Preferred Stock Warrants, Outstanding Convertible preferred stock warrants Represents the fair value written certificate that gives the holder the right to purchase shares of a stock for a specified price within a specified period of time. Series F amount Series F Amount. Carrying value as of the balance sheet date of advances from Preferred Stockholders. Used to reflect the current portion of the liabilities (due within one year or within the normal operating cycle if longer). Non-controlling interest Non controlling Interest Equity Preferred Fair Value As of the reporting date, the fair value of noncontrolling interests. The noncontrolling interest holder's ownership (or holders' ownership) may be in the form of preferred shares (regardless of class), preferred partnership units (regardless of class), preferential membership interests, or any other form of preferred equity regardless of investee entity legal form. Issuance of Common Stock Warrants Issuance of common stock warrants in connection with equipment financing loans Represents the equity impact of the issue of warrants by the reporting entity. Adjustments to Additional Paid in Capital, Temporary Equity Interest, Amount Series F amount interest Represents change in additional paid-in capital resulting from the interest amount on preferred stock. Modification of Stock Warrants Common stock warrant modification Represents the effect of modification in terms and conditions on stock warrants. Effect of Change in Par Value of Stock Change in par value Represents the effect of change in par value on equity of the reporting entity. Return of Series C stock as a result of license agreement Return of Series C Stock, During Period, Value Equity impact of the value of Series C stock that was returned by stockholders as consideration for a license agreement. Return of Series C Stock, During Period, Shares Number of Shares of Series C stock that was returned by stockholders as consideration for a license agreement. Return of Series C stock as a result of license agreement (in shares) Gain (loss) on Mark to Market on Preferred Stock Warrants (Gain) loss on mark-to-market on preferred stock warrants and contingent consideration Loss (gain) on mark-to-market on convertible preferred stock warrants Represents the unrealized gain or loss on the remeasurement of fair value of preferred stock warrants. Amendment Description Amortization of Deferred Lease Benefits and Tax Incentives Amortization of deferred lease benefits and tax incentives Represents the amortization of deferred lease benefits and tax incentive of the reporting entity during the reporting period. Amendment Flag Increase (Decrease) in Lease Benefits Deferred lease benefits The increase (decrease) during the reporting period in deferred lease benefits yet to be earned and recognized as a reduction of rental expense over the lease term. Deferred tax incentive Increase (Decrease) in Deferred Credits The increase (decrease) during the reporting period, excluding the portion taken into income, in the liability reflecting credits yet to be earned for which cash or other forms of consideration was received or recorded as a receivable. Proceeds Received in Advance for Issuance of Common Stock Proceeds received in advance of Series F issuance Represents the cash inflows in advance for issuance of convertible preferred stock. Accrued Interest on Preferred Stock Relieved to Additional Paid in Capital Accrued interest on Series F amount relieved to additional paid-in capital (Note 13) Represents the accrued interest on unissued preferred stock that was relieved to additional paid-in capital in a noncash transaction. Issuance of Shares from Advance Preferred Stock, Amount Issuance of shares from Series F amount (Note 13) Represents the issuance of shares from the amount received in advance in a noncash transaction. Preferred Stock Issuable for Consulting Services Rendered Series F convertible preferred stock issuable for consulting services rendered Represents the convertible preferred stock issuable for consulting services rendered in a noncash transaction. Preferred Stock Received for Technology License Series C convertible preferred stock received for technology license Represents the convertible preferred stock received for technology license in a noncash transaction. License and Collaboration Agreements Consolidated subsidiaries Consolidated Subsidiaries [Text Block] The entire disclosure for a business combination (or series of individually immaterial business combinations) completed during the period, including background, timing, and recognized assets and liabilities. The disclosure may include leverage buyout transactions (as applicable). This discolsure also includes information regarding variable interest entities (VIE) in aggregate, including how similar entities are aggregated, if separate reporting would not provide more useful information, distinguished between (1) VIEs that are not consolidated because the enterprise is not the primary beneficiary but has a significant variable interest or is the sponsor that holds a variable interest, and (2) VIEs that are consolidated. Presented in a manner that clearly and fully explains to financial statement users the nature and extent of an enterprise's involvement with