the statutory federal income tax rate is as follows:
		Year Ended December 31,
		2013			2012			2011
	Federal income tax at statutory federal rate		35	%		35	%		35	%
	State taxes		4.3			4.5			4.2
	Permanent differences		(2.0	)		(0.2	)		(0.4	)
	Stock-based compensation		(0.6	)		(0.3	)		(1.2	)
	Tax credits		12.4			1.1			3.9
	Foreign rate differential		(2.8	)		(4.1	)		(4.4	)
	Other		(1.5	)		(0.5	)		(0.8	)
	Change in valuation allowance		(44.8	)		(35.5	)		(36.3	)
			—	%		—	%		—	%
	During the year ended December 31, 2013, the Company recorded a deferred tax liability related to the embedded conversion option of the 4.50% convertible senior notes through equity. This deferred tax liability is reflected in the deferred tax table below, but is appropriately excluded from the effective tax rate.
	Temporary differences that give rise to significant net deferred tax assets as of December 31, 2013 and 2012 are as follows:
	(in thousands)	2013			2012
	Deferred tax assets
	Net operating losses	$	113,843		$	77,806
	Capitalized research and development expenses		32,771			40,083
	Credit carryforwards		37,304			14,398
	Depreciation		2,735			2,931
	Deferred compensation		8,249			5,068
	Accrued expenses		841			1,184
	Deferred revenue		28,381			29,936
	Other temporary differences		5,885			1,934
	Total gross deferred tax asset		230,009			173,340
	Valuation allowance		(207,304	)		(169,651	)
	Net deferred tax asset		22,705			3,689
	Deferred tax liabilities
	Intangible assets		(3,234	)		(3,689	)
	Debt discount		(19,471	)		—
	Net deferred taxes	$	—		$	—
	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, 2010 through 2013, the tax years which remain subject to examination by major tax jurisdictions as of December 31, 2013. However, to the extent the Company utilizes net operating losses from years prior to 2010, 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, 2013.
	At December 31, 2013, the Company had net operating loss carryforwards for federal and state income tax purposes of $304.2 million and $230.7 million, respectively. Included in the federal and state net operating loss carryforwards is approximately $11.6 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 will expire in years through 2033. The Company also has available research and development credits for federal and state income tax purposes of approximately $18.6 million and $6.3 million, respectively. The federal and state research and development credits will begin to expire in 2022 and 2024, respectively. As of December 31, 2013, the Company also had available investment tax credits for state income tax purposes of $0.3 million, which will expire in years through 2016 if unused. In addition we have federal orphan drug credits of $14.4 million which begin to expire in 2031. 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, 2013 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 has not yet conducted a study of its domestic research and development credit carryforwards and orphan drug credits. This study may result in an increase or decrease to the Company’s credit carryforwards; however, until a study is completed and any adjustment is known, no amounts are being presented as an uncertain tax position. A full valuation allowance has been provided against the Company’s credits, and if an adjustment is required, this adjustment would be offset by an adjustment to the valuation allowance. As a result, there would be no impact to the statement of operations and comprehensive loss or cash flows if an adjustment were required.
	The change in the valuation allowance against the deferred tax assets in the years ended December 31, 2013, 2012 and 2011 was as follows:
	(in thousands)	Balance at			Additions			Deductions			Balance at
		beginning									end of
		of period									period
	December 31, 2011	$	103,881			28,802			—		$	132,683
	December 31, 2012	$	132,683			36,968			—		$	169,651
	December 31, 2013	$	169,651			37,653			—		$	207,304

Commitments_and_Contingencies

Commitments and Contingencies	12 Months Ended
	Dec. 31, 2013
Commitments And Contingencies Disclosure [Abstract]	'
Commitments and Contingencies	'
	16. Commitments and Contingencies
	Operating Leases
	The Company leases its office, laboratory and manufacturing space under non-cancelable operating leases. Total rent expense under these operating leases was $5.5 million, $4.3 million and $3.2 million for the years ended December 31, 2013, 2012 and 2011, respectively.
	During August 2012, the Company entered into an Indenture of Lease (the “Amended Lease”), which amended and restated its facility lease, and agreed to occupy approximately a total of 109,000 square feet (the “Leased Space”), all of which is leased until June 30, 2019. In March and September 2013, the Company entered into facility lease amendments that are co-terminus with the existing lease. The aggregate minimum lease payments due over the seven-year term of the Amended Lease are approximately $35.0 million. As part of the Amended Lease and subsequent amendments, the landlord agreed to reimburse the Company for a portion of tenant improvements made to the facility, up to approximately $8.2 million. As of December 31, 2013, the Company has received $5.5 million of these tenant improvement reimbursements, with the remaining reimbursable tenant improvements recorded within other current assets on the consolidated balance sheets as of December 31, 2013. 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, 2013 are as follows:
	Years ended December 31,	(in thousands)
	2014		5,095
	2015		5,208
	2016		5,354
	2017		5,470
	2018 and thereafter		8,399

Related_Party_Transactions

Related Party Transactions	12 Months Ended
	Dec. 31, 2013
Related Party Transactions [Abstract]	'
Related Party Transactions	'
	17. 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, February 2013 and December 2013, Silver Creek entered into an aggregate $2.5 million convertible note payable, of which $0.3 million was with directors, officers, scientific advisory board members and related parties of the Company. As of December 31, 2013, these notes and related accrued interest of $0.1 million converted into shares of Silver Creek Series A preferred stock.

Retirement_Plan

Retirement Plan	12 Months Ended
	Dec. 31, 2013
Postemployment Benefits [Abstract]	'
Retirement Plan	'
	18. 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, 2013, 2012 and 2011, the Company made contributions of $0.7 million, $0.6 million and $0.5 million, respectively, to the plan.

Selected_Quarterly_Financial_D

Selected Quarterly Financial Data	12 Months Ended
	Dec. 31, 2013
Quarterly Financial Information Disclosure [Abstract]	'
Selected Quarterly Financial Data	'
	19. Selected Quarterly Financial Data (Unaudited)
	The following table contains quarterly financial information for 2013 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			Second			Third			Fourth
		Quarter			Quarter			Quarter			Quarter
		(in thousands, except per share data)
	2013
	Collaboration revenues	$	14,655		$	18,452		$	6,856		$	7,823
	Total operating expenses		41,921			47,560			42,780			36,065
	Net loss		(28,323	)		(30,251	)		(39,763	)		(32,348	)
	Net loss attributable to Merrimack Pharmaceuticals, Inc.		(28,153	)		(30,082	)		(39,631	)		(33,059	)
	Net loss per share available to common stockholders—basic and diluted	$	(0.29	)	$	(0.31	)	$	(0.39	)	$	(0.33	)
		First			Second			Third			Fourth
		Quarter			Quarter			Quarter			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	)

Subsequent_Events

Subsequent Events	12 Months Ended
	Dec. 31, 2013
Subsequent Events [Abstract]	'
Subsequent Events	'
	20. Subsequent Events
	In February 2014, the Company received notice of an award of $0.6 million of tax incentives from the MLSC, which will allow the Company to monetize approximately $0.6 million of state research and development tax credits. In exchange for these incentives, the Company pledged to hire an incremental 31 employees and to maintain the additional headcount through at least December 31, 2018. Failure to do so could result in the Company being required to repay some or all of these incentives.
	In February 2014, Hercules exercised warrants to purchase 302,143 shares of common stock for proceeds of $1.1 million.

