Merrimack Pharmaceuticals (MACK) 10-K 27 Feb 15 2014 FY Annual report Financial data

Document_and_Entity_Informatio

Document and Entity Information (USD $)	12 Months Ended
	Dec. 31, 2014	Feb. 13, 2015	Jun. 30, 2014
Document And Entity Information [Abstract]
Document Type	10-K
Amendment Flag	FALSE
Document Period End Date	31-Dec-14
Document Fiscal Year Focus	2014
Document Fiscal Period Focus	FY
Trading Symbol	MACK
Entity Registrant Name	MERRIMACK PHARMACEUTICALS INC
Entity Central Index Key	1274792
Current Fiscal Year End Date	-19
Entity Well-known Seasoned Issuer	No
Entity Current Reporting Status	Yes
Entity Voluntary Filers	No
Entity Filer Category	Large Accelerated Filer
Entity Common Stock, Shares Outstanding		106,934,746
Entity Public Float			$707,644,419

Consolidated_Balance_Sheets

Consolidated Balance Sheets (USD $)	Dec. 31, 2014	Dec. 31, 2013
In Thousands, unless otherwise specified
Current assets:
Cash and cash equivalents	$35,688	$65,086
Available-for-sale securities	88,340	90,116
Restricted cash	101	101
Accounts receivable	3,313	5,857
Prepaid expenses and other current assets	4,654	5,484
Total current assets	132,096	166,644
Restricted cash	584	584
Property and equipment, net	14,502	13,364
Other assets	144	175
Intangible assets, net	1,525	1,845
In-process research and development	6,200	6,200
Goodwill	3,605	3,605
Total assets	158,656	192,417
Current liabilities:
Accounts payable, accrued expenses and other	37,236	38,814
Deferred revenues	59,275	9,336
Deferred rent	1,285	1,336
Long-term debt, current portion	13,346	8,248
Total current liabilities	111,142	57,734
Deferred revenues, net of current portion	35,682	66,139
Deferred rent, net of current portion	5,401	6,538
Deferred tax incentives, net of current portion	496	507
Long-term debt, net of current portion	106,806	103,427
Accrued interest	1,200	1,200
Total liabilities	260,727	235,545
Commitments and contingencies
Non-controlling interest	69	337
Stockholders' deficit:
Preferred stock, $0.01 par value: 10,000 shares authorized at December 31, 2014 and 2013, respectively; no shares issued or outstanding at December 31, 2014 or 2013
Common stock, $0.01 par value: 200,000 shares authorized at December 31, 2014 and 2013, respectively, 106,697 and 102,523 issued and outstanding at December 31, 2014 and 2013, respectively	1,067	1,025
Additional paid-in capital	552,037	527,779
Accumulated other comprehensive loss	-74	-24
Accumulated deficit	-655,170	-572,245
Total stockholders' deficit	-102,140	-43,465
Total liabilities, non-controlling interest and stockholders' deficit	$158,656	$192,417

Consolidated_Balance_Sheets_Pa

Consolidated Balance Sheets (Parenthetical) (USD $)	Dec. 31, 2014	Dec. 31, 2013
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	106,697,000	102,523,000
Common stock, shares outstanding	106,697,000	102,523,000

Consolidated_Statements_of_Com

Consolidated Statements of Comprehensive Loss (USD $)	12 Months Ended
In Thousands, except Per Share data, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Statement of Comprehensive Income [Abstract]
Collaboration revenues	$102,756	$47,786	$48,921
Operating expenses:
Research and development	138,495	147,139	125,858
General and administrative	30,517	21,187	15,805
Total operating expenses	169,012	168,326	141,663
Loss from operations	-66,256	-120,540	-92,742
Other income and expenses
Interest income	114	166	184
Interest expense	-18,230	-10,938	-553
Other, net	813	627	1,357
Net loss	-83,559	-130,685	-91,754
Less net income (loss) attributable to non-controlling interest	-268	240	-477
Net loss attributable to Merrimack Pharmaceuticals, Inc.	-83,291	-130,925	-91,277
Other comprehensive income (loss):
Unrealized loss on available-for-sale securities	-50	14	-38
Other comprehensive income (loss)	-50	14	-38
Comprehensive loss	($83,341)	($130,911)	($91,315)
Net loss per share available to common stockholders-basic and diluted	($0.80)	($1.32)	($1.28)
Weighted-average common shares used in computing net loss per share available to common stockholders-basic and diluted	104,410	98,919	72,831

Consolidated_Statements_of_Con

Consolidated Statements of Convertible Preferred Stock, Non-Controlling Interest and Stockholders' Deficit (USD $)	Total	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		Series B-G Convertible Preferred Stock [Member]		Series B-G Convertible Preferred Stock [Member]
Balance at Dec. 31, 2011	($290,490)	$268,225	$574		$118	$60,231		($350,839)
Balance (in shares) at Dec. 31, 2011		64,151
Balance (in shares) at Dec. 31, 2011					11,834
Exercise of stock options and common stock warrants	5,427				27	5,400
Exercise of stock options and common stock warrants (shares)					2,693
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			66,256
Initial public offering, net of issuance costs	98,081				150	97,931
Initial public offering, net of issuance costs (shares)					15,042
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
Loss attributable to non-controlling interest	477		-477					477
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					95,825
Conversion of Silver Creek convertible notes payable			796
Exercise of stock options and common stock warrants	2,036				9	2,027
Exercise of stock options and common stock warrants (shares)					948
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
Conversion option of convertible senior notes	51,875					51,875
Other comprehensive (loss) income	14						14
Loss attributable to non-controlling interest	556		-556					556
Net loss	-130,685							-130,685
Balance at Dec. 31, 2013	-43,465		337		1,025	527,779	-24	-572,245
Balance (in shares) at Dec. 31, 2013					102,523
Conversion of Silver Creek convertible notes payable				366
Exercise of stock options and common stock warrants	10,425				42	10,383
Exercise of stock options and common stock warrants (shares)					4,174
Stock-based compensation	13,197					13,197
Conversion of convertible preferred stock into common stock	678					678
Other comprehensive (loss) income	-50						-50
Loss attributable to non-controlling interest	634		-634					634
Net loss	-83,559							-83,559
Balance at Dec. 31, 2014	($102,140)		$69		$1,067	$552,037	($74)	($655,170)
Balance (in shares) at Dec. 31, 2014					106,697

Consolidated_Statements_of_Cas

Consolidated Statements of Cash Flows (USD $)	12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Cash flows from operating activities
Net loss	($83,559)	($130,685)	($91,754)
Adjustments to reconcile net loss to net cash used in operating activities
Non-cash interest expense	8,511	4,548	78
Depreciation and amortization	3,223	2,589	3,664
Stock-based compensation	13,197	10,733	6,889
Purchased premiums and interest on available-for-sale securities	858	-1,809	-2,354
Other non-cash items		579	-587
Changes in operating assets and liabilities:
Accounts receivable	2,587	3,410	-1,841
Prepaid expenses and other current assets	827	4,037	-2,477
Accounts payable, accrued expenses and other	-646	13,304	6,985
Deferred revenues	19,482	-4,989	-5,281
Deferred rent and tax incentives	712	2,076	7,892
Other assets and liabilities, net		1,032	-1,030
Net cash used in operating activities	-34,808	-95,175	-79,816
Cash flows from investing activities
Purchase of available-for-sale securities	-111,832	-112,923	-115,665
Proceeds from sales and maturities of available-for-sale securities	111,858	95,100	43,880
Purchase of property and equipment	-6,035	-9,857	-3,189
Assignment of restricted cash		-57	-628
Release of restricted cash			381
Other investing activities, net	-2	-2
Net cash used in investing activities	-6,011	-27,739	-75,221
Cash flows from financing activities
Proceeds from public offerings, net of offering costs		26,716	100,025
Proceeds from exercise of common stock and warrants	10,384	2,036	5,427
Proceeds from issuance of debt, net of issuance costs	1,044	121,537	41,128
Payments of dividends on Series B convertible preferred stock	-7	-3	-4,235
Other financing activities			-48
Net cash provided by financing activities	11,421	150,286	142,297
Net increase (decrease) in cash and cash equivalents	-29,398	27,372	-12,740
Cash and cash equivalents, beginning of period	65,086	37,714	50,454
Cash and cash equivalents, end of period	35,688	65,086	37,714
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
Property, plant and equipment in accounts payable and accrued expenses	704		412
Disposals of fully depreciated assets	1,603	210	671
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	1,044	2,603
Supplemental disclosure of cash flows
Cash paid for interest	$9,510	$3,915	$169

Nature_of_the_Business

Nature of the Business	12 Months Ended
	Dec. 31, 2014
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 for the treatment of cancer. The Company currently has six targeted therapeutic oncology candidates in clinical development (MM-398, MM-302, MM-121, MM-111, MM-151 and MM-141). The Company’s most advanced program is its investigational agent MM-398. The Company has initiated a New Drug Application submission with the U.S. Food and Drug Administration for MM-398 as a treatment for metastatic pancreatic cancer in combination with 5-fluorouracil and leucovorin in patients who have been previously treated with gemcitabine. The Company also has multiple product candidates in preclinical development and a discovery effort advancing additional candidate medicines. The Company also has 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 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.
	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, 2014
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 2011, and Merrimack Pharmaceuticals (Bermuda) Ltd., which was merged with and into the Company during the third quarter of 2014. 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, but may not be limited to, revenue recognition, including the estimated percentage of billable expenses in any particular budget period, periods of meaningful use of licensed products, estimated service periods and services to be completed under a collaboration, 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, valuation of convertible debt, tax valuation reserves and accrued expenses. 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 region.
	Cash, Cash Equivalents and Restricted Cash
	Cash and cash equivalents are short-term, highly liquid investments with original maturities of three months or less at the date of purchase. Investments qualifying as cash equivalents primarily consist of money market funds, commercial paper, corporate notes and bonds and certificates of deposit.
	Cash accounts with any type of restriction are classified as restricted cash. If restrictions are expected to be lifted in the next twelve months, the restricted cash account is classified as current. As of both December 31, 2014 and 2013, the Company recorded restricted cash of $685,000, which was 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, 2014, 2013 or 2012.
	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, 2014 and 2013, Sanofi represented approximately 90% and 98% of collaboration revenues, respectively. As of December 31, 2014 and 2013, Sanofi represented approximately 50% and 98% of accounts receivable, respectively. Baxter International Inc., Baxter Healthcare Corporation and Baxter Healthcare SA (collectively, “Baxter”) represented approximately 49% of accounts receivable as of December 31, 2014.
	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 a component of mezzanine equity.
	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 license and collaboration agreement with Baxter in September 2014, which was evaluated under the accounting guidance on revenue recognition for multiple element arrangements. The Company determined that the obligations under this agreement represent a single unit of accounting and that the agreement represents a services agreement. As a result, the Company has estimated the level of effort expected to be completed as a result of providing the identified deliverables and will recognize revenue related to the agreement based on proportional performance as effort is completed over the expected services period.
	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.
	Whenever the Company determines that an arrangement should be accounted for as a single unit of accounting, it determines the period over which the performance obligations would be performed and revenue would be recognized. If the Company cannot reasonably estimate the timing and the level of effort to complete its performance obligations under the arrangement, then revenue under the arrangement is recognized on a straight-line basis over the period the Company expects to complete its performance obligations.
	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 divided by 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.
	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, research-related 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 services have 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 to 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 and expense.
	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. For the years ended December 31, 2014, 2013 and 2012, 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, 2014, 2013 and 2012, the Company recognized $0.2 million of benefit in other income in each period.
	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 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, 2014 and 2013, the Company recognized $0.1 million of benefit in other income in each period.
	In May 2014, the MLSC awarded the Company an additional $0.6 million of tax incentives under its Life Science Tax Incentive Program, which allows 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. The Company deferred and is amortizing 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 was achieved. For the year ended December 31, 2014, the Company recognized $0.1 million of benefit in other income.
	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 2014. The Company’s next annual impairment evaluation will be made in the third quarter of 2015 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, further testing is not 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. It was determined that it was not more likely than not that an impairment existed as of the third quarter of 2014 and, therefore, impairment evaluations were not performed.
	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, 2014.
	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 during the year ended December 31, 2014.
	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 did not have a material impact on the Company’s consolidated financial statements.
In May 2014, the FASB issued guidance which supersedes all existing revenue recognition requirements, including most industry-specific guidance. The new standard requires a company to recognize revenue when it transfers goods or services to customers in an amount that reflects the consideration that the company expects to receive for those goods or services. This guidance will be effective prospectively for fiscal years, and interim periods within those years, beginning after December 15, 2016, and early adoption is not permitted. The Company is currently evaluating the potential impact that the adoption of this guidance and the related transition guidance may have on its consolidated financial statements.
In August 2014, the FASB issued guidance outlining management’s responsibility to perform interim and annual assessments of an entity’s ability to continue as a going concern within one year of the date the financial statements are issued and providing guidance on determining when and how to disclose going concern uncertainties in the financial statements. This guidance will be effective for annual and interim reporting periods ending after December 15, 2016, with early adoption permitted. The Company does not anticipate a material impact to its consolidated financial statements as a result of this change.