variable interest entities. Accrued Expenses and other Accrued and Other Current Liabilities Disclosure [Text Block] The entire disclosure for accrued and other current liabilities at the end of the reporting period. Series F amount Series F amount The entire disclosure for Series F convertible preferred stock. Series F Amount Disclosure [Text Block] Stock Warrants Disclosure [Text Block] Stock warrants The entire disclosure for the common stock warrants activity of the company. Document and Entity Information Increase Decrease Deferred Lease Benefits and Tax Incentive The increase (decrease) during the reporting period in deferred lease benefits and tax incentives yet to be earned and recognized as a reduction of rental expense over the lease term and in the liability reflecting credits yet to be earned for which cash or other forms of consideration was received or recorded as a receivable. Deferred lease benefits and tax incentive Current Fiscal Year End Date Amortization of premiums on marketable securities Accretion (Amortization) of Discounts and Premiums, Investments Document Period End Date Entity Well-known Seasoned Issuer Entity Voluntary Filers Entity Current Reporting Status Entity Filer Category Entity Public Float Entity Registrant Name Entity Central Index Key Entity Common Stock, Shares Outstanding Document Fiscal Year Focus Document Fiscal Period Focus Document Type Additional paid-in capital Additional Paid in Capital Condensed Consolidated Balance Sheets Capital lease obligations Capital Lease Obligations, Current Capital lease obligations Capital Lease Obligations, Noncurrent Cash acquired in acquisition Cash Acquired from Acquisition Cash and cash equivalents Cash and Cash Equivalents, at Carrying Value Cash and cash equivalents, beginning of period Cash and cash equivalents, end of period Interest Paid Cash paid for interest Valuation Allowance, Deferred Tax Asset, Change in Amount Noncash benefit on release of tax valuation allowance Noncash interest expense Increase (Decrease) in Interest Payable, Net Prepaid expenses and other current assets Increase (Decrease) in Prepaid Expense and Other Assets Accounts receivable Increase (Decrease) in Receivables Changes in operating assets and liabilities Increase (Decrease) in Operating Capital [Abstract] Increase (Decrease) in Accounts Payable Accounts payable Commitments and Contingencies Disclosure [Text Block] Commitments and Contingencies Common stock, authorized shares Common Stock, Shares Authorized Common stock, issued shares Common Stock, Shares, Issued Balance (in shares) Balance (in shares) Common stock, outstanding shares Common Stock, Shares, Outstanding Common stock, 138,500 and 200,000 authorized $0.01 par value shares at December 31, 2011 and March 31, 2012, respectively, 11,834 and 11,907 issued and outstanding at December 31, 2011 and March 31, 2012, respectively Common Stock, Value, Issued Total current liabilities Liabilities, Current Liabilities, Current [Abstract] Current liabilities: Debt Disclosure [Text Block] Debt Deferred revenue Deferred Revenue, Current Deferred revenues Deferred Revenue, Noncurrent Cash and Cash Equivalents Disclosure [Text Block] Cash equivalents Stock-based compensation Share-based Compensation Fair value of assets acquired in acquisition Fair Value of Assets Acquired Gain (Loss) on Sale of Property Plant Equipment (Gain) loss on disposal of property and equipment General and Administrative Expense General and administrative Condensed Consolidated Statements of Operations Income Tax Disclosure [Text Block] Income taxes (Assignment) release of restricted cash Increase (Decrease) in Restricted Cash Intangible assets, net Finite-Lived Intangible Assets, Net Goodwill Goodwill In-process research and development Indefinite-Lived Intangible Assets (Excluding Goodwill) Interest Expense Interest expense Total liabilities Liabilities Total liabilities, convertible preferred stock, non-controlling interest and stockholders' deficit Liabilities and Equity Liabilities, Convertible Preferred Stock, Non-controlling Interest and Stockholders' Deficit Liabilities and Equity [Abstract] Net cash used in operating activities Net Cash Provided by (Used in) Operating Activities, Continuing Operations Cash flows from operating activities Net Cash Provided by (Used in) Operating Activities, Continuing Operations [Abstract] Other income and expenses Nonoperating Income (Expense) [Abstract] Operating Income (Loss) Loss from operations Collaboration revenues Revenues Other assets and liabilities, net Increase (Decrease) in Other Operating Assets and Liabilities, Net Other investing activities, net Payments for (Proceeds from) Other Investing Activities Other Nonoperating Income (Expense) Other, net Proceeds from issuance of common stock Proceeds from Issuance of Common Stock Proceeds from issuance of convertible preferred stock of Silver Creek Pharmaceuticals, Inc. Proceeds from Issuance of Convertible Preferred Stock Proceeds from Issuance of Preferred Stock and Preference Stock Proceeds received in advance of Series G issuance Sale of marketable securities Proceeds from Sale and Maturity of Marketable Securities Proceeds from sale of property and equipment Proceeds from Sale of Property, Plant, and Equipment Property and equipment, net