Summary_of_Significant_Account1

Summary of Significant Accounting Policies (Policies)	12 Months Ended
	Dec. 31, 2013
Accounting Policies [Abstract]	'
Basis of Presentation and Consolidation	'
	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 2009, and Merrimack Pharmaceuticals (Bermuda) Ltd., which was incorporated during 2011. The Company also consolidates its majority owned subsidiary, Silver Creek Pharmaceuticals, Inc. (“Silver Creek”). All intercompany transactions and balances have been eliminated in consolidation.
Use of Estimates	'
	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 most significant estimates in these consolidated financial statements include revenue recognition, including the estimated percentage of billable expenses in any particular budget period, periods of meaningful use of licensed products, estimates used in accounting for revenue separability and recognition, useful lives with respect to long-lived assets and intangibles, accounting for stock-based compensation, convertible preferred stock warrants, contingencies, intangible assets, goodwill, in-process research and development, derivative liability, valuation of convertible debt, tax valuation reserves and accrued expenses, including clinical research costs. 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	'
	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, 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 typically 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, 2013 and 2012, the Company recorded restricted cash of $685,000 and $628,000, respectively, which were primarily related to the Company’s facility lease.
Available-for-Sale Securities	'
	Available-for-Sale Securities
	The Company classifies marketable securities with a remaining maturity when purchased of greater than three months as available-for-sale. Available-for-sale 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. 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’ deficit 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. There were no realized gains or losses recognized on the sale or maturity of available-for-sale securities during the years ended December 31, 2013, 2012 or 2011.
Concentration of Credit Risk	'
	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 each of the years ended December 31, 2013 and 2012, Sanofi represented greater than 98% of collaboration revenues. As of December 31, 2013 and 2012, Sanofi represented greater than 98% and 99% of accounts receivable, respectively.
Property and Equipment	'
	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		7-Mar
	IT equipment		7-Mar
	Leaseholds improvements		Lesser of useful life or lease term
	Furniture and fixtures		7-Mar
	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
	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. 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, 2013 and 2012, respectively, as a component of mezzanine equity.
Derivative Liability	'
	Derivative Liability
	In December 2012, the Company’s majority owned subsidiary, Silver Creek, entered into a Note Purchase Agreement pursuant to which it issued convertible notes to various lenders in aggregate principal amounts of $1.6 million in December 2012, $0.3 million in February 2013 and $0.6 million in December 2013. Upon issuance, the Company determined that the underlying convertible notes represented share-settled debt and the potential conversion of the convertible notes into Silver Creek’s next qualifying series of preferred stock at a discount met the definition of a derivative, which was classified as a liability and remeasured each reporting period. Prior to conversion on December 31, 2013, the derivative was remeasured with changes in fair value recognized in earnings. The aggregate principal of $2.5 million and accrued interest of $0.1 million converted into shares of Silver Creek Series A preferred stock upon maturity on December 31, 2013. The Company estimated the value of the derivative liability issued in connection with the convertible notes payable at zero upon full conversion as of December 31, 2013 and $196,000 as of December 31, 2012.
Revenue Recognition	'
	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 or profit-sharing 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 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, separates and allocates consideration in a multiple element arrangement according to the relative selling price of each deliverable. 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 entered into a collaboration agreement with Watson Laboratories, Inc. (“Actavis”) in November 2013, which was evaluated under the accounting guidance on revenue recognition for multiple element arrangements. See Note 5, “License and Collaboration Agreements,” for additional information.
	The Company’s license and collaboration agreements executed prior to January 1, 2011 continue to be accounted for under previously issued revenue recognition guidance for multiple element arrangements and milestone revenue recognition. 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 counterparty 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.
	The Company did not materially modify any of its previously-existing multiple element arrangements during the years ended December 31, 2013 and 2012.
Research and Development Expenses	'
	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	'
	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 non-employees 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.
Comprehensive Income (Loss)	'
	Comprehensive Income (Loss)
	Comprehensive income (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 and Expense	'
	Other Income and Expense
	The Company records gains and losses on the remeasurement of fair value of convertible preferred stock warrants and other derivative liabilities, the federal and state sponsored tax incentives and other one-time income or expense-related items in other income.
	In January 2010, the Massachusetts Life Sciences Center (“MLSC”), an independent agency of the Commonwealth of Massachusetts, awarded the Company $1.5 million of tax incentives under its Life Sciences Tax Incentive Program. These incentives allowed the Company to monetize approximately $1.4 million of state research and development tax credits. The Company received this payment 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 some or all of these incentives. The Company deferred and is amortizing the benefit of this incentive 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, 2013, 2012 and 2011, the Company recognized $0.3 million of benefit in other income in each period.
	In January 2011, the MLSC awarded the Company an additional $1.3 million of tax incentives under its Life Sciences Tax Incentive Program, which allowed the Company to monetize approximately $1.2 million of state research and development tax credits. The Company received this payment 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 some or all of these incentives. The Company deferred and is amortizing the benefit of this incentive 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, 2013 and 2012, the Company recognized $0.2 million and $0.5 million, respectively, of benefit in other income. For the year ended December 31, 2011, the Company did not recognize any benefit in other income.
	In January 2013, the MLSC awarded the Company an additional $0.5 million of tax incentives under its Life Science Tax Incentive Program, which allows the Company to monetize approximately $0.4 million of state research and development tax credits. The Company received this payment in the fourth quarter of 2013. 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. The Company has deferred and will amortize the benefit of this incentive on a straight-line basis over the five-year performance period, commencing with a cumulative catch-up when the pledge is achieved.
Deferred Financing Costs	'
	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.
	In April 2012, the Company closed the initial public offering of its common stock. Upon closing, $2.7 million of deferred financing costs were netted against the equity proceeds within stockholders’ deficit.
	In July 2013, the Company closed a follow-on underwritten public offering of additional shares of common stock and issued convertible senior notes in concurrent public offerings. Upon closing, $0.6 million of aggregate deferred financing costs were netted against the proceeds of these offerings.
Income Taxes	'
	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 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 resulted from the Company’s most recent evaluation, which occurred in the third quarter of 2013. The Company’s next annual impairment evaluation will be made in the third quarter of 2014 unless indicators arise that would require the Company to evaluate at an earlier date.
	When performing an evaluation of goodwill impairment, the Company has the option to first assess qualitative factors to determine whether it is necessary to perform the quantitative two-step impairment test. If the Company elects this option and finds, as a result of the qualitative assessment, that it is more likely than not that the fair value of the reporting unit is less than its carrying amount, the quantitative two-step impairment test is required; otherwise, no further testing is required. This requires the Company to assess the impact of significant events, milestones and changes to expectations and activities that may have occurred since the last impairment evaluation. Significant changes to these estimates, judgments and assumptions could materially change the outcome of the impairment assessment. Alternatively, the Company may elect to not first assess qualitative factors and immediately perform the quantitative two-step impairment test. If such an election occurs, in the first step, the fair value of the Company’s reporting unit is compared to the carrying value. If the carrying value of the net assets assigned to the reporting unit exceeds the fair value of the reporting unit, then the second step of the impairment test is performed in order to determine the implied fair value of the reporting unit’s goodwill. If the carrying value of the reporting unit’s goodwill exceeds the implied fair value, then the Company would record an impairment loss equal to the difference. As described above, the Company operates in one operating segment, which is considered the only reporting unit.
	The Company’s evaluation of IPR&D impairment included a qualitative assessment to determine whether further impairment testing of indefinite-lived intangible assets was necessary. For all but one IPR&D asset, it was determined that it was not more likely than not that an impairment existed as of August 31, 2013 and, therefore, impairment evaluations were not performed. In this one case, because it was determined that it was more likely than not that an impairment of one IPR&D asset existed as of August 31, 2013, an impairment evaluation was performed. These determinations and the evaluation 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, estimates of expected intellectual property protection and discount rates. Significant changes to these estimates, judgments and assumptions could materially change the outcome of management’s impairment assessment. The impairment evaluation resulted in the Company recognizing a $0.8 million impairment charge related to an early-stage preclinical program, which was charged to research and development expense. See Note 8, “Goodwill and Intangible Assets, Net,” for additional information.
	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 lives. Amortization of remaining IPR&D has not commenced as of December 31, 2013.
	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 lives. The Company has not recorded any impairment charges related to definite-lived intangible assets.
Subsequent Events	'
	Subsequent Events
	The Company has evaluated all events or transactions that occurred after December 31, 2013 up through the date the Company issued these financial statements. Other than the items discussed at Note 20, “Subsequent Events,” the Company did not have any other material recognizable or unrecognizable subsequent events.
Recent Accounting Pronouncements	'
	Recent Accounting Pronouncements
	In February 2013, the Financial Accounting Standards Board (“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 adoption of this guidance did not have a material impact on the Company’s consolidated financial statements.
	In July 2013, the FASB issued guidance to address the diversity in practice related to the financial statement presentation of unrecognized tax benefits as either a reduction of a deferred tax asset or a liability when a net operating loss carryforward, a similar tax loss or a tax credit carryforward exists. This guidance is effective prospectively for fiscal years, and interim periods within those years, beginning after December 15, 2013. The adoption of this guidance is not expected to have a material impact on the Company’s consolidated financial statements.

Summary_of_Significant_Account2

Summary of Significant Accounting Policies (Tables)	12 Months Ended
	Dec. 31, 2013
Accounting Policies [Abstract]	'
Schedule of Estimated Useful Lives of Property and Equipment Including Leasehold Improvements	'
	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		7-Mar
	IT equipment		7-Mar
	Leaseholds improvements		Lesser of useful life or lease term
	Furniture and fixtures		7-Mar

Marketable_Securities_Tables

Marketable Securities (Tables)	12 Months Ended
	Dec. 31, 2013
Investments Debt And Equity Securities [Abstract]	'
Schedule of Available-For-Sale Securities, All of Which Have Maturities of Twelve Months or Less	'
	Available-for-sale securities, all of which have maturities of twelve months or less, as of December 31, 2013 consisted of the following:
		Amortized		Unrealized			Unrealized			Fair
		Cost		Gains			Losses			Value
		(in thousands)
	December 31, 2013:
	Commercial paper	$	49,685	$	—		$	(5	)	$	49,680
	Corporate debt securities		40,455		—			(19	)		40,436
	Total	$	90,140	$	—		$	(24	)	$	90,116
Schedule of Marketable Securities in an Unrealized Loss Position	'
	Available-for-sale securities in an unrealized loss position as of December 31, 2013 consisted of the following:
		Aggregate		Unrealized
		Fair Value		Losses
		(in thousands)
	December 31, 2013:
	Commercial paper	$	36,683	$	(5	)
	Corporate debt securities		40,436		(19	)
		$	77,119	$	(24	)

Net_Loss_Per_Common_Share_Tabl

Net Loss Per Common Share (Tables)	12 Months Ended
	Dec. 31, 2013
Earnings Per Share [Abstract]	'
Schedule of Computation of Basic and Diluted Net Loss Per Share Available to Common Stockholders	'
	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)	2013			2012			2011
	Net Loss Per Share:
	Numerator:
	Net loss attributable to Merrimack Pharmaceuticals, Inc.	$	(130,925	)	$	(91,277	)	$	(79,223	)
	Plus: Unaccreted dividends on convertible preferred stock		—			(2,107	)		(7,789	)
	Net loss available to common stockholders—basic and diluted		(130,925	)		(93,384	)		(87,012	)
	Denominator:
	Weighted-average common shares—basic and diluted		98,919			72,831			11,343
	Net loss per share available to common stockholders—basic and diluted	$	(1.32	)	$	(1.28	)	$	(7.67	)
Schedule of Potentially Dilutive Securities Excluded from Computation of Diluted Weighted Average Shares	'
	The potential dilutive effect of these securities is shown in the chart below:
		Years ended December 31,
	(in thousands)	2013			2012			2011
	Common stock warrants		2,777			2,842			2,640
	Convertible preferred stock		—			—			66,256
	Options to purchase common stock		20,107			18,066			17,617
	Convertible preferred stock warrants		—			—			302
	Conversion of the Notes		25,000			—			—

License_and_Collaboration_Agre1

License and Collaboration Agreements (Tables)	12 Months Ended
	Dec. 31, 2013
Organization Consolidation And Presentation Of Financial Statements [Abstract]	'
Schedule of Revenue Recognized and Assets and Liabilities under Collaborative Arrangements	'
	During the years ended December 31, 2013, 2012 and 2011, the Company recognized revenue based on the following components of the Sanofi agreement:
		Years ended December 31,
	(in thousands)	2013		2012		2011
	Upfront payment	$	5,000	$	5,000	$	5,000
	Milestone payment		2,083		2,975		2,616
	Development services		36,283		36,905		25,053
	Manufacturing services and other		3,867		3,307		1,456
	Total	$	47,233		48,187	$	34,125
	As of December 31, 2013 and 2012, the Company maintained the following assets and liabilities related to the Sanofi agreement:
		December 31,
	(in thousands)	2013		2012
	Accounts receivable, billed	$	2,357	$	1,577
	Accounts receivable, unbilled		3,417		7,690
	Deferred revenues		73,392		79,913

Fair_Value_of_Financial_Instru1

Fair Value of Financial Instruments (Tables)	12 Months Ended
	Dec. 31, 2013
Fair Value Disclosures [Abstract]	'
Schedule of Assets and Liabilities Measured at Fair Value on a Recurring Basis	'
	The following tables show assets and liabilities measured at fair value on a recurring basis as of December 31, 2013 and 2012 and the input categories associated with those assets and liabilities:
	As of December 31, 2013	Level 1			Level 2			Level 3
	(in thousands)
	Assets:
	Cash equivalents—money market funds	$	47,740		$	—		$	—
	Cash equivalents—commercial paper		—			13,998			—
	Investments—commercial paper		—			49,680			—
	Investments—corporate debt securities		—			40,436			—
	As of December 31, 2012	Level 1			Level 2			Level 3
	(in thousands)
	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	'
	The following table provides a roll-forward of the fair value of the liabilities categorized as Level 3 instruments, for the years ended December 31, 2013 and 2012:
	(in thousands)	Convertible			Derivative
		preferred			Liability
		stock warrants
	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
	Portion of convertible note allocated to derivative		—			35
	Remeasurement of derivative liability upon conversion of underlying note		—			(231	)
	Balance, December 31, 2013	$	—		$	—
Schedule of Quantitative Information Associated with the Fair Value Measurement of the Company's Non-Recurring Level 3 Inputs	'
	The following table provides quantitative information associated with the fair value measurement of the Company’s non-recurring Level 3 inputs:
		Fair Value as of			Valuation		Unobservable Input	Percentage
		August 31, 2013			Technique
		(in thousands)
	IPR&D asset	$	—		Income approach—Probability weighted discounted cash flow analysis		Discount rate		25.7	%