Marketable_Securities

Marketable Securities	12 Months Ended
	Dec. 31, 2014
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, 2014 consisted of the following:
		Amortized		Unrealized			Unrealized			Fair
		Cost		Gains			Losses			Value
		(in thousands)
	December 31, 2014:
	Commercial paper	$	6,493	$	—		$	(2	)	$	6,491
	Corporate debt securities		81,921		—			(72	)		81,849
	Total	$	88,414	$	—		$	(74	)	$	88,340
	The aggregate fair value of securities held by the Company in an unrealized loss position for less than 12 months as of December 31, 2014 was $88.4 million, representing 35 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, 2014 consisted of the following:
		Aggregate		Unrealized
		Fair Value		Losses
		(in thousands)
	December 31, 2014:
	Commercial paper	$	6,493	$	(2	)
	Corporate debt securities		81,921		(72	)
		$	88,414	$	(74	)
	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, 2014.

Net_Loss_Per_Common_Share

Net Loss Per Common Share	12 Months Ended
	Dec. 31, 2014
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)	2014			2013			2012
	Net Loss Per Share:
	Numerator:
	Net loss attributable to Merrimack Pharmaceuticals, Inc.	$	(83,291	)	$	(130,925	)	$	(91,277	)
	Plus: Unaccreted dividends on convertible preferred stock		—			—			(2,107	)
	Net loss available to common stockholders—basic and diluted		(83,291	)		(130,925	)		(93,384	)
	Denominator:
	Weighted-average common shares—basic and diluted		104,410			98,919			72,831
	Net loss per share available to common stockholders—basic and diluted	$	(0.80	)	$	(1.32	)	$	(1.28	)
	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, 2014, 2013 and 2012 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)	2014			2013			2012
	Common stock warrants		2,381			2,777			2,842
	Options to purchase common stock		19,567			20,107			18,066
	Conversion of the Notes		25,000			25,000			—

License_and_Collaboration_Agre

License and Collaboration Agreements	12 Months Ended
	Dec. 31, 2014
Organization, Consolidation and Presentation of Financial Statements [Abstract]
License and Collaboration Agreements	5. License and Collaboration Agreements
	Baxter
	On September 23, 2014, the Company and Baxter entered into a license and collaboration agreement (the “Baxter Agreement”) for the development and commercialization of MM-398 outside of the United States and Taiwan (the “Licensed Territory”). As part of the Baxter Agreement, the Company granted Baxter an exclusive, royalty-bearing right and license under the Company’s patent rights and know-how to develop and commercialize MM-398 in the Licensed Territory. Baxter is responsible for using commercially reasonable efforts to develop, obtain regulatory approvals for and, following regulatory approval, commercializing MM-398 in the Licensed Territory. A joint steering committee comprised of an equal number of representatives from each of Baxter and the Company is responsible for approving changes to the global development plan for MM-398, including all budgets, and overseeing the parties’ development and commercialization activities with respect to MM-398. Unless otherwise agreed, the Company will be responsible for conducting all clinical trials contemplated by the global development plan for MM-398 and manufacturing all clinical material needed for such trials.
	Under the terms of the Baxter Agreement, the Company received a $100.0 million upfront, non-refundable cash payment. In addition, the Company is eligible to receive from Baxter (i) up to an aggregate of $100.0 million upon the achievement of specified research and development milestones, (ii) up to an aggregate of $520.0 million upon the achievement of specified regulatory milestones and (iii) up to an aggregate of $250.0 million upon the achievement of specified sales milestones. Under the terms of the Baxter Agreement, the Company will bear up to the first $98.8 million of costs related to the development of MM-398 for pancreatic cancer patients who have not previously received gemcitabine; however, the Company expects most of these costs to be offset by payments received upon the achievement of clinical trial-related milestones. The Company and Baxter will share equally all other clinical trial costs contemplated by the global development plan. The Company is also entitled to tiered, escalating royalties ranging from sub-teen double-digits to low twenties percentages of net sales of MM-398 in the Licensed Territory.
	The Company and Baxter expect to enter into a commercial supply agreement pursuant to which the Company will supply MM-398 bulk drug substance to Baxter and, at Baxter’s option, may manage fill and finish activities to be conducted by a third party contract manufacturer for Baxter. Baxter also has the option to manufacture MM-398 itself, in which case the Company will perform a technology transfer of its manufacturing process to Baxter.
	Under the Baxter Agreement, the Company granted Baxter a right of first negotiation to obtain a license to develop and commercialize MM-111, MM-141 and MM-302 outside of the United States.
	If not terminated earlier by either party, the Baxter Agreement will expire upon expiration of all royalty and other payment obligations of Baxter under the Baxter Agreement. Either party may terminate the Baxter Agreement in the event of an uncured material breach by the other party. Baxter may also terminate the Baxter Agreement on a product-by-product, country-by-country or sub-territory-by-sub-territory basis or in its entirety, for its convenience, upon 180 days’ prior written notice. In addition, the Company may terminate the Baxter Agreement if Baxter challenges or supports any challenge of the Company’s licensed patent rights.
	At the inception of the collaboration, the Company identified the following deliverables as part of the Baxter Agreement: (i) license to develop and commercialize MM-398 in Baxter’s territories, (ii) discovery, research, development and manufacturing services required to complete ongoing clinical trials related to MM-398, (iii) discovery, research, development and manufacturing services needed to complete future clinical trials in further indications related to MM-398, (iv) the option to perform a technology transfer of the Company’s manufacturing process related to the production of MM-398 to Baxter and (v) participation on the joint steering committee.
	The Company concluded that none of the deliverables identified at the inception of the collaboration has standalone value from the other undelivered elements. As such, all deliverables represent a single unit of accounting.
	The Company has determined that the collaboration represents a services agreement and as such has estimated the level of effort expected to be completed as a result of providing the identified deliverables. The Company will recognize revenue from the non-refundable upfront payment, forecasted non-substantive milestone payments and estimated payments related to discovery, research, development and technology transfer services based on proportional performance as effort is completed over the expected services period, which is estimated to be primarily complete by December 31, 2019. The Company will periodically review and, if necessary, revise the estimated service period related to its collaboration with Baxter.
	Research, development and regulatory milestones that are considered substantive on the basis of the contingent nature of the milestone will be recognized as revenue in full in the period in which the associated milestone is achieved, assuming all other revenue recognition criteria are met. All sales milestones will be accounted for in the same manner as royalties and recorded as revenue upon achievement of the milestone, assuming all other revenue recognition criteria are met.
	During the year ended December 31, 2014, the Company recognized $10.5 million of revenue associated with these payment amounts. As of December 31, 2014, the Company had $91.2 million of deferred revenue related to the Baxter Agreement, $59.3 million of which is classified as current in the consolidated balance sheets based upon the Company’s estimate of revenue that will be recognized as a result of effort expected to be completed within the next twelve months. As of December 31, 2014, the Company had $1.6 million of unbilled accounts receivable outstanding with Baxter.
	Sanofi
	On September 30, 2009, the Company and Sanofi entered into a license and collaboration agreement (the “Sanofi Agreement”) for the development and commercialization of MM-121. The Sanofi Agreement became effective on November 10, 2009, and Sanofi paid the Company a nonrefundable, noncreditable upfront license fee of $60.0 million. On June 17, 2014, the Company and Sanofi agreed to terminate the Sanofi Agreement effective December 17, 2014. In connection with the agreement to terminate the Sanofi Agreement, among other things, Sanofi transferred ownership of the investigational new drug application for MM-121 back to the Company in July 2014, and the Company waived Sanofi’s obligation to reimburse the Company for MM-121 development costs incurred after the effective termination date. Effective upon the termination of the Sanofi Agreement, the Company will not be entitled to receive any additional fees, milestone payments or reimbursements from the collaboration.
	From the effective date of the Sanofi Agreement through December 31, 2014, the Company had received total milestone payments of $25.0 million pursuant to the Sanofi Agreement. Under the Sanofi Agreement, Sanofi was responsible for all MM-121 development and manufacturing costs. Sanofi reimbursed the Company for direct costs incurred in both development and manufacturing and compensated the Company for its internal development efforts based on a full time equivalent rate.
	The Company recognized cost reimbursements for MM-121 development services within the period they were incurred and billable. Billable expenses were identified during each specified budget period. For the year ended December 31, 2014, this specified budget period was determined to end December 17, 2014. In the event that total development services expense incurred and expected to be incurred during the same period exceeded the total contractually allowed billable amount for development services during that period, the Company recognized only a percentage of the development services incurred as revenue during that period. This percentage was 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.
	At the inception of the collaboration, the Company determined that the license, the right to future technology, back-up compounds, participation on steering committees and manufacturing services performance obligations comprising the Sanofi Agreement represented a single unit of accounting. As the Company could not reasonably estimate its level of effort over the collaboration, the Company recognized revenue from the upfront payment, milestone payment and manufacturing services payments using the contingency-adjusted performance model over the expected development period, which was initially estimated at 12 years from the effective date of the Sanofi Agreement.
	As a result of the Company and Sanofi agreeing to terminate the Sanofi Agreement, the development period was revised to end as of December 17, 2014. Accordingly, the balance of the deferred revenue remaining on April 1, 2014 was recognized prospectively on a straight-line basis over the remaining development period, ending on December 17, 2014, in accordance with current generally accepted principles on revenue recognition.
	During the years ended December 31, 2014, 2013 and 2012, the Company recognized revenue based on the following components of the Sanofi agreement:
		Years ended December 31,
	(in thousands)	2014		2013		2012
	Upfront payment	$	39,306	$	5,000	$	5,000
	Milestone payment		16,377		2,083		2,975
	Development services		18,904		36,283		36,905
	Manufacturing services and other		17,709		3,867		3,307
	Total	$	92,296	$	47,233	$	48,187
	The Company performed development services for which revenue was recognized under the Sanofi agreement in accordance with the specified budget period. During the year and specified budget periods ended December 31, 2013, the Company performed $10.1 million of development services in excess of recognized revenue. Of this amount, approximately $5.8 million was recognized as increased revenue in the year ended December 31, 2014 related to expenses incurred prior to December 31, 2013 upon the Company receiving budget approval for these overruns. During the year ended December 31, 2012, development services approximated recognized revenue.
	In July 2014, the Company agreed with Sanofi to purchase certain existing drug supply of MM-121. The estimated total cost of this drug supply is approximately $5.2 million. This drug supply is expected to be released in 2015 and is expected to supply the Company’s currently projected drug needs for MM-121 into the second half of 2016.
	As of December 31, 2014 and 2013, the Company maintained the following assets and liabilities related to the Sanofi agreement:
		December 31,
	(in thousands)	2014		2013
	Accounts receivable, billed	$	369	$	2,357
	Accounts receivable, unbilled		1,282		3,417
	Deferred revenues	$	—	$	73,392
	PharmaEngine, Inc.
	On May 5, 2011, the Company and PharmaEngine, Inc. (“PharmaEngine”) entered into an assignment, sublicense and collaboration agreement (the “PharmaEngine Agreement”) under which the Company reacquired rights in Europe and certain countries in Asia to MM-398. In exchange, the Company agreed to pay PharmaEngine a nonrefundable, noncreditable upfront payment of $10.0 million and up to an additional $80.0 million in aggregate development and regulatory milestones and $130.0 million in aggregate sales milestones. During the first quarter of 2012, the Company paid a development milestone of $5.0 million under the PharmaEngine Agreement 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. PharmaEngine is not responsible for any future development costs of MM-398 except those required specifically for regulatory approval in Taiwan.
	On September 22, 2014, the Company amended the PharmaEngine Agreement to redefine sublicense revenue and reduce the portion of sublicense revenue that the Company is required to pay to PharmaEngine. As a result of this amendment, the Company made a $7.0 million milestone payment to PharmaEngine in September 2014. Additionally, as a result of this amendment, a previously contingent $5.0 million milestone payment is now payable to PharmaEngine upon the earlier of either the U.S. Food and Drug Administration’s acceptance of a new drug application for MM-398 or April 30, 2015. Prior to the amendment of the PharmaEngine Agreement, this milestone payment was contingent upon the award of certain specified regulatory designations. These milestone payments were recognized as research and development expense during the year ended December 31, 2014.
	During the years ended December 31, 2014, 2013 and 2012, the Company recognized research and development expenses of $12.6 million, $1.5 million and $6.2 million, respectively, related to the PharmaEngine Agreement.
	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. In January 2015, we amended our agreement with Actavis to transfer certain responsibilities from us to Actavis in exchange for reducing the aggregate milestone payments that we are eligible to receive by $0.4 million.
	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 $3.8 million and $2.1 million as of December 31, 2014 and 2013, respectively.