Goodwill_and_Intangible_Assets1

Goodwill and Intangible Assets, Net (Tables)	12 Months Ended
	Dec. 31, 2013
Goodwill And Intangible Assets Disclosure [Abstract]	'
Schedule of Changes in the Carrying Value of Goodwill, IPR&D and Intangible Assets	'
	Changes in the carrying value of goodwill, IPR&D and intangible assets for the years ended December 31, 2013, 2012 and 2011 were as follows:
	(in thousands)	Intangible			IPR&D			Goodwill
		assets
	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
	Amortization		(320	)		—			—
	Impairment		—			(810	)		—
	Balance, December 31, 2013	$	1,845		$	6,200		$	3,605
Schedule of Expected Amortization Expense for Intangible Assets for the Next Five-Year Period	'
	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)
	2014	$	320
	2015		320
	2016		320
	2017		320
	2018		320

Property_and_Equipment_Net_Tab

Property and Equipment, Net (Tables)	12 Months Ended
	Dec. 31, 2013
Property Plant And Equipment [Abstract]	'
Schedule of Property and Equipment	'
	Property and equipment consisted of the following:
		December 31,
	(in thousands)	2013			2012
	Lab equipment	$	13,714		$	12,616
	IT equipment		2,701			2,346
	Leasehold improvements		10,523			8,200
	Furniture and fixtures		340			330
	Construction in process		7,635			1,774
			34,913			25,266
	Less: Accumulated depreciation and amortization		(21,549	)		(18,969	)
		$	13,364		$	6,297

Accounts_Payables_Accrued_Expe1

Accounts Payables, Accrued Expenses and Other (Tables)	12 Months Ended
	Dec. 31, 2013
Payables And Accruals [Abstract]	'
Schedule of Accounts Payable, Accrued Expenses and Other	'
	Accounts payable, accrued expenses and other as of December 31, 2013 and 2012 consisted of the following:
		December 31,
	(in thousands)	2013		2012
	Accounts payable	$	1,889	$	283
	Accrued goods and services		26,031		17,615
	Accrued payroll and related benefits		7,255		5,853
	Accrued interest		2,926		306
	Accrued dividends payable		25		28
	Deferred tax incentives		688		512
	Derivative liability		—		196
	Other contractual liability		—		851
	Total accounts payable, accrued expenses and other	$	38,814	$	25,644

Borrowings_Tables

Borrowings (Tables)	12 Months Ended
	Dec. 31, 2013
Debt Disclosure [Abstract]	'
Schedule of Future Minimum Payments under the Loans Payable	'
	Future minimum payments under indebtedness agreements outstanding as of December 31, 2013 are as follows:
	Years Ending December 31:	4.50% Convertible			Loan
		Senior Notes			Agreement
	(in thousands)
	2014	$	5,594		$	12,353
	2015		5,625			18,130
	2016		5,625			17,916
	2017		5,625			—
	2018 and thereafter		141,875			—
		$	164,344		$	48,399
	Less interest		(39,344	)		(7,199	)
	Less unamortized discount		(52,422	)		(2,103	)
	Less current portion		—			(8,248	)
	Loans payable, net of current portion	$	72,578		$	30,849

Stock_Warrants_Tables

Stock Warrants (Tables)	12 Months Ended
	Dec. 31, 2013
Text Block [Abstract]	'
Schedule of Common and Convertible Preferred Stock Warrant Activity of the Company	'
	The following is a description of the common and convertible preferred stock warrant activity of the Company:
	(in thousands, except per share amounts)	Warrants			Weighted		Warrants			Weighted
		for the			Average		for the			Average
		Purchase of			Exercise		Purchase of			Exercise
		Common			Price		Convertible			Price
		Stock					Preferred
							Stock
	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		$	3.5
	Conversion		302		$	3.5		(302	)	$	3.5
	Exercised		(100	)	$	2.63		—			—
	Balance—December 31, 2012		2,842		$	3.05		—			—
	Exercised		(65	)	$	2.82
	Balance—December 31, 2013		2,777		$	3.05		—			—

Common_Stock_Tables

Common Stock (Tables)	12 Months Ended
	Dec. 31, 2013
Equity [Abstract]	'
Schedule of Shares Reserved for Future Issuance	'
	The shares reserved for future issuance as of December 31, 2013 and 2012 consisted of the following:
	(in thousands)	December 31, 2013		December 31, 2012
	Common stock warrants		2,777		2,842
	Options to purchase common stock		20,107		18,066
	Conversion of the Notes		25,000		—

StockBased_Compensation_Tables

Stock-Based Compensation (Tables)	12 Months Ended
	Dec. 31, 2013
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]	'
Schedule of Recognized Stock-Based Compensation Expense	'
	The Company recognized stock-based compensation expense as follows:
		Years ended December 31,
	(in thousands)	2013			2012			2011
	Employee awards:
	Research and development	$	5,954		$	4,234		$	3,597
	General and administrative		4,808			2,510			2,875
	Stock-based compensation for employee awards		10,762			6,744			6,472
	Stock-based compensation for non-employee awards		(29	)		145			480
	Total stock-based compensation	$	10,733		$	6,889		$	6,952
Schedule of Assumptions Used to Calculate Fair Value of Options Granted to Employees	'
	The fair value of employee options granted during the years ended December 31, 2013, 2012 and 2011 was estimated at the date of grant using the following assumptions:
		Years ended December 31,
		2013			2012			2011
	Risk-free interest rate		0.1 – 1.9	%		0.7 – 1.1	%		1.3 –2.5	%
	Expected dividend yield		0	%		0	%		0	%
	Expected term		5.3 – 5.9 years			5 – 5.9 years			5 – 5.9 years
	Expected volatility		67 – 70	%		66 – 72	%		71 – 73	%
Summary of Stock Option Activity	'
	The following table summarizes stock option activity:
		Shares			Weighted Average			Weighted Average			Aggregate
					Exercise Price			Remaining			Intrinsic Value
								Contractual Term
	Outstanding at December 31, 2012		18,066		$	3.5			6.54		$	51,486
	Granted		3,286		$	6.04
	Exercised		(913	)	$	2.23
	Cancelled		(332	)	$	6.02
	Outstanding at December 31, 2013		20,107		$	3.93			6.11		$	38,348
	Vested and expected to vest at December 31, 2013		19,829		$	3.89			6.07		$	38,307
	Exercisable at December 31, 2013		15,672		$	3.19			5.33		$	37,861

Income_Taxes_Tables

Income Taxes (Tables)	12 Months Ended
	Dec. 31, 2013
Income Tax Disclosure [Abstract]	'
Schedule of Reconciliation of the Company's Effective Tax Rate to the Statutory Federal Income Tax Rate	'
	A reconciliation of the Company’s effective tax rate to the statutory federal income tax rate is as follows:
		Year Ended December 31,
		2013			2012			2011
	Federal income tax at statutory federal rate		35	%		35	%		35	%
	State taxes		4.3			4.5			4.2
	Permanent differences		(2.0	)		(0.2	)		(0.4	)
	Stock-based compensation		(0.6	)		(0.3	)		(1.2	)
	Tax credits		12.4			1.1			3.9
	Foreign rate differential		(2.8	)		(4.1	)		(4.4	)
	Other		(1.5	)		(0.5	)		(0.8	)
	Change in valuation allowance		(44.8	)		(35.5	)		(36.3	)
			—	%		—	%		—	%
Schedule of Temporary Differences That Give Rise to Significant Net Deferred Tax Assets	'
	Temporary differences that give rise to significant net deferred tax assets as of December 31, 2013 and 2012 are as follows:
	(in thousands)	2013			2012
	Deferred tax assets
	Net operating losses	$	113,843		$	77,806
	Capitalized research and development expenses		32,771			40,083
	Credit carryforwards		37,304			14,398
	Depreciation		2,735			2,931
	Deferred compensation		8,249			5,068
	Accrued expenses		841			1,184
	Deferred revenue		28,381			29,936
	Other temporary differences		5,885			1,934
	Total gross deferred tax asset		230,009			173,340
	Valuation allowance		(207,304	)		(169,651	)
	Net deferred tax asset		22,705			3,689
	Deferred tax liabilities
	Intangible assets		(3,234	)		(3,689	)
	Debt discount		(19,471	)		—
	Net deferred taxes	$	—		$	—
Schedule of Changes in the Valuation Allowance	'
	The change in the valuation allowance against the deferred tax assets in the years ended December 31, 2013, 2012 and 2011 was as follows:
	(in thousands)	Balance at			Additions			Deductions			Balance at
		beginning									end of
		of period									period
	December 31, 2011	$	103,881			28,802			—		$	132,683
	December 31, 2012	$	132,683			36,968			—		$	169,651
	December 31, 2013	$	169,651			37,653			—		$	207,304

Commitments_and_Contingencies_

Commitments and Contingencies (Tables)	12 Months Ended
	Dec. 31, 2013
Commitments And Contingencies Disclosure [Abstract]	'
Schedule of Future Minimum Lease Payments Under Noncancelable Operating Leases	'
	Future minimum lease payments under noncancelable operating leases at December 31, 2013 are as follows:
	Years ended December 31,	(in thousands)
	2014		5,095
	2015		5,208
	2016		5,354
	2017		5,470
	2018 and thereafter		8,399

Selected_Quarterly_Financial_D1

Selected Quarterly Financial Data (Tables)	12 Months Ended
	Dec. 31, 2013
Quarterly Financial Information Disclosure [Abstract]	'
Schedule of Quarterly Financial Information	'
	The following table contains quarterly financial information for 2013 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			Second			Third			Fourth
		Quarter			Quarter			Quarter			Quarter
		(in thousands, except per share data)
	2013
	Collaboration revenues	$	14,655		$	18,452		$	6,856		$	7,823
	Total operating expenses		41,921			47,560			42,780			36,065
	Net loss		(28,323	)		(30,251	)		(39,763	)		(32,348	)
	Net loss attributable to Merrimack Pharmaceuticals, Inc.		(28,153	)		(30,082	)		(39,631	)		(33,059	)
	Net loss per share available to common stockholders—basic and diluted	$	(0.29	)	$	(0.31	)	$	(0.39	)	$	(0.33	)
		First			Second			Third			Fourth
		Quarter			Quarter			Quarter			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_Additio

Nature of the Business - Additional Information (Detail) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2013
	Candidate
Organization Consolidation And Presentation Of Financial Statements [Abstract]	'
Number of novel therapeutic oncology candidates	6
Unrestricted cash and cash equivalents and available-for-sale securities	$155.20

Summary_of_Significant_Account3

Summary of Significant Accounting Policies - Additional Information (Detail) (USD $)

1 Months Ended

3 Months Ended

12 Months Ended

1 Months Ended

12 Months Ended

1 Months Ended

12 Months Ended

1 Months Ended

0 Months Ended

1 Months Ended

2 Months Ended

12 Months Ended

Aug. 31, 2013

Apr. 30, 2012

Sep. 30, 2013

Dec. 31, 2013

Dec. 31, 2012

Dec. 31, 2011

Jul. 31, 2013

Jan. 31, 2010

Dec. 31, 2013

Dec. 31, 2012

Dec. 31, 2011

Jan. 31, 2011

Dec. 31, 2013

Dec. 31, 2012

Dec. 31, 2011

Jan. 31, 2013

Dec. 31, 2013

Dec. 31, 2013

Feb. 28, 2013

Dec. 31, 2012

Dec. 31, 2013

Feb. 28, 2013

Dec. 31, 2012

Feb. 28, 2013

Dec. 31, 2013

Dec. 31, 2012

Dec. 31, 2013

Dec. 31, 2013

Dec. 31, 2012

Dec. 31, 2013

Dec. 31, 2012

Item

Life Sciences Tax Incentive Program 2010 [Member]

Life Sciences Tax Incentive Program 2010 [Member]

Life Sciences Tax Incentive Program 2010 [Member]

Life Sciences Tax Incentive Program 2010 [Member]

Life Sciences Tax Incentive Program 2011 [Member]

Life Sciences Tax Incentive Program 2011 [Member]

Life Sciences Tax Incentive Program 2011 [Member]

Life Sciences Tax Incentive Program 2011 [Member]

Life Sciences Tax Incentive Program 2013 [Member]

Silver Creek [Member]

Silver Creek Convertible Note [Member]

Silver Creek Convertible Note [Member]

Silver Creek Convertible Note [Member]

Silver Creek Convertible Note [Member]

Silver Creek Convertible Note [Member]

Silver Creek Convertible Note [Member]

Silver Creek Convertible Note [Member]

Silver Creek Convertible Note [Member]

Silver Creek Convertible Note [Member]

Minimum [Member]

Minimum [Member]

Minimum [Member]

Minimum [Member]

Minimum [Member]

Segment

Employees

Employees

Employees

Silver Creek [Member]

Silver Creek [Member]

Silver Creek [Member]

Silver Creek [Member]

Silver Creek [Member]

Silver Creek [Member]

Silver Creek [Member]

Silver Creek [Member]

Silver Creek [Member]

Collaboration Revenue [Member]

Collaboration Revenue [Member]

Accounts Receivable [Member]

Accounts Receivable [Member]

Customer Concentration Risk [Member]

Customer Concentration Risk [Member]

Credit Concentration Risk [Member]

Credit Concentration Risk [Member]

Sanofi [Member]

Sanofi [Member]

Sanofi [Member]

Sanofi [Member]

Summary Of Significant Accounting Policies [Line Items]

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Number of operating segments

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Number of geographic segments

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Original maturities

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'Three months or less

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Restricted cash

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$685,000

$628,000

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Expected average maturities period of marketable securities classified as available-for-sale

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'

'

'

'

'

'

'

'

'

'

'

'

'3 months

'

'

'

'

Realized gains (losses) recognized on the sale or maturity of securities

'

'

'

0

0

0

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Number of issues with limited investment as per investment policy

'

'

'

1

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Concentration risk (as a percent)

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

98.00%

98.00%

98.00%

99.00%

Aggregate principal amounts

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

600,000

300,000

1,600,000

2,500,000

2,500,000

2,500,000

300,000

600,000

1,600,000

'

'

'

'

'

Aggregate principal amount

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

2,500,000

'

'

2,500,000

'

'

'

2,500,000

'

'

'

'

'

'

Accrued interest

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

100,000

100,000

'

'

100,000

'

'

'

100,000

'

'

'

'

'

'

Derivative liability

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

0

'

196,000

0

'

196,000

'

0

196,000

'

'

'

'

'

Tax incentive awarded

'

'

'

'

'

'

'

1,500,000

'

'

'

1,300,000

'

'

'

500,000

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

State research and development tax credits monetized

'

'

'

'

'

'

'

1,400,000

'

'

'

1,200,000

'

'

'

400,000

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Number of incremental employees pledged to be hired

'

'

'

'

'

'

'

50

'

'

'

50

'

'

'

20

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Performance period

'

'

'

'

'

'

'

'5 years

'

'

'

'5 years

'

'

'

'5 years

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Amount of benefit recognized

'

'

'

'

'

'

'

'

300,000

300,000

300,000

'

200,000

500,000

0

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Deferred financing costs netted against the equity proceeds within stockholders' equity

'

2,700,000

'

278,000

2,748,000

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Aggregate deferred financing costs

'

'

'

'

'

'

600,000

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Impairment charge

$810,000,000

'

$0

$810,000

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Summary_of_Significant_Account4

Summary of Significant Accounting Policies - Schedule of Estimated Useful Lives of Property and Equipment Including Leasehold Improvements (Detail)	12 Months Ended
	Dec. 31, 2013
Leasehold Improvements [Member]	'
Property, Plant and Equipment [Line Items]	'
Estimated useful life	'Lesser of useful life or lease term
Minimum [Member] | Lab Equipment [Member]	'
Property, Plant and Equipment [Line Items]	'
Estimated useful life	'3 years
Minimum [Member] | IT Equipment [Member]	'
Property, Plant and Equipment [Line Items]	'
Estimated useful life	'3 years
Minimum [Member] | Furniture and Fixtures [Member]	'
Property, Plant and Equipment [Line Items]	'
Estimated useful life	'3 years
Maximum [Member] | Lab Equipment [Member]	'
Property, Plant and Equipment [Line Items]	'
Estimated useful life	'7 years
Maximum [Member] | IT Equipment [Member]	'
Property, Plant and Equipment [Line Items]	'
Estimated useful life	'7 years
Maximum [Member] | Furniture and Fixtures [Member]	'
Property, Plant and Equipment [Line Items]	'
Estimated useful life	'7 years

Marketable_Securities_Schedule

Marketable Securities - Schedule of Available-For-Sale Securities, All of Which Have Maturities of Twelve Months or Less (Detail) (USD $)	Dec. 31, 2013
In Thousands, unless otherwise specified
Schedule of Available-for-sale Securities [Line Items]	'
Amortized Cost	$90,140
Unrealized Gains	'
Unrealized Losses	-24
Fair Value	90,116
Commercial Paper [Member]	'
Schedule of Available-for-sale Securities [Line Items]	'
Amortized Cost	49,685
Unrealized Gains	'
Unrealized Losses	-5
Fair Value	49,680
Corporate Debt Securities [Member]	'
Schedule of Available-for-sale Securities [Line Items]	'
Amortized Cost	40,455
Unrealized Gains	'
Unrealized Losses	-19
Fair Value	$40,436

Marketable_Securities_Addition

Marketable Securities - Additional Information (Detail) (USD $)	12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2013
Schedule of Available-for-sale Securities [Line Items]	'
Aggregate fair value of securities held by the Company in an unrealized loss position for less than 12 months	77,119
Number of securities in unrealized loss position for less than 12 months	21
Maximum [Member]	'
Schedule of Available-for-sale Securities [Line Items]	'
Expected average maturities period of marketable securities classified as available-for-sale	'12 months

Marketable_Securities_Schedule1

Marketable Securities - Schedule of Marketable Securities in an Unrealized Loss Position (Detail) (USD $)	Dec. 31, 2013
In Thousands, unless otherwise specified
Schedule of Available-for-sale Securities [Line Items]	'
Aggregate Fair Value	$77,119
Unrealized Losses	-24
Commercial Paper [Member]	'
Schedule of Available-for-sale Securities [Line Items]	'
Aggregate Fair Value	36,683
Unrealized Losses	-5
Corporate Debt Securities [Member]	'
Schedule of Available-for-sale Securities [Line Items]	'
Aggregate Fair Value	40,436
Unrealized Losses	($19)

Net_Loss_Per_Common_Share_Sche

Net Loss Per Common Share - Schedule of Computation of Basic and Diluted Net Loss Per Share Available to Common Stockholders (Detail) (USD $)	3 Months Ended								12 Months Ended
In Thousands, except Per Share data, unless otherwise specified	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Numerator:	'	'	'	'	'	'	'	'	'	'	'
Net loss attributable to Merrimack Pharmaceuticals, Inc.	($33,059)	($39,631)	($30,082)	($28,153)	($24,768)	($23,199)	($20,026)	($23,284)	($130,925)	($91,277)	($79,223)
Plus: Unaccreted dividends on convertible preferred stock	'	'	'	'	'	'	'	'	'	-2,107	-7,789
Net loss available to common stockholders-basic and diluted	'	'	'	'	'	'	'	'	($130,925)	($93,384)	($87,012)
Denominator:	'	'	'	'	'	'	'	'	'	'	'
Weighted-average common shares-basic and diluted	'	'	'	'	'	'	'	'	98,919	72,831	11,343
Net loss per share available to common stockholders-basic and diluted	($0.33)	($0.39)	($0.31)	($0.29)	($0.26)	($0.25)	($0.22)	($2.14)	($1.32)	($1.28)	($7.67)

Net_Loss_Per_Common_Share_Addi

Net Loss Per Common Share - Additional Information (Detail) (USD $)	1 Months Ended
In Millions, unless otherwise specified	Jul. 31, 2013	Dec. 31, 2013
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]	'	'
Interest rate (as a percent)	'	4.50%
4.50% Convertible Senior Notes [Member]	'	'
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]	'	'
Aggregate principal amount	$125	'
Interest rate (as a percent)	4.50%	4.50%