Fair_Value_of_Financial_Instru

Fair Value of Financial Instruments

12 Months Ended

Dec. 31, 2014

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.

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, 2014 and 2013 and the input categories associated with those assets and liabilities:

As of December 31, 2014

Level 1

Level 2

Level 3

(in thousands)

Assets:

Cash equivalents—money market funds

$

33,199

$

—

$

—

Investments—commercial paper

—

6,491

—

Investments—corporate debt securities

—

81,849

—

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

—

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.

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 in the third quarter of 2013.

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

Discount rate

25.7

%

weighted discounted cash flow

analysis

Other Fair Value Measurements

The estimated fair value and carrying value of the $125.0 million aggregate principal amount of the Notes was $232.4 million and $125.0 million, respectively, as of December 31, 2014. 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 $40.6 million and $40.5 million, respectively, as of December 31, 2014. 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, 2014
Text Block [Abstract]
Consolidated Subsidiaries	7. Consolidated Subsidiaries
	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, these 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 as of December 31, 2013, the non-controlling interest in Silver Creek increased by $0.8 million. During the year ended December 31, 2014, Silver Creek issued convertible notes to various lenders in the aggregate principal amount of $1.0 million. As of December 31, 2014, these outstanding borrowings and an immaterial amount of related accrued interest 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 as of December 31, 2014, the non-controlling interest in Silver Creek increased by approximately $0.4 million. As of December 31, 2014 and 2013, the Company owned approximately 60% and 64% of the voting stock of Silver Creek, respectively, and recorded a non-controlling interest of approximately $0.1 million and $0.3 million, 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, 2014, the Company consolidated Silver Creek’s total assets and total liabilities of $0.3 million and $0.2 million, respectively. 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, 2014 and 2013, employees and directors of the Company owned approximately 6% and 7% 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 and merged with and into the Company during the third quarter of 2014.

Goodwill_and_Intangible_Assets

Goodwill and Intangible Assets, Net	12 Months Ended
	Dec. 31, 2014
Goodwill and Intangible Assets Disclosure [Abstract]
Goodwill and Intangible Assets, Net	8. Goodwill and Intangible Assets, Net
	As part of the acquisition of Hermes on October 6, 2009 (the “Acquisition Date”), 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, 2014 and 2013, 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, 2014 and 2013. 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, 2014, 2013 and 2012 were as follows:
	(in thousands)	Intangible			IPR&D			Goodwill
		assets
	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
	Amortization		(320	)		—			—
	Balance, December 31, 2014	$	1,525		$	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)
	2015	$	320
	2016		320
	2017		320
	2018		320
	2019	$	245

Property_and_Equipment_Net

Property and Equipment, Net	12 Months Ended
	Dec. 31, 2014
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)	2014			2013
	Lab equipment	$	16,214		$	13,714
	IT equipment		3,113			2,701
	Leasehold improvements		18,219			10,523
	Furniture and fixtures		624			340
	Construction in process		472			7,635
			38,642			34,913
	Less: Accumulated depreciation		(24,140	)		(21,549	)
		$	14,502		$	13,364
	Depreciation expense was $4.2 million, $2.8 million and $3.5 million for the years ended December 31, 2014, 2013 and 2012, respectively. Capitalized interest costs were immaterial for the years ended December 31, 2014, 2013 and 2012.
	During the years ended December 31, 2014, 2013 and 2012, the Company disposed of $1.6 million, $0.2 million and $0.7 million, respectively, of fully depreciated assets. There were no recognized impairment charges related to fixed assets in the years ended December 31, 2014, 2013 or 2012.

Accounts_Payables_Accrued_Expe

Accounts Payables, Accrued Expenses and Other	12 Months Ended
	Dec. 31, 2014
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, 2014 and 2013 consisted of the following:
		December 31,
	(in thousands)	2014		2013
	Accounts payable	$	2,510	$	1,889
	Accrued goods and services		17,481		7,958
	Accrued clinical trial costs		7,637		18,073
	Accrued payroll and related benefits		6,166		7,255
	Accrued interest		2,956		2,926
	Accrued dividends payable		19		25
	Deferred tax incentives		467		688
	Total accounts payable, accrued expenses and other	$	37,236	$	38,814

Borrowings

Borrowings	12 Months Ended
	Dec. 31, 2014
Debt Disclosure [Abstract]
Borrowings	11. Borrowings
	Future minimum payments under indebtedness agreements outstanding as of December 31, 2014 are as follows:
	Years Ending December 31:		4.50% Convertible			Loan
	(in thousands)		Senior Notes			Agreement
	2015		$	5,625		$	16,991
	2016			5,625			29,501
	2017			5,625			—
	2018			5,625			—
	2019 and thereafter			136,250
			$	158,750		$	46,492
	Less interest			(33,750	)		(5,292	)
	Less unamortized discount			(44,405	)		(1,643	)
	Less current portion			—			(13,346	)
	Loans payable, net of current portion		$	80,595		$	26,211
	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.
	During the fourth quarter of 2014, the last reported sale price of the common stock for at least 20 trading days (whether or not consecutive) during the period of 30 consecutive trading days ending on the last trading day of the calendar quarter ended December 31, 2014 was greater than 130% of the conversion price for the Notes on each applicable trading day. As a result, holders may convert their Notes at their option at any time from January 1, 2015 through March 31, 2015.
	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, 2014, interest expense related to the outstanding principal balance of the Notes was $13.7 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. The term loans bear interest at an annual rate equal to the greater of 10.55% or 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. As permitted under the Loan Agreement, the Company had previously extended the interest-only payment period with the aggregate principal balance of the loans to be repaid in monthly installments starting on June 1, 2014 and continuing through November 1, 2016. On June 25, 2014, the Company entered into an amendment to the Loan Agreement, whereby the Company and Hercules agreed to extend until October 1, 2014 the period during which the Company makes interest-only payments. On November 6, 2014, the Company entered into a further amendment to the Loan Agreement, whereby the Company and Hercules agreed to extend by four additional months the period during which the Company makes interest-only payments. As a result, the Company will repay the aggregate outstanding principal balance of the loan in equal monthly installments of principal and interest (based on a 30 month amortization schedule) beginning on February 1, 2015. The remaining principal balance and interest will be due and payable on November 1, 2016. This amendment was treated as a debt modification for accounting purposes.
	At the Company’s option, the Company may elect to prepay all or any part of the outstanding term loans without penalty.
	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 granted 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. This option expired on November 8, 2013.
	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, 2014, 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, 2014 and 2013, interest expense related to the Hercules loans payable were $4.7 million and $4.9 million, respectively.
	Convertible Notes—Silver Creek
	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, $0.9 million during the year ended December 31, 2013 and $1.0 million during the year ended December 31, 2014. The notes issued pursuant to the Note Purchase Agreement bore interest at 6% per annum. Upon issuance, these convertible notes contained a feature wherein if at any time prior to maturity Silver Creek enters into a qualifying equity financing, defined as a sale or series of related sales of equity securities prior to the maturity date and resulting in at least $4.0 million of gross proceeds, the notes 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 and was recorded as derivative liability on the consolidated balance sheets. For the years ended December 31, 2014 and 2014, the derivative was remeasured upon conversion of the notes with the gain in remeasurement recognized in other income. The notes matured and converted, along with an immaterial amount of accrued interest into shares of Silver Creek Series A preferred stock on both December 31, 2014 and 2013. Upon conversion, the Company’s ownership percentage of Silver Creek outstanding preferred stock decreased from 74% as of December 31, 2012 to 64% as of December 31, 2013 to 60% as of December 31, 2014, and a $0.4 million and $0.8 million increase to non-controlling interest was recognized as of December 31, 2014 and December 31, 2013, respectively.

Stock_Warrants

Stock Warrants	12 Months Ended
	Dec. 31, 2014
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, 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		—			—
	Exercised		(396	)	$	3.38		—			—
	Balance—December 31, 2014		2,381		$	3		—			—
	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. During the year ended December 31, 2014, warrants to purchase approximately 75,000 shares of common stock were cashless exercised and 38,000 shares of common stock were issued.

Common_Stock

Common Stock	12 Months Ended
	Dec. 31, 2014
Equity [Abstract]
Common Stock	13. Common Stock
	In July 2013, the Company sold an aggregate of 5.8 million 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, 2014 and 2013, the Company had 200.0 million shares of $0.01 par value common stock authorized. There were approximately 106.7 million and 102.5 million shares of common stock issued and outstanding as of December 31, 2014 and 2013, respectively. The shares reserved for future issuance as of December 31, 2014 and 2013 consisted of the following:
	(in thousands)	December 31, 2014		December 31, 2013
	Common stock warrants		2,381		2,777
	Options to purchase common stock		19,567		20,107
	Conversion of the Notes		25,000		25,000

StockBased_Compensation

Stock-Based Compensation	12 Months Ended
	Dec. 31, 2014
Disclosure of Compensation Related Costs, Share-based 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-qualified stock options, stock appreciation rights, restricted stock, restricted stock units and other stock-based awards. The 2011 Plan increased the total number of shares of common stock available to be issued by 3.5 million, 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. The Company registered 3.4 million and 3.6 million of additional shares of common stock related to the 2011 Plan in February 2013 and March 2014, respectively. As of December 31, 2014, there were 2.0 million shares of common stock available to be issued under the 2011 Plan.
	During the years ended December 31, 2014, 2013 and 2012, the Company issued options to purchase 3.9 million, 3.3 million and 3.3 million shares of common stock, respectively. These options generally vest over a three-year period for employees. Options granted to directors during the period from April 2012 through December 2013 vest over a one-year period. All other options granted to directors vest immediately. During the years ended December 31, 2013 and 2012, the Company also issued options to purchase less than 0.1 million shares of common stock to non-employees in each period. The assumptions used to estimate the fair value of options granted to non-employees at the date of grant were materially consistent with those used for employee and director grants.
	The Company recognized stock-based compensation expense as follows:
		Years ended December 31,
	(in thousands)	2014			2013			2012
	Employee awards:
	Research and development	$	6,864		$	5,954		$	4,234
	General and administrative		6,065			4,808			2,510
	Stock-based compensation for employee awards		12,929			10,762			6,744
	Stock-based compensation for non-employee awards		268			(29	)		145
	Total stock-based compensation	$	13,197		$	10,733		$	6,889
	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, 2014, 2013 and 2012 was estimated at the date of grant using the following assumptions:
		Years ended December 31,
		2014		2013	2012
	Risk-free interest rate	1.6 – 2.0%		0.1 – 1.9%	0.7 – 1.1%
	Expected dividend yield	0%		0%	0%
	Expected term	5.0 – 5.9 years		5.3 – 5.9 years	5.0 – 5.9 years
	Expected volatility	64 – 72%		67 – 70%	66 –72%
	The Company uses the simplified method to calculate the expected term, as it does not have sufficient historical exercise data to provide a reasonable basis upon which to estimate expected term. The computation of expected volatility is based on the historical volatility of comparable companies from a representative peer group selected based on industry and market capitalization. The risk-free interest rate is based on a treasury instrument whose term is consistent with the expected life of the stock options. Management estimates expected forfeitures based on historical experience and recognizes compensation costs only for those equity awards expected to vest.
	The following table summarizes stock option activity:
	(in thousands, except per share amounts)	Shares			Weighted Average			Weighted Average		Aggregate
					Exercise Price			Remaining		Intrinsic Value
								Contractual Term
	Outstanding at December 31, 2013		20,107		$	3.93			6.11	$	38,348
	Granted		3,858		$	5.56
	Exercised		(3,814	)	$	2.44
	Cancelled		(595	)	$	6.21
	Outstanding at December 31, 2014		19,556		$	4.47			6.17	$	133,599
	Vested and expected to vest at December 31, 2014		19,296		$	4.45			6.13	$	132,172
	Exercisable at December 31, 2014		15,009		$	4.02			5.38	$	109,347
	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 2014, 2013 and 2012 was $19.8 million, $2.7 million and $13.7 million, respectively.
	As of December 31, 2014, there was $15.1 million of total unrecognized compensation cost related to nonvested employee stock awards. As of December 31, 2014, the Company expects to recognize those costs over a weighted average period of approximately 1.7 years.

Income_Taxes

Income Taxes	12 Months Ended
	Dec. 31, 2014
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, 2014, 2013 and 2012. A reconciliation of the Company’s effective tax rate to the statutory federal income tax rate is as follows:
		Year Ended December 31,
		2014			2013			2012
	Federal income tax at statutory federal rate		35	%		35	%		35	%
	State taxes		3.1			4.3			4.5
	Permanent differences		(9.1	)		(2.0	)		(0.2	)
	Stock-based compensation		(1.7	)		(0.6	)		(0.3	)
	Tax credits		30.5			12.4			1.1
	Foreign rate differential		(2.3	)		(2.8	)		(4.1	)
	Other		4.5			(1.5	)		(0.5	)
	Change in valuation allowance		(60.0	)		(44.8	)		(35.5	)
			—	%		—	%		—	%
	During the year ended December 31, 2014, the Company recorded a deferred tax liability related to the embedded conversion option of the 4.50% convertible senior notes though 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, 2014 and 2013 are as follows:
	(in thousands)	2014			2013
	Deferred tax assets
	Net operating losses	$	160,333		$	113,843
	Capitalized research and development expenses		30,929			32,771
	Credit carryforwards		62,362			37,304
	Depreciation		2,579			2,735
	Deferred compensation		10,682			8,249
	Accrued expenses		12			841
	Deferred revenue		960			28,381
	Other temporary differences		9,195			5,885
	Total gross deferred tax asset		277,052			230,009
	Valuation allowance		(257,489	)		(207,304	)
	Net deferred tax asset		19,563			22,705
	Deferred tax liabilities
	Intangible assets		(3,106	)		(3,234	)
	Debt discount		(16,457	)		(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, 2011 through 2014, the tax years which remain subject to examination by major tax jurisdictions as of December 31, 2014. 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’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, 2014.
	At December 31, 2014, the Company had net operating loss carryforwards for federal and state income tax purposes of $432.0 million and $337.5 million, respectively. Included in the federal and state net operating loss carryforwards is approximately $21.1 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 2034. The Company also has available research and development credits for federal and state income tax purposes of approximately $20.8 million and $7.9 million, respectively. The federal and state research and development credits will begin to expire in 2022 and 2024, respectively. As of December 31, 2014, the Company also had available investment tax credits for state income tax purposes of $0.6 million, which will expire in years through 2017 if unused. In addition we have federal orphan drug credits of $36.1 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, 2014 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, 2014, 2013 and 2012 was as follows:
	(in thousands)	Balance at			Additions			Deductions			Balance at
		beginning									end of
		of period									period
	December 31, 2012	$	132,683		$	36,968		$	—		$	169,651
	December 31, 2013		169,651			37,653			—			207,304
	December 31, 2014	$	207,304		$	50,185		$	—		$	257,489

Commitments_and_Contingencies

Commitments and Contingencies	12 Months Ended
	Dec. 31, 2014
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 $5.9 million, $5.5 million and $4.3 million for the years ended December 31, 2014, 2013 and 2012, 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, 2014 and 2013, the Company has received $8.2 million and $5.5 million, respectively, 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 non-cancelable operating leases at December 31, 2014 are as follows:
	Years ended December 31,	(in thousands)
	2015		5,208
	2016		5,354
	2017		5,470
	2018		5,587
	2019 and thereafter		2,813

Related_Party_Transactions

Related Party Transactions	12 Months Ended
	Dec. 31, 2014
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 purchased 5,217,391 shares of the Company’s common stock in April 2012.
	Directors, officers, scientific advisory board members and related parties of the Company held approximately 1.3 million and 1.2 million shares of Silver Creek Series A preferred stock as of December 31, 2014 and 2013, respectively. These shares amount to 6% and 7% of the outstanding shares of Silver Creek Series A preferred stock as of December 31, 2014 and 2013, respectively.

Retirement_Plan

Retirement Plan	12 Months Ended
	Dec. 31, 2014
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, 2014, 2013 and 2012, the Company made contributions of $0.8 million, 0.7 million and $0.6 million, respectively, to the plan.

Selected_Quarterly_Financial_D

Selected Quarterly Financial Data (Unaudited)	12 Months Ended
	Dec. 31, 2014
Quarterly Financial Information Disclosure [Abstract]
Selected Quarterly Financial Data (Unaudited)	19. Selected Quarterly Financial Data (Unaudited)
	The following table contains quarterly financial information for 2014 and 2013. 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)
	2014
	Collaboration revenues	$	13,034		$	27,815		$	28,002		$	33,905
	Total operating expenses		36,548			41,716			51,727			39,021
	Net loss		(27,754	)		(18,290	)		(28,038	)		(9,477	)
	Net loss attributable to Merrimack Pharmaceuticals, Inc.		(27,585	)		(18,109	)		(27,901	)		(9,696	)
	Net loss per share available to common stockholders—basic and diluted	$	(0.27	)	$	(0.17	)	$	(0.27	)	$	(0.09	)
		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	)

Subsequent_Events

Subsequent Events	12 Months Ended
	Dec. 31, 2014
Subsequent Events [Abstract]
Subsequent Events	20. Subsequent Events
	On February 25, 2015, the Company entered into a fourth amendment to the Loan Agreement with Hercules pursuant to which the Company and Hercules agreed to extend the maturity date and the period during which the Company makes interest-only payments on its current term loan in the aggregate principal amount of $40.0 million. As a result of this amendment, the Company will repay the outstanding aggregate principal balance of the term loan beginning on June 1, 2016 and continuing through November 1, 2018. If the FDA approves the Company’s NDA for MM-398 by May 1, 2016, the Company may elect to extend the interest-only period by an additional six months so that the Company would repay the outstanding aggregate principal balance of the term loan beginning on December 1, 2016 and continuing through November 1, 2018. In addition, if the FDA approves the Company’s NDA for MM-398 by May 1, 2016, the Company may elect to draw, at any time until August 1, 2016, an additional term loan advance of up to $15.0 million. Principal and interest payments on the additional term loan advance would be made in the same manner as the Company’s current term loan in the aggregate principal amount of $40.0 million.
	On February 23, 2015, the Company entered into an amendment to its facility lease. The lease amendment provides an additional 31,620 square feet of leased space at the Company’s current facility in Cambridge, Massachusetts, with a termination date of June 30, 2019, which is co-terminous with the Company’s existing lease. As a result of this amendment, the Company has additional future minimum lease payments of $1.0 million, $1.8 million, $1.8 million $1.9 million and $0.9 million in fiscal years 2015, 2016, 2017, 2018 and 2019, respectively. In addition, under the terms of the amendment, the landlord will provide the Company with an aggregate leasehold improvement allowance of up to $0.8 million.