Net_Loss_Per_Common_Share_Sche1

Net Loss Per Common Share - Schedule of Potentially Dilutive Securities Excluded from Computation of Diluted Weighted Average Shares (Detail)	12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Common Stock Warrants [Member]	'	'	'
Dilutive Securities Included And Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]	'	'	'
Potentially dilutive securities (in shares)	2,777	2,842	2,640
Series B-G Convertible Preferred Stock [Member]	'	'	'
Dilutive Securities Included And Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]	'	'	'
Potentially dilutive securities (in shares)	'	'	66,256
Options to Purchase Common Stock [Member]	'	'	'
Dilutive Securities Included And Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]	'	'	'
Potentially dilutive securities (in shares)	20,107	18,066	17,617
Convertible Preferred Stock Warrants [Member]	'	'	'
Dilutive Securities Included And Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]	'	'	'
Potentially dilutive securities (in shares)	'	'	302
4.50% Convertible Senior Notes [Member]	'	'	'
Dilutive Securities Included And Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]	'	'	'
Potentially dilutive securities (in shares)	25,000	'	'

License_and_Collaboration_Agre2

License and Collaboration Agreements - Additional Information (Detail) (USD $)

12 Months Ended

3 Months Ended

0 Months Ended

12 Months Ended

0 Months Ended

12 Months Ended

Dec. 31, 2013

Dec. 31, 2012

Dec. 31, 2011

Dec. 31, 2013

Mar. 31, 2013

5-May-11

Dec. 31, 2013

Dec. 31, 2012

Dec. 31, 2011

5-May-11

5-May-11

5-May-12

Nov. 10, 2009

Dec. 31, 2013

Dec. 31, 2010

Dec. 31, 2011

Dec. 31, 2012

Actavis [Member]

License and Collaboration Agreements [Member]

License and Collaboration Agreements [Member]

License and Collaboration Agreements [Member]

License and Collaboration Agreements [Member]

License and Collaboration Agreements [Member]

License and Collaboration Agreements [Member]

License and Collaboration Agreements [Member]

License and Collaboration Agreements [Member]

License and Collaboration Agreements [Member]

License and Collaboration Agreements [Member]

License and Collaboration Agreements [Member]

License and Collaboration Agreements [Member]

License and Collaboration Agreements [Member]

GTC [Member]

PharmaEngine [Member]

PharmaEngine [Member]

PharmaEngine [Member]

PharmaEngine [Member]

PharmaEngine [Member]

PharmaEngine [Member]

PharmaEngine [Member]

Sanofi [Member]

Sanofi [Member]

Sanofi [Member]

Sanofi [Member]

Sanofi [Member]

Development and Regulatory Milestone [Member]

Sales Milestones [Member]

Phase 3 Clinical Trial in Pancreatic Cancer [Member]

Phase 2 Clinical Trial in Breast Cancer [Member]

Phase 2 Clinical Trial in Non-small Cell Lung Cancer [Member]

Phase 2 Clinical Trial in Ovarian Cancer [Member]

Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Upfront license fee received

'

'

'

'

'

'

'

'

'

'

'

'

$60,000,000

'

'

'

'

Milestone payments received

'

'

'

15,500,000

'

'

'

'

'

'

'

'

'

25,000,000

10,000,000

10,000,000

5,000,000

Budget period

'

'

'

'

'

'

'

'

'

'

'

'

'

'12 months

'

'

'

Expected development period from the effective date of agreement

'

'

'

'

'

'

'

'

'

'

'

'

'12 years

'

'

'

'

Development services performed

'

'

'

'

'

'

'

'

'

'

'

'

'

10,100,000

'

'

'

Upfront license fees paid

'

'

'

'

'

10,000,000

'

'

'

'

'

'

'

'

'

'

'

Maximum milestone payment obligation

'

'

'

'

'

'

'

'

'

80,000,000

130,000,000

'

'

'

'

'

'

Milestone payment

'

'

'

'

'

'

'

'

'

'

'

5,000,000

'

'

'

'

'

Research and development expenses

147,139,000

125,858,000

100,630,000

'

'

'

1,500,000

6,200,000

11,200,000

'

'

'

'

'

'

'

'

Milestone payment expensed

'

'

'

'

'

'

'

5,000,000

'

'

'

'

'

'

'

'

'

Milestone payment expense

'

'

'

'

'

'

'

'

10,000,000

'

'

'

'

'

'

'

'

Amounts accrued and payable related to the agreement

'

'

'

'

'

'

600,000

300,000

'

'

'

'

'

'

'

'

'

Milestone license fee received

'

'

'

2,000,000

'

'

'

'

'

'

'

'

'

'

'

'

'

Agreement expiration term respect to each product

'

'

'

'10 years

'

'

'

'

'

'

'

'

'

'

'

'

'

Additional renewal term

'

'

'

'2 years

'

'

'

'

'

'

'

'

'

'

'

'

'

Notice period of termination

'

'

'

'90 days

'

'

'

'

'

'

'

'

'

'

'

'

'

Milestones and development expenses

'

'

'

2,100,000

'

'

'

'

'

'

'

'

'

'

'

'

'

Deferred revenue recognized due to a change in estimate of expected development term

'

'

'

'

$600,000

'

'

'

'

'

'

'

'

'

'

'

'

License_and_Collaboration_Agre3

License and Collaboration Agreements - Schedule of Revenue Recognized and Assets and Liabilities under Collaborative Arrangements (Detail) (License and Collaboration Agreements [Member], Sanofi [Member], USD $)	12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
License and Collaboration Agreements [Member] | Sanofi [Member]	'	'	'
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]	'	'	'
Upfront payment	$5,000	$5,000	$5,000
Milestone payment	2,083	2,975	2,616
Development services	36,283	36,905	25,053
Manufacturing services and other	3,867	3,307	1,456
Total	47,233	48,187	34,125
Accounts receivable, billed	2,357	1,577	'
Accounts receivable, unbilled	3,417	7,690	'
Deferred revenues	$73,392	$79,913	'

Fair_Value_of_Financial_Instru2

Fair Value of Financial Instruments - Schedule of Assets and Liabilities Measured at Fair Value on a Recurring Basis (Detail) (USD $)

Dec. 31, 2013

Dec. 31, 2013

Dec. 31, 2013

Dec. 31, 2013

Dec. 31, 2013

Dec. 31, 2013

Dec. 31, 2013

Dec. 31, 2013

Dec. 31, 2013

Dec. 31, 2012

Dec. 31, 2012

Dec. 31, 2012

Dec. 31, 2012

Dec. 31, 2012

Dec. 31, 2012

Dec. 31, 2012

Dec. 31, 2012

Dec. 31, 2012

Dec. 31, 2012

Dec. 31, 2012

In Thousands, unless otherwise specified

Level 1 [Member]

Level 3 [Member]

Corporate Debt Securities [Member]

Corporate Debt Securities [Member]

Corporate Debt Securities [Member]

Commercial Paper [Member]

Commercial Paper [Member]

Commercial Paper [Member]

Recurring Basis [Member]

Recurring Basis [Member]

Recurring Basis [Member]

Recurring Basis [Member]

Recurring Basis [Member]

Recurring Basis [Member]

Recurring Basis [Member]

Recurring Basis [Member]

Recurring Basis [Member]

Recurring Basis [Member]

Recurring Basis [Member]

Level 2 [Member]

Level 3 [Member]

Level 2 [Member]

Level 3 [Member]

Level 1 [Member]

Level 2 [Member]

Level 3 [Member]

Certificates of Deposit [Member]

Certificates of Deposit [Member]

Certificates of Deposit [Member]

Corporate Debt Securities [Member]

Corporate Debt Securities [Member]

Corporate Debt Securities [Member]

Commercial Paper [Member]

Commercial Paper [Member]

Level 1 [Member]

Level 2 [Member]

Level 3 [Member]

Level 1 [Member]

Level 2 [Member]

Level 3 [Member]

Level 1 [Member]

Level 2 [Member]

Assets:

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Cash equivalents - money market funds

'

$47,740

'

'

'

'

'

'

'

$25,668

'

'

'

'

'

'

'

'

'

'

Cash equivalents

'

'

'

'

'

'

'

13,998

'

'

'

'

'

480

'

'

5,017

'

'

'

Investments

90,116

'

'

40,436

40,436

'

49,680

49,680

'

'

'

'

'

240

'

'

59,533

'

'

12,465

Liabilities:

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Derivative liability

'

'

'

'

'

'

'

'

'

'

'

$196

'

'

'

'

'

'

'

'

Fair_Value_of_Financial_Instru3

Fair Value of Financial Instruments - Additional Information (Detail) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2013
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'
Marketable securities with an original maturity	'greater than three months
Gross proceeds from qualified equity financing	$4
Impairment charge of IPR&D	0.8
Principal amount	125
Maximum [Member]	'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'
Liquid investments with an original maturity	'3 months
Fair Value [Member] | Level 2 [Member]	'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'
Notes	127.5
Fair Value [Member] | Level 3 [Member] | Hercules [Member]	'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'
Notes	39.5
Carrying Value [Member] | Level 2 [Member]	'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'
Notes	125
Carrying Value [Member] | Level 3 [Member] | Hercules [Member]	'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'
Notes	$40.30

Fair_Value_of_Financial_Instru4

Fair Value of Financial Instruments - Schedule of Fair Value of the Liabilities Categorized as Level 3 Instruments (Detail) (USD $)	12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2013	Dec. 31, 2012
Convertible Preferred Stock Warrants [Member]	'	'
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]	'	'
Balance at the beginning of the period	'	$1,516
Unrealized gain included in other income (expense)	'	-587
Reclassification to common stock warrants	'	-929
Balance at the end of the period	'	'
Derivative Liability [Member]	'	'
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]	'	'
Balance at the beginning of the period	196	'
Portion of convertible note allocated to derivative	35	196
Remeasurement of derivative liability upon conversion of underlying note	-231	'
Balance at the end of the period	'	$196

Fair_Value_of_Financial_Instru5

Fair Value of Financial Instruments - Schedule of Quantitative Information Associated with the Fair Value Measurement of the Company's Non-Recurring Level 3 Inputs (Detail) (Level 3 [Member], USD $)	0 Months Ended
	Aug. 31, 2013
In Process Research and Development [Member]	'
Fair Value Inputs, Assets, Quantitative Information [Line Items]	'
IPR&D asset	'
Non-recurring [Member] | Income Approach - Probability Weighted Discounted Cash Flow Analysis [Member]	'
Fair Value Inputs, Assets, Quantitative Information [Line Items]	'
Discount rate (as a percent)	25.70%