Summary_of_Significant_Account1

Summary of Significant Accounting Policies (Policies)	12 Months Ended
	Dec. 31, 2014
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 2011, and Merrimack Pharmaceuticals (Bermuda) Ltd., which was merged with and into the Company during the third quarter of 2014. 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, but may not be limited to, revenue recognition, including the estimated percentage of billable expenses in any particular budget period, periods of meaningful use of licensed products, estimated service periods and services to be completed under a collaboration, 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, valuation of convertible debt, tax valuation reserves and accrued expenses. 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 region.
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 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 both December 31, 2014 and 2013, the Company recorded restricted cash of $685,000, which was 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, 2014, 2013 or 2012.
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, 2014 and 2013, Sanofi represented approximately 90% and 98% of collaboration revenues, respectively. As of December 31, 2014 and 2013, Sanofi represented approximately 50% and 98% of accounts receivable, respectively. Baxter International Inc., Baxter Healthcare Corporation and Baxter Healthcare SA (collectively, “Baxter”) represented approximately 49% of accounts receivable as of December 31, 2014.
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 a component of mezzanine equity.
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 license and collaboration agreement with Baxter in September 2014, which was evaluated under the accounting guidance on revenue recognition for multiple element arrangements. The Company determined that the obligations under this agreement represent a single unit of accounting and that the agreement represents a services agreement. As a result, the Company has estimated the level of effort expected to be completed as a result of providing the identified deliverables and will recognize revenue related to the agreement based on proportional performance as effort is completed over the expected services period.
	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.
	Whenever the Company determines that an arrangement should be accounted for as a single unit of accounting, it determines the period over which the performance obligations would be performed and revenue would be recognized. If the Company cannot reasonably estimate the timing and the level of effort to complete its performance obligations under the arrangement, then revenue under the arrangement is recognized on a straight-line basis over the period the Company expects to complete its performance obligations.
	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 divided by 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.
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, research-related 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 services have 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 to 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 and expense.
	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. For the years ended December 31, 2014, 2013 and 2012, 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, 2014, 2013 and 2012, the Company recognized $0.2 million of benefit in other income in each period.
	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 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, 2014 and 2013, the Company recognized $0.1 million of benefit in other income in each period.
	In May 2014, the MLSC awarded the Company an additional $0.6 million of tax incentives under its Life Science Tax Incentive Program, which allows 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. The Company deferred and is amortizing 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 was achieved. For the year ended December 31, 2014, the Company recognized $0.1 million of benefit in other income.
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 2014. The Company’s next annual impairment evaluation will be made in the third quarter of 2015 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, further testing is not 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. It was determined that it was not more likely than not that an impairment existed as of the third quarter of 2014 and, therefore, impairment evaluations were not performed.
	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, 2014.
	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 during the year ended December 31, 2014.
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 did not have a material impact on the Company’s consolidated financial statements.
	In May 2014, the FASB issued guidance which supersedes all existing revenue recognition requirements, including most industry-specific guidance. The new standard requires a company to recognize revenue when it transfers goods or services to customers in an amount that reflects the consideration that the company expects to receive for those goods or services. This guidance will be effective prospectively for fiscal years, and interim periods within those years, beginning after December 15, 2016, and early adoption is not permitted. The Company is currently evaluating the potential impact that the adoption of this guidance and the related transition guidance may have on its consolidated financial statements.
	In August 2014, the FASB issued guidance outlining management’s responsibility to perform interim and annual assessments of an entity’s ability to continue as a going concern within one year of the date the financial statements are issued and providing guidance on determining when and how to disclose going concern uncertainties in the financial statements. This guidance will be effective for annual and interim reporting periods ending after December 15, 2016, with early adoption permitted. The Company does not anticipate a material impact to its consolidated financial statements as a result of this change.

Summary_of_Significant_Account2

Summary of Significant Accounting Policies (Tables)	12 Months Ended
	Dec. 31, 2014
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, 2014
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, 2014 consisted of the following:
		Amortized		Unrealized			Unrealized			Fair
		Cost		Gains			Losses			Value
		(in thousands)
	December 31, 2014:
	Commercial paper	$	6,493	$	—		$	(2	)	$	6,491
	Corporate debt securities		81,921		—			(72	)		81,849
	Total	$	88,414	$	—		$	(74	)	$	88,340
Schedule of Marketable Securities in an Unrealized Loss Position	Available-for-sale securities in an unrealized loss position as of December 31, 2014 consisted of the following:
		Aggregate		Unrealized
		Fair Value		Losses
		(in thousands)
	December 31, 2014:
	Commercial paper	$	6,493	$	(2	)
	Corporate debt securities		81,921		(72	)
		$	88,414	$	(74	)

Net_Loss_Per_Common_Share_Tabl

Net Loss Per Common Share (Tables)	12 Months Ended
	Dec. 31, 2014
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)	2014			2013			2012
	Net Loss Per Share:
	Numerator:
	Net loss attributable to Merrimack Pharmaceuticals, Inc.	$	(83,291	)	$	(130,925	)	$	(91,277	)
	Plus: Unaccreted dividends on convertible preferred stock		—			—			(2,107	)
	Net loss available to common stockholders—basic and diluted		(83,291	)		(130,925	)		(93,384	)
	Denominator:
	Weighted-average common shares—basic and diluted		104,410			98,919			72,831
	Net loss per share available to common stockholders—basic and diluted	$	(0.80	)	$	(1.32	)	$	(1.28	)
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)	2014			2013			2012
	Common stock warrants		2,381			2,777			2,842
	Options to purchase common stock		19,567			20,107			18,066
	Conversion of the Notes		25,000			25,000			—

License_and_Collaboration_Agre1

License and Collaboration Agreements (Tables) (Sanofi [Member])	12 Months Ended
	Dec. 31, 2014
Sanofi [Member]
Schedule of Revenue Recognized and Assets and Liabilities under Collaborative Arrangements	During the years ended December 31, 2014, 2013 and 2012, the Company recognized revenue based on the following components of the Sanofi agreement:
		Years ended December 31,
	(in thousands)	2014		2013		2012
	Upfront payment	$	39,306	$	5,000	$	5,000
	Milestone payment		16,377		2,083		2,975
	Development services		18,904		36,283		36,905
	Manufacturing services and other		17,709		3,867		3,307
	Total	$	92,296	$	47,233	$	48,187
	As of December 31, 2014 and 2013, the Company maintained the following assets and liabilities related to the Sanofi agreement:
		December 31,
	(in thousands)	2014		2013
	Accounts receivable, billed	$	369	$	2,357
	Accounts receivable, unbilled		1,282		3,417
	Deferred revenues	$	—	$	73,392

Fair_Value_of_Financial_Instru1

Fair Value of Financial Instruments (Tables)	12 Months Ended
	Dec. 31, 2014
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, 2014 and 2013 and the input categories associated with those assets and liabilities:
	As of December 31, 2014	Level 1		Level 2				Level 3
	(in thousands)
	Assets:
	Cash equivalents—money market funds	$	33,199	$	—			$	—
	Investments—commercial paper		—		6,491				—
	Investments—corporate debt securities		—		81,849				—
	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				—
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			Discount rate				25.7	%
				weighted discounted cash flow
				analysis

Goodwill_and_Intangible_Assets1

Goodwill and Intangible Assets, Net (Tables)	12 Months Ended
	Dec. 31, 2014
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, 2014, 2013 and 2012 were as follows:
	(in thousands)	Intangible			IPR&D			Goodwill
		assets
	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
	Amortization		(320	)		—			—
	Balance, December 31, 2014	$	1,525		$	6,200		$	3,605
Schedule of Expected Amortization Expense for Intangible Assets for the Next Five-Year Period	Amortization expense is expected to be as follows for the next five-year period:
	Years Ended December 31,	(in thousands)
	2015	$	320
	2016		320
	2017		320
	2018		320
	2019	$	245

Property_and_Equipment_Net_Tab

Property and Equipment, Net (Tables)	12 Months Ended
	Dec. 31, 2014
Property, Plant and Equipment [Abstract]
Schedule of Property and Equipment	Property and equipment consisted of the following:
		December 31,
	(in thousands)	2014			2013
	Lab equipment	$	16,214		$	13,714
	IT equipment		3,113			2,701
	Leasehold improvements		18,219			10,523
	Furniture and fixtures		624			340
	Construction in process		472			7,635
			38,642			34,913
	Less: Accumulated depreciation		(24,140	)		(21,549	)
		$	14,502		$	13,364

Accounts_Payables_Accrued_Expe1

Accounts Payables, Accrued Expenses and Other (Tables)	12 Months Ended
	Dec. 31, 2014
Payables and Accruals [Abstract]
Schedule of Accounts Payable, Accrued Expenses and Other	Accounts payable, accrued expenses and other as of December 31, 2014 and 2013 consisted of the following:
		December 31,
	(in thousands)	2014		2013
	Accounts payable	$	2,510	$	1,889
	Accrued goods and services		17,481		7,958
	Accrued clinical trial costs		7,637		18,073
	Accrued payroll and related benefits		6,166		7,255
	Accrued interest		2,956		2,926
	Accrued dividends payable		19		25
	Deferred tax incentives		467		688
	Total accounts payable, accrued expenses and other	$	37,236	$	38,814

Borrowings_Tables

Borrowings (Tables)	12 Months Ended
	Dec. 31, 2014
Debt Disclosure [Abstract]
Schedule of Future Minimum Payments under the Loans Payable	Future minimum payments under indebtedness agreements outstanding as of December 31, 2014 are as follows:
	Years Ending December 31:	4.50% Convertible			Loan
	(in thousands)	Senior Notes			Agreement
	2015	$	5,625		$	16,991
	2016		5,625			29,501
	2017		5,625			—
	2018		5,625			—
	2019 and thereafter		136,250
		$	158,750		$	46,492
	Less interest		(33,750	)		(5,292	)
	Less unamortized discount		(44,405	)		(1,643	)
	Less current portion		—			(13,346	)
	Loans payable, net of current portion	$	80,595		$	26,211

Stock_Warrants_Tables

Stock Warrants (Tables)	12 Months Ended
	Dec. 31, 2014
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, 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		—			—
	Exercised		(396	)	$	3.38		—			—
	Balance—December 31, 2014		2,381		$	3		—			—

Common_Stock_Tables

Common Stock (Tables)	12 Months Ended
	Dec. 31, 2014
Equity [Abstract]
Schedule of Shares Reserved for Future Issuance	The shares reserved for future issuance as of December 31, 2014 and 2013 consisted of the following:
	(in thousands)	December 31, 2014		December 31, 2013
	Common stock warrants		2,381		2,777
	Options to purchase common stock		19,567		20,107
	Conversion of the Notes		25,000		25,000

StockBased_Compensation_Tables

Stock-Based Compensation (Tables)	12 Months Ended
	Dec. 31, 2014
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Schedule of Recognized Stock-Based Compensation Expense	The Company recognized stock-based compensation expense as follows:
		Years ended December 31,
	(in thousands)	2014			2013			2012
	Employee awards:
	Research and development	$	6,864		$	5,954		$	4,234
	General and administrative		6,065			4,808			2,510
	Stock-based compensation for employee awards		12,929			10,762			6,744
	Stock-based compensation for non-employee awards		268			(29	)		145
	Total stock-based compensation	$	13,197		$	10,733		$	6,889
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, 2014, 2013 and 2012 was estimated at the date of grant using the following assumptions:
		Years ended December 31,
		2014		2013	2012
	Risk-free interest rate	1.6 – 2.0%		0.1 – 1.9%	0.7 – 1.1%
	Expected dividend yield	0%		0%	0%
	Expected term	5.0 – 5.9 years		5.3 – 5.9 years	5.0 – 5.9 years
	Expected volatility	64 – 72%		67 – 70%	66 –72%
Summary of Stock Option Activity	The following table summarizes stock option activity:
	(in thousands, except per share amounts)	Shares			Weighted Average			Weighted Average		Aggregate
					Exercise Price			Remaining		Intrinsic Value
								Contractual Term
	Outstanding at December 31,2013		20,107		$	3.93			6.11	$	38,348
	Granted		3,858		$	5.56
	Exercised		(3,814	)	$	2.44
	Cancelled		(595	)	$	6.21
	Outstanding at December 31, 2014		19,556		$	4.47			6.17	$	133,599
	Vested and expected to vest at December 31, 2014		19,296		$	4.45			6.13	$	132,172
	Exercisable at December 31, 2014		15,009		$	4.02			5.38	$	109,347

Income_Taxes_Tables

Income Taxes (Tables)	12 Months Ended
	Dec. 31, 2014
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,
		2014			2013			2012
	Federal income tax at statutory federal rate		35	%		35	%		35	%
	State taxes		3.1			4.3			4.5
	Permanent differences		(9.1	)		(2.0	)		(0.2	)
	Stock-based compensation		(1.7	)		(0.6	)		(0.3	)
	Tax credits		30.5			12.4			1.1
	Foreign rate differential		(2.3	)		(2.8	)		(4.1	)
	Other		4.5			(1.5	)		(0.5	)
	Change in valuation allowance		(60.0	)		(44.8	)		(35.5	)
			—	%		—	%		—	%
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, 2014 and 2013 are as follows:
	(in thousands)	2014			2013
	Deferred tax assets
	Net operating losses	$	160,333		$	113,843
	Capitalized research and development expenses		30,929			32,771
	Credit carryforwards		62,362			37,304
	Depreciation		2,579			2,735
	Deferred compensation		10,682			8,249
	Accrued expenses		12			841
	Deferred revenue		960			28,381
	Other temporary differences		9,195			5,885
	Total gross deferred tax asset		277,052			230,009
	Valuation allowance		(257,489	)		(207,304	)
	Net deferred tax asset		19,563			22,705
	Deferred tax liabilities
	Intangible assets		(3,106	)		(3,234	)
	Debt discount		(16,457	)		(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, 2014, 2013 and 2012 was as follows:
	(in thousands)	Balance at			Additions			Deductions			Balance at
		beginning									end of
		of period									period
	December 31, 2012	$	132,683		$	36,968		$	—		$	169,651
	December 31, 2013		169,651			37,653			—			207,304
	December 31, 2014	$	207,304		$	50,185		$	—		$	257,489

Commitments_and_Contingencies_

Commitments and Contingencies (Tables)	12 Months Ended
	Dec. 31, 2014
Commitments and Contingencies Disclosure [Abstract]
Schedule of Future Minimum Lease Payments Under Non-cancelable Operating Leases	Future minimum lease payments under non-cancelable operating leases at December 31, 2014 are as follows:
	Years ended December 31,	(in thousands)
	2015		5,208
	2016		5,354
	2017		5,470
	2018		5,587
	2019 and thereafter		2,813

Selected_Quarterly_Financial_D1

Selected Quarterly Financial Data (Unaudited) (Tables)	12 Months Ended
	Dec. 31, 2014
Quarterly Financial Information Disclosure [Abstract]
Schedule of Quarterly Financial Information	The following table contains quarterly financial information for 2014 and 2013. 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)
	2014
	Collaboration revenues	$	13,034		$	27,815		$	28,002		$	33,905
	Total operating expenses		36,548			41,716			51,727			39,021
	Net loss		(27,754	)		(18,290	)		(28,038	)		(9,477	)
	Net loss attributable to Merrimack Pharmaceuticals, Inc.		(27,585	)		(18,109	)		(27,901	)		(9,696	)
	Net loss per share available to common stockholders—basic and diluted	$	(0.27	)	$	(0.17	)	$	(0.27	)	$	(0.09	)
		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	)

Nature_of_the_Business_Additio

Nature of the Business - Additional Information (Detail)	12 Months Ended
	Dec. 31, 2014
	Candidate
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Number of novel therapeutic oncology candidates	6

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
	Apr. 30, 2012	Sep. 30, 2014	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Jan. 31, 2010	Jan. 31, 2011	Jan. 31, 2013	31-May-14	Jul. 31, 2013
			Item			Employees	Employees	Employees	Employees
			Segment
			Region
Summary Of Significant Accounting Policies [Line Items]
Number of operating segments			1
Number of geographic regions			1
Original maturities			Three months or less
Restricted cash			$685,000	$685,000
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
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 of goodwill		0		810,000
Life Sciences Tax Incentive Program 2010 [Member]
Summary Of Significant Accounting Policies [Line Items]
Tax incentive awarded						1,500,000
State research and development tax credits monetized						1,400,000
Number of incremental employees pledged to be hired						50
Amount of benefit recognized			300,000	300,000	300,000
Life Sciences Tax Incentive Program 2011 [Member]
Summary Of Significant Accounting Policies [Line Items]
Tax incentive awarded							1,300,000
State research and development tax credits monetized							1,200,000
Number of incremental employees pledged to be hired							50
Amount of benefit recognized			200,000	200,000	200,000
Performance period							5 years
Life Sciences Tax Incentive Program 2013 [Member]
Summary Of Significant Accounting Policies [Line Items]
Tax incentive awarded								500,000
State research and development tax credits monetized								400,000
Number of incremental employees pledged to be hired								20
Amount of benefit recognized			100,000	100,000
Performance period								5 years
Life Sciences Tax Incentive Program 2014 [Member]
Summary Of Significant Accounting Policies [Line Items]
Tax incentive awarded									600,000
State research and development tax credits monetized									600,000
Number of incremental employees pledged to be hired									31
Amount of benefit recognized			$100,000
Performance period									5 years
Collaboration Revenue [Member] | Customer Concentration Risk [Member] | Sanofi [Member]
Summary Of Significant Accounting Policies [Line Items]
Concentration risk (as a percent)			90.00%	98.00%
Accounts Receivable [Member] | Credit Concentration Risk [Member] | Sanofi [Member]
Summary Of Significant Accounting Policies [Line Items]
Concentration risk (as a percent)			50.00%	98.00%
Accounts Receivable [Member] | Credit Concentration Risk [Member] | Baxter [Member]
Summary Of Significant Accounting Policies [Line Items]
Concentration risk (as a percent)			49.00%
Minimum [Member]
Summary Of Significant Accounting Policies [Line Items]
Expected average maturities period of marketable securities classified as available-for-sale			3 months

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, 2014
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, 2014
In Thousands, unless otherwise specified
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$88,414
Unrealized Gains	0
Unrealized Losses	-74
Fair Value	88,340
Commercial Paper [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	6,493
Unrealized Gains	0
Unrealized Losses	-2
Fair Value	6,491
Corporate Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	81,921
Unrealized Gains	0
Unrealized Losses	-72
Fair Value	$81,849

Marketable_Securities_Addition

Marketable Securities - Additional Information (Detail) (USD $)	12 Months Ended
	Dec. 31, 2014
	Security
Schedule of Available-for-sale Securities [Line Items]
Aggregate Fair Value	$88,414,000
Number of securities in unrealized loss position for less than 12 months	35
Other-than-temporary-impairment investments	$0
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 $)	12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2014
Schedule of Available-for-sale Securities [Line Items]
Aggregate Fair Value	$88,414
Unrealized Losses	-74
Commercial Paper [Member]
Schedule of Available-for-sale Securities [Line Items]
Aggregate Fair Value	6,493
Unrealized Losses	-2
Corporate Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Aggregate Fair Value	81,921
Unrealized Losses	($72)

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, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Numerator:
Net loss attributable to Merrimack Pharmaceuticals, Inc.	($9,696)	($27,901)	($18,109)	($27,585)	($33,059)	($39,631)	($30,082)	($28,153)	($83,291)	($130,925)	($91,277)
Plus: Unaccreted dividends on convertible preferred stock											-2,107
Net loss available to common stockholders-basic and diluted									($83,291)	($130,925)	($93,384)
Denominator:
Weighted-average common shares-basic and diluted									104,410	98,919	72,831
Net loss per share available to common stockholders-basic and diluted	($0.09)	($0.27)	($0.17)	($0.27)	($0.33)	($0.39)	($0.31)	($0.29)	($0.80)	($1.32)	($1.28)

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, 2014
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, 2014	Dec. 31, 2013	Dec. 31, 2012
Common Stock Warrants [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Potentially dilutive securities (in shares)	2,381	2,777	2,842
Options to Purchase Common Stock [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Potentially dilutive securities (in shares)	19,567	20,107	18,066
Conversion of the Notes [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Potentially dilutive securities (in shares)	25,000	25,000

License_and_Collaboration_Agre2

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

3 Months Ended

12 Months Ended

0 Months Ended

3 Months Ended

0 Months Ended

1 Months Ended

Dec. 31, 2014

Sep. 30, 2014

Jun. 30, 2014

Mar. 31, 2014

Dec. 31, 2013

Sep. 30, 2013

Jun. 30, 2013

Mar. 31, 2013

Dec. 31, 2014

Dec. 31, 2013

Dec. 31, 2012

5-May-11

Mar. 31, 2012

Sep. 22, 2014

Nov. 10, 2009

Jan. 31, 2015

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

Company recognized revenue

$33,905,000

$28,002,000

$27,815,000

$13,034,000

$7,823,000

$6,856,000

$18,452,000

$14,655,000

$102,756,000

$47,786,000

$48,921,000

Deferred revenue, current

59,275,000

9,336,000

59,275,000

9,336,000

Agreement termination scheduled date

17-Dec-14

Research and development expenses

138,495,000

147,139,000

125,858,000

PharmaEngine [Member]

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

Upfront license fees paid

10,000,000

PharmaEngine [Member] | Development and Regulatory Milestone [Member]

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

Maximum milestone payment obligation

80,000,000

PharmaEngine [Member] | Sales Milestone [Member]

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

Maximum milestone payment obligation

130,000,000

PharmaEngine [Member] | Phase 3 Clinical Trial in Pancreatic Cancer [Member]

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

Milestone payment

5,000,000

PharmaEngine [Member] | License and Collaboration Agreements [Member]

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

Milestone payment

7,000,000

Milestone payment due date

30-Apr-15

Research and development expenses

12,600,000

1,500,000

6,200,000

PharmaEngine [Member] | License and Collaboration Agreements [Member] | New Drug Application [Member]

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

Collaboration milestone obligation, accrued

5,000,000

Baxter [Member] | License and Collaboration Agreements [Member]

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

Upfront license fee received

100,000,000

Royalty for improved products

The Company is also entitled to tiered, escalating royalties ranging from sub-teen double-digits to low twenties percentages of net sales of MM-398 in the Licensed Territory.

Notice period of termination

180 days

Company recognized revenue

10,500,000

Deferred revenue

91,200,000

91,200,000

Deferred revenue, current

59,300,000

59,300,000

Unbilled accounts receivables, outstanding

1,600,000

1,600,000

Revenue recognition completion period

12 months

Baxter [Member] | License and Collaboration Agreements [Member] | Sales Milestone [Member]

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

Maximum amount of milestone payments that can be received

250,000,000

Baxter [Member] | License and Collaboration Agreements [Member] | Research and Development Milestones [Member]

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

Maximum amount of milestone payments that can be received

100,000,000

Baxter [Member] | License and Collaboration Agreements [Member] | Regulatory Milestones [Member]

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

Maximum amount of milestone payments that can be received

520,000,000

Baxter [Member] | License and Collaboration Agreements [Member] | Clinical Trials in Pancreatic Cancer [Member]

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

Collaboration agreement costs

98,800,000

Sanofi [Member]

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

Upfront license fee received

60,000,000

Milestone license fee received

25,000,000

Expected development period from the effective date of agreement

12 years

Sanofi [Member] | License and Collaboration Agreements [Member]

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

Company recognized revenue

92,296,000

47,233,000

48,187,000

Deferred revenue

0

73,392,000

0

73,392,000

Spending in excess of budget

10,100,000

Recognized revenue related to excess spending

5,800,000

Estimated total cost of this drug supply

5,200,000

Actavis [Member]

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

Maximum amount of milestone payments that can be received

15,500,000

Notice period of termination

90 days

Milestone license fee received

2,000,000

Agreement expiration term respect to each product

10 years

Additional renewal term

2 years

Milestones and development expenses

3,800,000

2,100,000

3,800,000

2,100,000

Actavis [Member] | Subsequent Event [Member]

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

Aggregate milestone payments eligible to receive, decrease

$400,000

License_and_Collaboration_Agre3

License and Collaboration Agreements - Schedule of Revenue Recognized and Assets and Liabilities under Collaborative Arrangements (Detail) (USD $)	3 Months Ended								12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Total	$33,905	$28,002	$27,815	$13,034	$7,823	$6,856	$18,452	$14,655	$102,756	$47,786	$48,921
License and Collaboration Agreements [Member] | Sanofi [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Upfront payment									39,306	5,000	5,000
Milestone payment									16,377	2,083	2,975
Development services									18,904	36,283	36,905
Manufacturing services and other									17,709	3,867	3,307
Total									92,296	47,233	48,187
Accounts receivable, billed	369				2,357				369	2,357
Accounts receivable, unbilled	1,282				3,417				1,282	3,417
Deferred revenues	$0				$73,392				$0	$73,392