Consolidated_Subsidiaries_Addi

Consolidated Subsidiaries - Additional Information (Detail) (USD $)	12 Months Ended
	Dec. 31, 2013	Feb. 28, 2013	Dec. 31, 2012
Consolidation, Less than Wholly Owned Subsidiary, Parent Ownership Interest, Effects of Changes, Net [Line Items]	'	'	'
Aggregate principal amounts	$125,000,000	'	'
Outstanding borrowings and accrued interest	2,603,000	'	'
Non-controlling interest	337,000	'	97,000
Total assets of consolidated subsidiaries	192,417,000	'	148,974,000
Total liabilities of consolidated subsidiaries	235,545,000	'	155,394,000
Silver Creek [Member]	'	'	'
Consolidation, Less than Wholly Owned Subsidiary, Parent Ownership Interest, Effects of Changes, Net [Line Items]	'	'	'
Non-controlling interest increased	800,000	'	'
Ownership interest (as a percent)	64.00%	'	74.00%
Non-controlling interest	337,000	'	97,000
Total assets of consolidated subsidiaries	1,000,000	'	2,200,000
Total liabilities of consolidated subsidiaries	100,000	'	1,800,000
Silver Creek [Member] | Silver Creek Convertible Note [Member]	'	'	'
Consolidation, Less than Wholly Owned Subsidiary, Parent Ownership Interest, Effects of Changes, Net [Line Items]	'	'	'
Aggregate principal amounts	600,000	300,000	1,600,000
Outstanding borrowings and accrued interest	2,600,000	'	'
Preferred stock value, per share	$1	'	'
Silver Creek [Member] | Series A Preferred Stock [Member]	'	'	'
Consolidation, Less than Wholly Owned Subsidiary, Parent Ownership Interest, Effects of Changes, Net [Line Items]	'	'	'
Shares owned by employees and directors of the parent company (as a percent)	7.00%	'	6.00%

Goodwill_and_Intangible_Assets2

Goodwill and Intangible Assets, Net - Additional Information (Detail) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Business Acquisition [Line Items]	'	'	'
Impairment charge	$0	$0	$0
Hermes Bio Sciences Inc [Member]	'	'	'
Business Acquisition [Line Items]	'	'	'
Acquired IPR&D recognized in a business combination	7	'	'
Amortization period	'10 years	'	'
Impairment charge	0.8	'	'
Hermes Bio Sciences Inc [Member] | Core Nano Carrier Technology [Member]	'	'	'
Business Acquisition [Line Items]	'	'	'
Acquired intangible assets	3.2	'	'
Hermes Bio Sciences Inc [Member] | Antibody Targeted Nanotherapeutic Program with Chemotherapy Drug [Member]	'	'	'
Business Acquisition [Line Items]	'	'	'
Acquired IPR&D recognized in a business combination	2.8	'	'
Hermes Bio Sciences Inc [Member] | Nanotherapeutic Program with Chemotherapy Drug [Member]	'	'	'
Business Acquisition [Line Items]	'	'	'
Acquired IPR&D recognized in a business combination	3.4	'	'
Hermes Bio Sciences Inc [Member] | Other Inprocess Research and Development Program [Member]	'	'	'
Business Acquisition [Line Items]	'	'	'
Acquired IPR&D recognized in a business combination	$0.80	'	'

Goodwill_and_Intangible_Assets3

Goodwill and Intangible Assets, Net - Schedule of Changes in the Carrying Value of Goodwill, IPR&D and Intangible Assets (Detail) (USD $)	1 Months Ended	3 Months Ended	12 Months Ended
In Thousands, unless otherwise specified	Aug. 31, 2013	Sep. 30, 2013	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Goodwill And Intangible Assets Disclosure [Abstract]	'	'	'	'	'	'
Intangible assets	'	'	$1,845	$2,165	$2,485	$2,805
IPR&D	'	'	6,200	7,010	7,010	7,010
Goodwill	'	'	3,605	3,605	3,605	3,605
Amortization	'	'	-320	-320	-320	'
Impairment	($810,000)	$0	($810)	'	'	'

Goodwill_and_Intangible_Assets4

Goodwill and Intangible Assets, Net - Schedule of Expected Amortization Expense for Intangible Assets for the Next Five-Year Period (Detail) (USD $)	Dec. 31, 2013
In Thousands, unless otherwise specified
Goodwill And Intangible Assets Disclosure [Abstract]	'
2014	$320
2015	320
2016	320
2017	320
2018	$320

Property_and_Equipment_Net_Sch

Property and Equipment, Net - Schedule of Property and Equipment (Detail) (USD $)	Dec. 31, 2013	Dec. 31, 2012
In Thousands, unless otherwise specified
Property, Plant and Equipment [Line Items]	'	'
Property and equipment, gross	$34,913	$25,266
Less: Accumulated depreciation and amortization	-21,549	-18,969
Property and equipment, net	13,364	6,297
Lab Equipment [Member]	'	'
Property, Plant and Equipment [Line Items]	'	'
Property and equipment, gross	13,714	12,616
IT Equipment [Member]	'	'
Property, Plant and Equipment [Line Items]	'	'
Property and equipment, gross	2,701	2,346
Leasehold Improvements [Member]	'	'
Property, Plant and Equipment [Line Items]	'	'
Property and equipment, gross	10,523	8,200
Furniture and Fixtures [Member]	'	'
Property, Plant and Equipment [Line Items]	'	'
Property and equipment, gross	340	330
Construction in Process [Member]	'	'
Property, Plant and Equipment [Line Items]	'	'
Property and equipment, gross	$7,635	$1,774

Property_and_Equipment_Net_Add

Property and Equipment, Net - Additional Information (Detail) (USD $)	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Property Plant And Equipment Useful Life And Values [Abstract]	'	'	'
Depreciation expense	$2,800,000	$3,500,000	$5,000,000
Capitalized interest cost	'	'	0
Gross amount of fully depreciated fixed assets disposed	210,000	671,000	0
Recognized impairment charges	$0	$0	$0

Accounts_Payable_Accrued_Expen

Accounts Payable, Accrued Expenses and Other - Schedule of Accounts Payable, Accrued Expenses and Other (Detail) (USD $)	Dec. 31, 2013	Dec. 31, 2012
In Thousands, unless otherwise specified
Accounts Payable Accrued Expenses And Other [Line Items]	'	'
Accounts payable	$1,889	$283
Accrued goods and services	26,031	17,615
Accrued payroll and related benefits	7,255	5,853
Accrued interest	2,926	306
Accrued dividends payable	25	28
Deferred tax incentives	688	512
Derivative liability	'	196
Total accounts payable, accrued expenses and other	38,814	25,644
Sanofi [Member] | License and Collaboration Agreements [Member]	'	'
Accounts Payable Accrued Expenses And Other [Line Items]	'	'
Other contractual liability	'	$851

Borrowings_Schedule_of_Future_

Borrowings - Schedule of Future Minimum Payments under the Loans Payable (Detail) (USD $)	Dec. 31, 2013	Dec. 31, 2012
In Thousands, unless otherwise specified
Debt Instrument [Line Items]	'	'
Loans payable, net of current portion	$103,427	$37,482
4.50% Convertible Senior Notes [Member]	'	'
Debt Instrument [Line Items]	'	'
2014	5,594	'
2015	5,625	'
2016	5,625	'
2017	5,625	'
2018 and thereafter	141,875	'
Loans payable, gross	164,344	'
Less interest	-39,344	'
Less unamortized discount	-52,422	'
Loans payable, net of current portion	72,578	'
Loans Payable [Member] | Loan and Security Agreement [Member]	'	'
Debt Instrument [Line Items]	'	'
2014	12,353	'
2015	18,130	'
2016	17,916	'
Loans payable, gross	48,399	'
Less interest	-7,199	'
Less unamortized discount	-2,103	'
Less current portion	-8,248	'
Loans payable, net of current portion	$30,849	'

Borrowings_Additional_Informat

Borrowings - Additional Information (Detail) (USD $)

1 Months Ended

12 Months Ended

0 Months Ended

12 Months Ended

0 Months Ended

1 Months Ended

0 Months Ended

1 Months Ended

12 Months Ended

0 Months Ended

0 Months Ended

1 Months Ended

2 Months Ended

12 Months Ended

1 Months Ended

Jul. 31, 2013

Dec. 31, 2013

Nov. 30, 2012

Dec. 31, 2013

Dec. 31, 2012

Nov. 30, 2012

Dec. 31, 2013

Dec. 31, 2012

Nov. 30, 2012

Nov. 30, 2012

Jul. 31, 2013

Jul. 31, 2013

Jul. 31, 2013

Jul. 31, 2013

Dec. 31, 2013

Jul. 31, 2013

Jul. 31, 2013

Jul. 31, 2013

Jul. 31, 2013

Dec. 31, 2013

Dec. 31, 2013

Feb. 28, 2013

Dec. 31, 2012

Dec. 31, 2013

Feb. 28, 2013

Dec. 31, 2012

Feb. 28, 2013

Dec. 31, 2013

Dec. 31, 2012

Dec. 31, 2012

Silver Creek [Member]

Silver Creek [Member]

Loan and Security Agreement [Member]

Loan and Security Agreement [Member]

Loan and Security Agreement [Member]

Loan and Security Agreement [Member]

Second Alternative Interest Rate [Member]

Minimum [Member]

Minimum [Member]

4.50% Convertible Senior Notes [Member]

4.50% Convertible Senior Notes [Member]

4.50% Convertible Senior Notes [Member]

4.50% Convertible Senior Notes [Member]

4.50% Convertible Senior Notes [Member]

4.50% Convertible Senior Notes [Member]

4.50% Convertible Senior Notes [Member]

Silver Creek Convertible Note [Member]

Silver Creek Convertible Note [Member]

Silver Creek Convertible Note [Member]

Silver Creek Convertible Note [Member]

Silver Creek Convertible Note [Member]

Silver Creek Convertible Note [Member]

Silver Creek Convertible Note [Member]

Silver Creek Convertible Note [Member]

Silver Creek Convertible Note [Member]

Silver Creek Convertible Note [Member]

Silver Creek Convertible Note [Member]

Hercules [Member]

Loan and Security Agreement [Member]

Loan and Security Agreement [Member]

Conversion Terms, Fundamental Changes [Member]

Conversion Terms, Event of Default [Member]

Minimum [Member]

Maximum [Member]

Silver Creek [Member]

Silver Creek [Member]

Silver Creek [Member]

Silver Creek [Member]

Silver Creek [Member]

Silver Creek [Member]

Silver Creek [Member]

Silver Creek [Member]

Silver Creek [Member]

Minimum [Member]

Silver Creek [Member]

Schedule Of Debt Instruments [Line Items]

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Aggregate principal amounts

'

'

'

'

'

'

'

'

'

'

'

'

'

$125,000,000

'

'

'

'

'

'

$600,000

$300,000

$1,600,000

$2,500,000

$2,500,000

$2,500,000

$300,000

$600,000

$1,600,000

'

Proceeds from issuance of convertible senior notes, net

'

'

'

'

'

'

'

'

'

'

'

'

'

120,600,000

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Interest rate (as a percent)

'

4.50%

'

'

'

'

'

'

'

'

'

'

4.50%

4.50%

4.50%

'

'

'

'

'

'

'

6.00%

'

'

6.00%

'

'

6.00%

'