Fair_Value_of_Financial_Instru2

Fair Value of Financial Instruments - Schedule of Assets and Liabilities Measured at Fair Value on a Recurring Basis (Detail) (Recurring Basis [Member], USD $)	Dec. 31, 2014	Dec. 31, 2013
In Thousands, unless otherwise specified
Level 1 [Member] | Money Market Funds [Member]
Assets:
Cash equivalents	$33,199	$47,740
Level 2 [Member] | Commercial Paper [Member]
Assets:
Cash equivalents		13,998
Investments	6,491	49,680
Level 2 [Member] | Corporate Debt Securities [Member]
Assets:
Investments	$81,849	$40,436

Fair_Value_of_Financial_Instru3

Fair Value of Financial Instruments - Additional Information (Detail) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Marketable securities with an original maturity	Greater than three months
Impairment charge of IPR&D		$800,000
Level 3 [Member] | Hercules [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value of debt	40,600,000
Carrying value of debt	40,500,000
Senior Convertible Notes [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Principal amount	125,000,000
Carrying value of debt	125,000,000
Senior Convertible Notes [Member] | Level 2 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value of debt	232,400,000
Maximum [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Liquid investments with an original maturity	3 months

Fair_Value_of_Financial_Instru4

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
In Thousands, unless otherwise specified	Aug. 31, 2013
In Process Research and Development [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
IPR&D asset	0
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, 2014	Dec. 31, 2013	Dec. 31, 2012	Feb. 28, 2013
Consolidation, Less than Wholly Owned Subsidiary, Parent Ownership Interest, Effects of Changes, Net [Line Items]
Outstanding borrowings and accrued interest	$1,044,000	$2,603,000
Total assets of consolidated subsidiaries	158,656,000	192,417,000
Total liabilities of consolidated subsidiaries	260,727,000	235,545,000
Silver Creek [Member]
Consolidation, Less than Wholly Owned Subsidiary, Parent Ownership Interest, Effects of Changes, Net [Line Items]
Non-controlling interest increased	400,000	800,000
Ownership interest (as a percent)	60.00%	64.00%	74.00%
Non-controlling interest	100,000	300,000
Total assets of consolidated subsidiaries	300,000	1,000,000
Total liabilities of consolidated subsidiaries	200,000	100,000
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	1,000,000	600,000	1,600,000	300,000
Outstanding borrowings and accrued interest		2,600,000
Preferred stock value, per share	$1,000,000	$1,000,000
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)	6.00%	7.00%

Goodwill_and_Intangible_Assets2

Goodwill and Intangible Assets, Net - Additional Information (Detail) (USD $)	12 Months Ended			3 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Sep. 30, 2013
Business Acquisition [Line Items]
Impairment charge	$0	$0	$0
Hermes BioSciences, Inc. [Member]
Business Acquisition [Line Items]
Acquired IPR&D recognized in a business combination	7
Acquisition date	6-Oct-09
Amortization period	10 years
Impairment charge				0.8
Hermes BioSciences, Inc. [Member] | Core Nano Carrier Technology [Member]
Business Acquisition [Line Items]
Acquired intangible assets	3.2
Hermes BioSciences, 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 BioSciences, Inc. [Member] | Nanotherapeutic Program with Chemotherapy Drug [Member]
Business Acquisition [Line Items]
Acquired IPR&D recognized in a business combination	3.4
Hermes BioSciences, Inc. [Member] | Other In-process 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 $)	3 Months Ended	12 Months Ended
In Thousands, unless otherwise specified	Sep. 30, 2014	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Goodwill and Intangible Assets Disclosure [Abstract]
Intangible assets		$1,525	$1,845	$2,165	$2,485
IPR&D		6,200	6,200	7,010	7,010
Goodwill		3,605	3,605	3,605	3,605
Amortization		-320	-320	-320
Impairment	$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, 2014
In Thousands, unless otherwise specified
Goodwill and Intangible Assets Disclosure [Abstract]
2015	$320
2016	320
2017	320
2018	320
2019	$245

Property_and_Equipment_Net_Sch

Property and Equipment, Net - Schedule of Property and Equipment (Detail) (USD $)	Dec. 31, 2014	Dec. 31, 2013
In Thousands, unless otherwise specified
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$38,642	$34,913
Less: Accumulated depreciation	-24,140	-21,549
Property and equipment, net	14,502	13,364
Lab Equipment [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	16,214	13,714
IT Equipment [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	3,113	2,701
Leasehold Improvements [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	18,219	10,523
Furniture and Fixtures [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	624	340
Construction in Process [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$472	$7,635

Property_and_Equipment_Net_Add

Property and Equipment, Net - Additional Information (Detail) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Property Plant and Equipment Useful Life and Values [Abstract]
Depreciation expense	$4,200,000	$2,800,000	$3,500,000
Gross amount of fully depreciated fixed assets disposed	1,603,000	210,000	671,000
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, 2014	Dec. 31, 2013
In Thousands, unless otherwise specified
Property Plant and Equipment Useful Life and Values [Abstract]
Accounts payable	$2,510	$1,889
Accrued goods and services	17,481	7,958
Accrued clinical trial costs	7,637	18,073
Accrued payroll and related benefits	6,166	7,255
Accrued interest	2,956	2,926
Accrued dividends payable	19	25
Deferred tax incentives	467	688
Total accounts payable, accrued expenses and other	$37,236	$38,814

Borrowings_Schedule_of_Future_

Borrowings - Schedule of Future Minimum Payments under the Loans Payable (Detail) (USD $)	Dec. 31, 2014	Dec. 31, 2013
In Thousands, unless otherwise specified
Debt Instrument [Line Items]
Less current portion	$13,346	$8,248
Loans payable, net of current portion	106,806	103,427
4.50% Convertible Senior Notes [Member]
Debt Instrument [Line Items]
2015	5,625
2016	5,625
2017	5,625
2018	5,625
2019 and thereafter	136,250
Loans payable, gross	158,750
Less interest	-33,750
Less unamortized discount	-44,405
Loans payable, net of current portion	80,595
Loans Payable [Member] | Loan and Security Agreement [Member]
Debt Instrument [Line Items]
2015	16,991
2016	29,501
Loans payable, gross	46,492
Less interest	-5,292
Less unamortized discount	-1,643
Less current portion	-13,346
Loans payable, net of current portion	$26,211

Borrowings_450_Convertible_Sen

Borrowings - 4.50% Convertible Senior Notes - Additional Information (Detail) (USD $)	12 Months Ended			0 Months Ended		1 Months Ended	3 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Jul. 31, 2014	Jul. 31, 2013	Jul. 31, 2013	Dec. 31, 2014
Schedule Of Debt Instruments [Line Items]
Net proceeds from the debt issuance	$1,044,000	$121,537,000	$41,128,000
Interest rate (as a percent)	4.50%						4.50%
Debt Maturity Date	15-Jul-20
Public offering price (in dollars per share)					$5	$5
Interest expense	18,230,000	10,938,000	553,000
4.50% Convertible Senior Notes [Member]
Schedule Of Debt Instruments [Line Items]
Gross proceeds from the debt issuance						125,000,000
Net proceeds from the debt issuance						120,600,000
Interest rate (as a percent)	4.50%				4.50%	4.50%	4.50%
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		30 days
Convertibility of debt, closing price of stock test, percentage of stock price to conversion price for the notes that must be exceeded					130.00%		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	1,000
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%
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%
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	$13,700,000
4.50% Convertible Senior Notes [Member] | Conversion Terms, Fundamental Changes [Member]
Schedule Of Debt Instruments [Line Items]
Redemption price as percentage of principal amount of notes plus accrued and unpaid interest					10000.00%
4.50% Convertible Senior Notes [Member] | Conversion Terms, Event of Default [Member]
Schedule Of Debt Instruments [Line Items]
Percentage of principal amount due and payable upon event of default arising out of certain bankruptcy events					100.00%
4.50% Convertible Senior Notes [Member] | Minimum [Member]
Schedule Of Debt Instruments [Line Items]
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		20 days
4.50% Convertible Senior Notes [Member] | Maximum [Member]
Schedule Of Debt Instruments [Line Items]
Convertibility of debt, trading price of debt test, percentage of closing price of stock used in calculation				98.00%

Borrowings_Loan_Agreement_Addi

Borrowings - Loan Agreement - Additional Information (Detail) (USD $)	0 Months Ended	12 Months Ended			0 Months Ended
	Nov. 06, 2014	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Nov. 30, 2012
Schedule Of Debt Instruments [Line Items]
Discount on loans related to costs reimbursed by the entity					$39,600,000
Extension period for interest-only payment	4 months
Amortization schedule of loan agreement		30 months
Beginning date for equal monthly installments of loan agreement		1-Feb-15
Interest expense		18,230,000	10,938,000	553,000
Loan and Security Agreement [Member]
Schedule Of Debt Instruments [Line Items]
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
Interest expense		4,700,000	4,900,000
Second Alternative Interest Rate [Member] | Loan and Security Agreement [Member]
Schedule Of Debt Instruments [Line Items]
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%
Hercules [Member]
Schedule Of Debt Instruments [Line Items]
Less unamortized discount		1,600,000
Hercules [Member] | Loan and Security Agreement [Member]
Schedule Of Debt Instruments [Line Items]
Aggregate principal amount of loans received					$40,000,000

Borrowings_Convertible_Notes_S

Borrowings - Convertible Notes - Silver Creek - Additional Information (Detail) (USD $)	12 Months Ended		0 Months Ended	1 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2012
Schedule Of Debt Instruments [Line Items]
Interest rate (as a percent)	4.50%
Silver Creek [Member]
Schedule Of Debt Instruments [Line Items]
Ownership interest (as a percent)	60.00%	64.00%	74.00%	74.00%
Non-controlling interest	$0.40	$0.80
Silver Creek Convertible Note [Member] | Silver Creek [Member]
Schedule Of Debt Instruments [Line Items]
Gross proceeds from the debt issuance	1	0.9	1.6
Interest rate (as a percent)	6.00%
Discount on automatic conversion into the next qualifying equity financing (as a percent)			25.00%	25.00%
Derivative liability			0.2	0.2
Silver Creek Convertible Note [Member] | Silver Creek [Member] | Minimum [Member]
Schedule Of Debt Instruments [Line Items]
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

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, 2014	Dec. 31, 2013	Dec. 31, 2012
Common Stock Warrant [Member]
Class of Warrant or Right [Line Items]
Beginning Balance	2,777	2,842	2,640
Conversion (in shares)			302
Exercised (in shares)	-396	-65	-100
Ending Balance	2,381	2,777	2,842
Beginning Balance	$3.05	$3.05	$2.98
Conversion (per share)			$3.50
Exercised (per share)	$3.38	$2.82	$2.63
Ending Balance	$3	$3.05	$3.05
Convertible Preferred Stock Warrants [Member]
Class of Warrant or Right [Line Items]
Beginning Balance			302
Conversion (in shares)			-302
Beginning Balance			$3.50
Conversion (per share)			$3.50