Number of days within 30 consecutive trading days in which the closing price of the entity's common stock must exceed the conversion price for the notes to be convertible

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'20 days

'

'

'

'

'

'

'

'

'

'

'

'

Number of consecutive trading days during which the closing price of the entity's common stock must exceed the conversion price for at least 20 days in order for the notes to be convertible

'

'

'

'

'

'

'

'

'

'

'

'

'30 days

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Convertibility of debt, closing price of stock test, percentage of stock price to conversion price that must be exceeded

'

'

'

'

'

'

'

'

'

'

'

'

130.00%

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Number of consecutive business days immediately after any five consecutive trading day period during the note measurement period

'

'

'

'

'

'

'

'

'

'

'

'

'5 days

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Number of consecutive trading days before five consecutive business days during the note measurement period

'

'

'

'

'

'

'

'

'

'

'

'

'5 days

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Conversion ratio, principal amount

'

'

'

'

'

'

'

'

'

'

'

'

1,000

1,000

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Convertibility of debt, trading price of debt test, percentage of closing price of stock used in calculation

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

98.00%

'

'

'

'

'

'

'

'

'

'

'

Conversion rate of common stock shares per $1,000 principal amount

'

'

'

'

'

'

'

'

'

'

'

'

160

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Initial conversion price of shares (in dollars per share)

'

'

'

'

'

'

'

'

'

'

'

'

$6.25

$6.25

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Premium over public offering price (as a percent)

'

'

'

'

'

'

'

'

'

'

'

'

25.00%

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Public offering price (in dollars per share)

$5

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Redemption price as percentage of principal amount of notes plus accrued and unpaid interest

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

100.00%

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Minimum percentage of aggregate principal amount held by bondholders to declare notes due and payable

'

'

'

'

'

'

'

'

'

'

'

'

25.00%

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Percentage of principal amount due and payable upon event of default

'

'

'

'

'

'

'

'

'

'

'

'

100.00%

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Percentage of principal amount due and payable upon event of default arising out of certain bankruptcy events

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

100.00%

'

'

'

'

'

'

'

'

'

'

'

'

'

Effective interest rate used to compute initial fair value (as a percent)

'

'

'

'

'

'

'

'

'

'

'

'

15.00%

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Fair value of indebtedness

'

'

'

'

'

'

'

'

'

'

'

'

71,200,000

71,200,000

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Embedded conversion option

'

'

'

'

'

'

'

'

'

'

'

'

53,800,000

53,800,000

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Underwriting discounts and commissions and offering expenses

'

'

'

'

'

'

'

'

'

'

'

'

4,400,000

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Debt issuance costs allocated to unamortized debt discount

'

'

'

'

'

'

'

'

'

'

'

'

2,500,000

2,500,000

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Debt issuance costs attributable to embedded conversion option

'

'

'

'

'

'

'

'

'

'

'

'

1,900,000

1,900,000

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Interest expense

'

'

'

'

'

'

4,900,000

600,000

'

'

'

'

'

'

6,200,000

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Aggregate principal amount of loans received

'

125,000,000

'

'

'

'

'

'

40,000,000

'

'

'

'

'

125,000,000

'

'

'

'

'

600,000

300,000

1,600,000

600,000

300,000

1,600,000

300,000

600,000

1,600,000

'

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

'

'

'

'

'

'

'

'

'

'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%

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Percentage points subtracted from alternative interest rate

'

'

'

'

'

'

'

'

'

5.25%

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Maximum interest rate on term loan (as a percent)

'

'

'

'

'

'

'

'

'

12.55%

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Discount on loans related to costs reimbursed by the entity

'

'

39,600,000

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Period of interest-only payment

'

'

'

'

'

'

'12 months

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

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 payment

'

'

'

'

'

'

'6 months

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Shares of common stock sold to public

5,750,000

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Net proceeds from underwritten public offering and issuance of debt

'

'

'

'

'

'

'

'

'

'

75,000,000

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

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

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Discount on loan recorded as a non-current liability to be paid upon full repayment or maturity of the loans

'

'

'

'

'

'

1,200,000

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Less unamortized discount

'

'

'

'

'

'

'

'

'

'

'

'

'

'

52,422,000

'

'

'

'

1,600,000

'

'

'

'

'

'

'

'

'

'

Amount of gross proceeds from qualifying equity financing in which holders would automatically convert into the next qualifying equity financing at a 25% discount

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

4,000,000

Discount on automatic conversion into the next qualifying equity financing (as a percent)

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

25.00%

'

'

25.00%

'

'

25.00%

'

Derivative liability

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

0

'

196,000

0

'

196,000

'

0

196,000

'

Accrued interest expense

'

'

'

100,000

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

100,000

'

'

100,000

'

'

'

100,000

'

'

Ownership interest (as a percent)

'

'

'

64.00%

74.00%

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Non-controlling interest

'

'

'

$800,000

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Stock_Warrants_Schedule_of_Com

Stock Warrants - Schedule of Common and Convertible Preferred Stock Warrant Activity of the Company (Detail) (USD $)	12 Months Ended
In Thousands, except Per Share data, unless otherwise specified	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Common Stock Warrant [Member]	'	'	'
Class of Warrant or Right [Line Items]	'	'	'
Beginning Balance	2,842	2,640	2,937
Conversion (in shares)	'	302	'
Conversion (per share)	'	$3.50	'
Expired (in shares)	'	'	-1
Exercised (in shares)	-65	-100	-296
Ending Balance	2,777	2,842	2,640
Beginning Balance	3.05	2.98	2.93
Expired (per share)	'	'	$2.47
Exercised (per share)	$2.82	$2.63	$2.46
Ending Balance	3.05	3.05	2.98
Convertible Preferred Stock Warrants [Member]	'	'	'
Class of Warrant or Right [Line Items]	'	'	'
Beginning Balance	'	302	306
Conversion (in shares)	'	-302	'
Conversion (per share)	'	$3.50	'
Exercised (in shares)	'	'	-4
Ending Balance	'	'	302
Beginning Balance	'	3.5	3.48
Exercised (per share)	'	'	$1.89
Ending Balance	'	'	3.5

Stock_Warrants_Additional_Info

Stock Warrants - Additional Information (Detail) (Common Stock Warrant [Member])	Dec. 31, 2013	Dec. 31, 2012
Common Stock Warrant [Member]	'	'
Class of Warrant or Right [Line Items]	'	'
Number of warrants cashless exercised	65,000	100,000
Number of common stock issued as a result of cashless exercise of warrants	35,000	71,000

Common_Stock_Additional_Inform

Common Stock - Additional Information (Detail) (USD $)	1 Months Ended
In Millions, except Share data, unless otherwise specified	Jul. 31, 2013	Dec. 31, 2013	Dec. 31, 2012	Mar. 31, 2012
Equity [Abstract]	'	'	'	'
Shares of common stock sold in underwritten public offering	5,750,000	'	'	'
Public offering price	$5	'	'	'
Net proceeds from sale of common stock and convertible senior notes	$26.70	'	'	'
Common stock, authorized shares	'	200,000,000	200,000,000	200,000,000
Common stock, par value	'	$0.01	$0.01	$0.01
Common stock, issued shares	'	102,523,000	95,825,000	'
Common stock, outstanding shares	'	102,523,000	95,825,000	'

Common_Stock_Schedule_of_Share

Common Stock - Schedule of Shares Reserved for Future Issuance (Detail)	Dec. 31, 2013	Dec. 31, 2012
In Thousands, unless otherwise specified
Equity [Abstract]	'	'
Common stock warrants	2,777	2,842
Options to purchase common stock	20,107	18,066
Conversion of the Notes	25,000	'

StockBased_Compensation_Additi

Stock-Based Compensation - Additional Information (Detail) (USD $)

12 Months Ended

1 Months Ended

12 Months Ended

In Millions, except Share data, unless otherwise specified

Dec. 31, 2013

Dec. 31, 2012

Dec. 31, 2011

Dec. 31, 2013

Dec. 31, 2012

Dec. 31, 2011

Dec. 31, 2013

Dec. 31, 2013

Dec. 31, 2012

Dec. 31, 2011

Feb. 28, 2013

Dec. 31, 2013

Dec. 31, 2013

Stock Incentive Plan 2008 Member And Stock Incentive Plan 2011 [Member]

Stock Incentive Plan 2008 Member And Stock Incentive Plan 2011 [Member]

Stock Incentive Plan 2008 Member And Stock Incentive Plan 2011 [Member]

Stock Incentive Plan 2008 Member And Stock Incentive Plan 2011 [Member]

Stock Incentive Plan 2008 Member And Stock Incentive Plan 2011 [Member]

Stock Incentive Plan 2008 Member And Stock Incentive Plan 2011 [Member]

Stock Incentive Plan 2008 Member And Stock Incentive Plan 2011 [Member]

Stock Incentive Plan 2011 [Member]

Stock Incentive Plan 2011 [Member]

Stock Incentive Plan 2011 [Member]

Employee [Member]

Non Employee [Member]

Non Employee [Member]

Non Employee [Member]

Director [Member]

Maximum [Member]

Maximum [Member]

Maximum [Member]

Share-based Compensation Arrangement by Share-based Payment Award [Line Items]

'

'

'

'

'

'

'

'

'

'

'

'

'

Additional common stock available for issuance (in shares)

'

'

'

'

'

'

'

'

'

'

3,400,000

3,500,000

'

Increase in Additional common stock available for issuance (in shares)

'

'

'

'

'

'

'

'

'

'

'

4,300,000

'

Common stock available for issuance (in shares)

'

'

'

'

'

'

'

'

'

'

'

1,700,000

'

Stock options issued (in shares)

3,286

'

'

3,300,000

3,300,000

2,300,000

'

100,000

100,000

100,000

'

'

'

Vesting period

'

'

'

'

'

'

'3 years

'

'

'

'

'

'1 year

Aggregate intrinsic value of options exercised

$2.70

$13.70

$1.40

'

'

'

'

'

'

'

'

'

'

Unrecognized compensation cost related to nonvested stock awards

$15.90

'

'

'

'

'

'

'

'

'

'

'

'

Weighted average period over which unrecognized compensation cost is expected to be recognized

'1 year 9 months 18 days

'

'

'

'

'

'

'

'

'

'

'

'

StockBased_Compensation_Schedu

Stock-Based Compensation - Schedule of Recognized Stock-Based Compensation Expense (Detail) (USD $)	12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]	'	'	'
Stock-based compensation expense for employee awards	$10,762	$6,744	$6,472
Stock-based compensation expense for non-employee awards	-29	145	480
Total stock-based compensation expense	10,733	6,889	6,952
Research and Development Expense [Member]	'	'	'
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]	'	'	'
Stock-based compensation expense for employee awards	5,954	4,234	3,597
General and Administrative Expense [Member]	'	'	'
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]	'	'	'
Stock-based compensation expense for employee awards	$4,808	$2,510	$2,875