Stock_Warrants_Additional_Info

Stock Warrants - Additional Information (Detail) (Common Stock Warrant [Member])	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Common Stock Warrant [Member]
Class of Warrant or Right [Line Items]
Number of warrants cashless exercised	75,000	65,000	100,000
Number of common stock issued as a result of cashless exercise of warrants	38,000	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, 2014	Dec. 31, 2013	Mar. 31, 2012
Equity [Abstract]
Shares of common stock sold in underwritten public offering	5,800,000
Public offering price	$5
Net proceeds from sale of common stock and convertible senior notes	$26.70
Common stock, shares authorized		200,000,000	200,000,000	200,000,000
Common stock, par value		$0.01	$0.01	$0.01
Common stock, shares issued		106,697,000	102,523,000
Common stock, shares outstanding		106,697,000	102,523,000

Common_Stock_Schedule_of_Share

Common Stock - Schedule of Shares Reserved for Future Issuance (Detail)	Dec. 31, 2014	Dec. 31, 2013
In Thousands, unless otherwise specified
Equity [Abstract]
Common stock warrants	2,381	2,777
Options to purchase common stock	19,567	20,107
Conversion of the Notes	25,000	25,000

StockBased_Compensation_Additi

Stock-Based Compensation - Additional Information (Detail) (USD $)	1 Months Ended		12 Months Ended
In Millions, except Share data, unless otherwise specified	Mar. 31, 2014	Feb. 28, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock options issued (in shares)			3,858,000
Aggregate intrinsic value of options exercised			19.8	2.7	13.7
Unrecognized compensation cost related to nonvested stock awards			15.1
Weighted average period over which unrecognized compensation cost is expected to be recognized			1 year 8 months 12 days
Stock Incentive Plan 2011 [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Additional common stock available for issuance (in shares)	3,600,000	3,400,000	3,500,000
Increase in Additional common stock available for issuance (in shares)			4,300,000
Shares of common stock available for grant (in shares)			2,000,000
Stock options issued (in shares)			3,900,000	3,300,000	3,300,000
Stock Incentive Plan 2011 [Member] | Non Employee [Member] | Maximum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock options issued (in shares)				100,000	100,000
Stock Incentive Plan 2011 [Member] | Director [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Options vesting period				1 year
Stock Incentive Plan 2011 [Member] | Employee [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Options vesting period			3 years

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, 2014	Dec. 31, 2013	Dec. 31, 2012
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Stock-based compensation expense for employee awards	$12,929	$10,762	$6,744
Stock-based compensation expense for non-employee awards	268	-29	145
Total stock-based compensation	13,197	10,733	6,889
Research and Development [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Stock-based compensation expense for employee awards	6,864	5,954	4,234
General and Administrative [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Stock-based compensation expense for employee awards	$6,065	$4,808	$2,510

StockBased_Compensation_Schedu1

Stock-Based Compensation - Schedule of Assumptions Used to Calculate Fair Value of Options Granted to Employees (Detail)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Minimum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Risk-free interest rate	1.60%	0.10%	0.70%
Expected term	5 years	5 years 3 months 18 days	5 years
Expected volatility	64.00%	67.00%	66.00%
Maximum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Risk-free interest rate	2.00%	1.90%	1.10%
Expected term	5 years 10 months 24 days	5 years 10 months 24 days	5 years 10 months 24 days
Expected volatility	72.00%	70.00%	72.00%
Options to Purchase Common Stock [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected dividend yield	0.00%	0.00%	0.00%

StockBased_Compensation_Summar

Stock-Based Compensation - Summary of Stock Option Activity (Detail) (USD $)	12 Months Ended
In Thousands, except Per Share data, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]
Number of Shares, Outstanding, Beginning balance	20,107
Number of Shares, Granted	3,858
Number of Shares, Exercised	-3,814
Number of Shares, Cancelled	-595
Number of Shares, Outstanding, Ending balance	19,567	20,107
Number of shares vested and expected to vest	19,296
Number of stock option exercisable	15,009
Weighted Average Exercise Price, Outstanding, Beginning balance	$3.93
Weighted Average Exercise Price, Granted	$5.56
Weighted Average Exercise Price, Exercised	$2.44
Weighted Average Exercise Price, Cancelled	$6.21
Weighted Average Exercise Price, Outstanding, Ending balance	$4.47	$3.93
Weighted average exercise price of shares vested and expected to vest (in dollars per share)	$4.45
Weighted average exercise price for exercisable option (in dollars per share)	$4.02
Weighted Average Remaining Contractual Term	6 years 2 months 1 day	6 years 1 month 10 days
Weighted Average Remaining Contractual Term, Vested and expected to vest	6 years 1 month 17 days
Weighted Average Remaining Contractual Term, Exercisable	5 years 4 months 17 days
Aggregate Intrinsic Value, Outstanding, Beginning balance	$38,348
Aggregate Intrinsic Value, Outstanding, Ending balance	133,599	38,348
Aggregate Intrinsic Value, Vested and expected to vest	132,172
Aggregate Intrinsic Value, Exercisable	$109,347

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, 2014	Dec. 31, 2013	Dec. 31, 2012
Income Tax Disclosure [Abstract]
Federal income tax at statutory federal rate	35.00%	35.00%	35.00%
State taxes	3.10%	4.30%	4.50%
Permanent differences	-9.10%	-2.00%	-0.20%
Stock-based compensation	-1.70%	-0.60%	-0.30%
Tax credits	30.50%	12.40%	1.10%
Foreign rate differential	-2.30%	-2.80%	-4.10%
Other	4.50%	-1.50%	-0.50%
Change in valuation allowance	-60.00%	-44.80%	-35.50%
Effective income tax rate	0.00%	0.00%	0.00%

Income_Taxes_Additional_Inform

Income Taxes - Additional Information (Detail) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014
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	$21.10
Federal Orphan Drug Credits	36.1
Federal [Member]
Operating Loss Carryforwards [Line Items]
NOL	432
Federal [Member] | Research and Development [Member]
Operating Loss Carryforwards [Line Items]
Amount of tax credit carryforward	20.8
State [Member]
Operating Loss Carryforwards [Line Items]
NOL	337.5
State [Member] | Research and Development [Member]
Operating Loss Carryforwards [Line Items]
Amount of tax credit carryforward	7.9
State [Member] | Investment [Member]
Operating Loss Carryforwards [Line Items]
Amount of tax credit carryforward	$0.60

Income_Taxes_Schedule_of_Tempo

Income Taxes - Schedule of Temporary Differences That Give Rise to Significant Net Deferred Tax Assets (Detail) (USD $)	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
In Thousands, unless otherwise specified
Deferred tax assets
Net operating losses	$160,333	$113,843
Capitalized research and development expenses	30,929	32,771
Credit carryforwards	62,362	37,304
Depreciation	2,579	2,735
Deferred compensation	10,682	8,249
Accrued expenses	12	841
Deferred revenues	960	28,381
Other temporary differences	9,195	5,885
Total gross deferred tax asset	277,052	230,009
Valuation allowance	-257,489	-207,304	-169,651	-132,683
Net deferred tax asset	19,563	22,705
Deferred tax liabilities
Intangible assets	-3,106	-3,234
Debt discount	-16,457	-19,471
Net deferred taxes	$0	$0

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, 2014	Dec. 31, 2013	Dec. 31, 2012
Income Tax Disclosure [Abstract]
Balance at beginning of period	$207,304	$169,651	$132,683
Additions	50,185	37,653	36,968
Deductions	0	0	0
Balance at end of period	$257,489	$207,304	$169,651

Commitments_and_Contingencies_1

Commitments and Contingencies - Additional Information (Detail) (USD $)	12 Months Ended			1 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Aug. 31, 2012
				sqft
Operating Leased Assets [Line Items]
Total rent expense	$5.90	$5.50	$4.30
August 2012 Amendment [Member]
Operating Leased Assets [Line Items]
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	8.2	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] | August 2012 Amendment [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 Non-cancelable Operating Leases (Detail) (August 2012 Amendment [Member], USD $)	Dec. 31, 2014
In Thousands, unless otherwise specified
August 2012 Amendment [Member]
Loss Contingencies [Line Items]
2015	$5,208
2016	5,354
2017	5,470
2018	5,587
2019 and thereafter	$2,813

Related_Party_Transactions_Add

Related Party Transactions - Additional Information (Detail)	1 Months Ended
	Apr. 30, 2012	Dec. 31, 2014	Dec. 31, 2013
Silver Creek [Member] | Series A Preferred Stock [Member]
Related Party Transaction [Line Items]
Shares owned by employees and directors of the parent company (as a percent)		6.00%	7.00%
Sanofi [Member]
Related Party Transaction [Line Items]
Shares of common stock purchased by related party in connection with initial public offering	5,217,391
Directors, Officers, Scientific Advisory Board Members and Related Parties [Member] | Silver Creek [Member] | Series A Preferred Stock [Member]
Related Party Transaction [Line Items]
Preferred stock shares held		1,300,000	1,200,000

Retirement_Plan_Additional_Inf

Retirement Plan - Additional Information (Detail) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Compensation and Retirement Disclosure [Abstract]
Company's contributions to 401(k) defined contribution savings plan	$0.80	$0.70	$0.60

Selected_Quarterly_Financial_D2

Selected Quarterly Financial Data (Unaudited) - Schedule of Quarterly Financial Information (Detail) (USD $)	3 Months Ended								12 Months Ended
In Thousands, except Per Share data, unless otherwise specified	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Quarterly Financial Information Disclosure [Abstract]
Collaboration revenues	$33,905	$28,002	$27,815	$13,034	$7,823	$6,856	$18,452	$14,655	$102,756	$47,786	$48,921
Total operating expenses	39,021	51,727	41,716	36,548	36,065	42,780	47,560	41,921	169,012	168,326	141,663
Net loss	-9,477	-28,038	-18,290	-27,754	-32,348	-39,763	-30,251	-28,323	-83,559	-130,685	-91,754
Net loss attributable to Merrimack Pharmaceuticals, Inc.	($9,696)	($27,901)	($18,109)	($27,585)	($33,059)	($39,631)	($30,082)	($28,153)	($83,291)	($130,925)	($91,277)
Net loss per share available to common stockholders-basic and diluted	($0.09)	($0.27)	($0.17)	($0.27)	($0.33)	($0.39)	($0.31)	($0.29)	($0.80)	($1.32)	($1.28)

Subsequent_Events_Additional_I

Subsequent Events - Additional Information (Detail) (USD $)	0 Months Ended	12 Months Ended
	Feb. 23, 2015	Dec. 31, 2014	Nov. 30, 2012
	sqft
Subsequent Event [Member] | February 23, 2015 Amendment [Member] | Cambridge, Massachusetts [Member]
Subsequent Event [Line Items]
Leased space	31,620
Additional future minimum lease payments, in fiscal year 2015	$1,000,000
Additional future minimum lease payments, in fiscal year 2016	1,800,000
Additional future minimum lease payments, in fiscal year 2017	1,800,000
Additional future minimum lease payments, in fiscal year 2018	1,900,000
Additional future minimum lease payments, in fiscal year 2019	900,000
Aggregate leasehold improvement allowance	800,000
Hercules [Member] | Loan and Security Agreement [Member]
Subsequent Event [Line Items]
Principal amount			40,000,000
Term loan, possible additional advance		$15,000,000
Term loan payment description		Company will repay the outstanding aggregate principal balance of the term loan beginning on June 1, 2016 and continuing through November 1, 2018. If the FDA approves the Companybs NDA for MM-398 by May 1, 2016, the Company may elect to extend the interest-only period by an additional six months so that the Company would repay the outstanding aggregate principal balance of the term loan beginning on December 1, 2016 and continuing through November 1, 2018.