StockBased_Compensation_Schedu1

Stock-Based Compensation - Schedule of Assumptions Used to Calculate Fair Value of Options Granted to Employees (Detail) (Options to Purchase Common Stock [Member])	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]	'	'	'
Expected dividend yield (as a percent)	0.00%	0.00%	0.00%
Minimum [Member]	'	'	'
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]	'	'	'
Risk-free interest rate (as a percent)	0.10%	0.70%	1.30%
Expected term	'5 years 3 months 18 days	'5 years	'5 years
Expected volatility (as a percent)	67.00%	66.00%	71.00%
Maximum [Member]	'	'	'
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]	'	'	'
Risk-free interest rate (as a percent)	1.90%	1.10%	2.50%
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)	70.00%	72.00%	73.00%

StockBased_Compensation_Summar

Stock-Based Compensation - Summary of Stock Option Activity (Detail) (USD $)	12 Months Ended
In Thousands, except Share data, unless otherwise specified	Dec. 31, 2013	Dec. 31, 2012
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]	'	'
Outstanding at the beginning of the period (in shares)	18,066	'
Granted (in shares)	3,286	'
Exercised (in shares)	-913	'
Cancelled (in shares)	-332	'
Outstanding at the end of the period (in shares)	20,107	18,066
Number of shares vested and expected to vest	19,829	'
Number of stock option exercisable	15,672	'
Outstanding at the beginning of the period (in dollars per share)	$3.50	'
Granted (in dollars per share)	$6.04	'
Exercised (in dollars per share)	$2.23	'
Cancelled (in dollars per share)	$6.02	'
Outstanding at the end of the period (in dollars per share)	$3.93	$3.50
Weighted average exercise price of shares vested and expected to vest (in dollars per share)	$3.89	'
Weighted average exercise price for exercisable option (in dollars per share)	$3.19	'
Outstanding at the beginning of the period	'6 years 1 month 10 days	'6 years 6 months 15 days
Vested and expected to vest	'6 years 26 days	'
Exercisable	'5 years 3 months 29 days	'
Outstanding at the beginning of the period (in dollars)	$51,486	'
Outstanding at the end of the period (in dollars)	38,348	51,486
Shares vested and expected to vest (in dollars)	38,307	'
Options exercisable (in dollars)	$37,861	'

Income_Taxes_Additional_Inform

Income Taxes - Additional Information (Detail) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2013
Operating Loss Carryforwards [Line Items]	'
Convertible senior notes, percentage	4.50%
Federal and state net operating loss carryforwards relate to deductions for stock option compensation	$11.60
Federal Orphan Drug Credits	14.4
Federal [Member]	'
Operating Loss Carryforwards [Line Items]	'
NOL	304.2
Federal [Member] | Research and Development [Member]	'
Operating Loss Carryforwards [Line Items]	'
Amount of tax credit carryforward	18.6
State [Member]	'
Operating Loss Carryforwards [Line Items]	'
NOL	230.7
State [Member] | Research and Development [Member]	'
Operating Loss Carryforwards [Line Items]	'
Amount of tax credit carryforward	6.3
State [Member] | Investment [Member]	'
Operating Loss Carryforwards [Line Items]	'
Amount of tax credit carryforward	$0.30

Income_Taxes_Schedule_of_Recon

Income Taxes - Schedule of Reconciliation of the Company's Effective Tax Rate to the Statutory Federal Income Tax Rate (Detail)	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Income Tax Disclosure [Abstract]	'	'	'
Federal income tax at statutory federal rate	35.00%	35.00%	35.00%
State taxes	4.30%	4.50%	4.20%
Permanent differences	-2.00%	-0.20%	-0.40%
Stock-based compensation	-0.60%	-0.30%	-1.20%
Tax credits	12.40%	1.10%	3.90%
Foreign rate differential	-2.80%	-4.10%	-4.40%
Other	-1.50%	-0.50%	-0.80%
Change in valuation allowance	-44.80%	-35.50%	-36.30%
Effective income tax rate	'	'	'

Income_Taxes_Schedule_of_Tempo

Income Taxes - Schedule of Temporary Differences That Give Rise to Significant Net Deferred Tax Assets (Detail) (USD $)	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
In Thousands, unless otherwise specified
Deferred tax assets	'	'	'	'
Net operating losses	$113,843	$77,806	'	'
Capitalized research and development expenses	32,771	40,083	'	'
Credit carryforwards	37,304	14,398	'	'
Depreciation	2,735	2,931	'	'
Deferred compensation	8,249	5,068	'	'
Accrued expenses	841	1,184	'	'
Deferred revenues	28,381	29,936	'	'
Other temporary differences	5,885	1,934	'	'
Total gross deferred tax asset	230,009	173,340	'	'
Valuation allowance	-207,304	-169,651	-132,683	-103,881
Net deferred tax asset	22,705	3,689	'	'
Deferred tax liabilities	'	'	'	'
Intangible assets	-3,234	-3,689	'	'
Debt discount	-19,471	'	'	'
Net deferred taxes	'	'	'	'

Income_Taxes_Schedule_of_Chang

Income Taxes - Schedule of Changes in the Valuation Allowance (Detail) (USD $)	12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Income Tax Disclosure [Abstract]	'	'	'
Balance at beginning of period	$169,651	$132,683	$103,881
Additions	37,653	36,968	28,802
Deductions	'	'	'
Balance at end of period	$207,304	$169,651	$132,683

Commitments_and_Contingencies_1

Commitments and Contingencies - Additional Information (Detail) (USD $)	1 Months Ended	12 Months Ended
In Millions, unless otherwise specified	Aug. 31, 2012	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
	sqft
Operating Leased Assets [Line Items]	'	'	'	'
Total rent expense	'	$5.50	$4.30	$3.20
Aggregate space that the entity will occupy under the amended lease agreement	109,000	'	'	'
Lease term of additional space	'7 years	'	'	'
Aggregate rent due over amended term of lease	'	35	'	'
Aggregate landlord reimbursable tenant improvements outstanding under the existing lease and the lease amendment	'	5.5	'	'
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	'	'	'
Maximum [Member]	'	'	'	'
Operating Leased Assets [Line Items]	'	'	'	'
Additional reimbursement for tenant improvements agreed by landlord per lease amendment	'	$8.20	'	'

Commitments_and_Contingencies_2

Commitments and Contingencies - Schedule of Future Minimum Lease Payments Under Noncancelable Operating Leases (Detail) (USD $)	Dec. 31, 2013
In Thousands, unless otherwise specified
Commitments And Contingencies Disclosure [Abstract]	'
2014	$5,095
2015	5,208
2016	5,354
2017	5,470
2018 and thereafter	$8,399

Related_Party_Transactions_Add

Related Party Transactions - Additional Information (Detail) (USD $)

Dec. 31, 2013

Dec. 31, 2013

Feb. 28, 2013

Dec. 31, 2012

Dec. 31, 2013

Feb. 28, 2013

Dec. 31, 2012

Feb. 28, 2013

Dec. 31, 2013

Dec. 31, 2012

Apr. 30, 2012

Dec. 31, 2013

Feb. 28, 2013

Dec. 31, 2012

In Millions, except Share data, unless otherwise specified

Silver Creek [Member]

Silver Creek Convertible Note [Member]

Silver Creek Convertible Note [Member]

Silver Creek Convertible Note [Member]

Silver Creek Convertible Note [Member]

Silver Creek Convertible Note [Member]

Silver Creek Convertible Note [Member]

Silver Creek Convertible Note [Member]

Silver Creek Convertible Note [Member]

Silver Creek Convertible Note [Member]

Sanofi [Member]

Directors, Officers, Scientific Advisory Board Members and Related Parties [Member]

Directors, Officers, Scientific Advisory Board Members and Related Parties [Member]

Directors, Officers, Scientific Advisory Board Members and Related Parties [Member]

Silver Creek [Member]

Silver Creek [Member]

Silver Creek [Member]

Silver Creek [Member]

Silver Creek [Member]

Silver Creek [Member]

Silver Creek [Member]

Silver Creek [Member]

Silver Creek [Member]

Silver Creek Convertible Note [Member]

Silver Creek Convertible Note [Member]

Silver Creek Convertible Note [Member]

Related Party Transaction [Line Items]

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Shares of common stock purchased by related party in connection with initial public offering

'

'

'

'

'

'

'

'

'

'

5,217,391

'

'

'

Debt issued

'

$0.60

$0.30

$1.60

$2.50

$2.50

$2.50

$0.30

$0.60

$1.60

'

$0.30

$0.30

$0.30

Notes and related accrued interest converted into shares of preferred stock

$0.10

$0.10

'

'

$0.10

'

'

'

$0.10

'

'

'

'

'

Retirement_Plan_Additional_Inf

Retirement Plan - Additional Information (Detail) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Schedule Of Sale Of Subsidiary [Abstract]	'	'	'
Company's contributions to 401(k) defined contribution savings plan	$0.70	$0.60	$0.50

Selected_Quarterly_Financial_D2

Selected Quarterly Financial Data - Schedule of Quarterly Financial Information (Detail) (USD $)	3 Months Ended								12 Months Ended
In Thousands, except Per Share data, unless otherwise specified	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Quarterly Financial Information Disclosure [Abstract]	'	'	'	'	'	'	'	'	'	'	'
Collaboration revenues	$7,823	$6,856	$18,452	$14,655	$14,191	$11,323	$12,063	$11,344	$47,786	$48,921	$34,215
Total operating expenses	36,065	42,780	47,560	41,921	38,719	35,197	32,368	35,379	168,326	141,663	115,084
Net loss	-32,348	-39,763	-30,251	-28,323	-24,893	-23,320	-20,139	-23,402	-130,685	-91,754	-79,676
Net loss attributable to Merrimack Pharmaceuticals, Inc.	($33,059)	($39,631)	($30,082)	($28,153)	($24,768)	($23,199)	($20,026)	($23,284)	($130,925)	($91,277)	($79,223)
Net loss per share available to common stockholders-basic and diluted	($0.33)	($0.39)	($0.31)	($0.29)	($0.26)	($0.25)	($0.22)	($2.14)	($1.32)	($1.28)	($7.67)

Subsequent_Events_Additional_I

Subsequent Events - Additional Information (Detail) (Subsequent Event [Member], USD $)	1 Months Ended
In Millions, except Share data, unless otherwise specified	Feb. 28, 2014
	Employees
Subsequent Event [Line Items]	'
Tax incentive awarded	$0.60
State research and development tax credits monetized	0.6
Number of incremental employees pledged to be hired	31
Hercules [Member]	'
Subsequent Event [Line Items]	'
warrants to purchase	302,143
common stock for proceeds	$1.10