Property, Plant and Equipment, Net Property and equipment, net Purchase of marketable securities Payments to Acquire Marketable Securities Purchase of property and equipment Payments to Acquire Property, Plant, and Equipment Accounts receivable Receivables, Net, Current Redeemable Convertible Preferred Stock [Member] Series B-F convertible preferred stock Repayments of Long-term Capital Lease Obligations Principal payment on capital lease obligations Principal payment of long-term debt Repayments of Long-term Debt Restricted cash Restricted Cash and Cash Equivalents, Current Accumulated deficit Retained Earnings (Accumulated Deficit) Sale Leaseback Transaction, Net Proceeds Proceeds from sale-lease back Condensed Consolidated Statements of Cash Flows Consolidated statements of convertible preferred stock, non-controlling interest and stockholders' deficit Stockholders' deficit: Stockholders' Equity Attributable to Parent [Abstract] Common Stock Stockholders' Equity Note Disclosure [Text Block] Common Stock Goodwill and Intangible Assets Disclosure [Text Block] Goodwill and intangible assets, net Supplemental disclosure of cash flows Supplemental Cash Flow Information [Abstract] Total current assets Assets, Current Current assets: Assets, Current [Abstract] Common stock warrants Warrant [Member] Common stock warrants Warrants and Rights Outstanding Weighted-average common shares used in computing pro forma net loss per share available to common stockholders-basic (unaudited) (in shares) Weighted Average Basic Shares Outstanding, Pro Forma Weighted Average Number of Shares Outstanding, Diluted Weighted-average common shares used in computing net loss per share available to common stockholders-diluted (in shares) Weighted-average common shares used in computing pro forma net loss per share available to common stockholders-diluted (unaudited) (in shares) Pro Forma Weighted Average Shares Outstanding, Diluted Weighted Average Number of Shares Outstanding, Basic Weighted-average common shares used in computing net loss per share available to common stockholders-basic (in shares) Restricted cash Restricted Cash and Cash Equivalents, Noncurrent Common stock Common Stock [Member] Property, Plant and Equipment Disclosure [Text Block] Property and equipment, net Research and Development Expense Research and development Total assets Assets Investment Income, Interest Interest income Deferred financing costs Deferred Finance Costs, Current, Net Disclosure of Compensation Related Costs, Share-based Payments [Text Block] Stock-Based Compensation Scenario, Unspecified [Domain] Statement [Table] Statement, Scenario [Axis] Assets Assets [Abstract] Statement Statement [Line Items] Convertible preferred stock Fair Value Disclosures [Text Block] Fair Value of Financial Instruments Temporary Equity, Carrying Amount Convertible preferred stock Cash flows from investing activities Net Cash Provided by (Used in) Investing Activities, Continuing Operations [Abstract] Cash flows from financing activities Net Cash Provided by (Used in) Financing Activities, Continuing Operations [Abstract] Net cash used in investing activities Net Cash Provided by (Used in) Investing Activities, Continuing Operations Net cash provided by (used in) financing activities Net Cash Provided by (Used in) Financing Activities, Continuing Operations Stock warrants Increase (Decrease) in Stockholders' Equity Increase (Decrease) in Stockholders' Equity [Roll Forward] Stockholders' Equity, Period Increase (Decrease) Deferred revenues Increase (Decrease) in Deferred Revenue Other assets Other Assets, Noncurrent Pro forma net loss per share available to common stockholders-basic (unaudited) (in dollars per share) Basic Earnings Per Share, Pro Forma Diluted Earnings Per Share Pro Forma Pro forma net loss per share available to common stockholders-diluted (unaudited) (in dollars per share) Net Loss per Common Share Income (Loss) from Continuing Operations before Equity Method Investments, Income Taxes, Extraordinary Items, Noncontrolling Interest Net loss before income taxes and non-controlling interest Deferred lease benefit Deferred Rent Credit, Current Deferred lease benefits Deferred Rent Credit, Noncurrent Common stock, par value (in dollars per share) Common Stock, Par or Stated Value Per Share Income Tax Expense (Benefit) Benefit from income taxes Statement, Equity Components [Axis] Additional paid-in capital Additional Paid-in Capital [Member] Accumulated deficit Retained Earnings [Member] Equity Component [Domain] Stock Issued During Period, Value, New Issues Issuance of Series F stock Stock Issued During Period, Value, Acquisitions Issuance of common stock in connection with acquisition Exercise of employee stock options Stock Issued During Period, Value, Stock Options Exercised Stock Issued During Period, Value, Conversion of Convertible Securities Issuance of Series C stock as a result of warrant exercises Issuance of Series F stock (in shares) Stock Issued During Period, Shares, New Issues Stock Issued During Period, Shares, Acquisitions Issuance of common stock in connection with acquisition (in shares) Exercise of employee stock options (in shares) Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercises in Period Stock Issued During Period, Shares, Conversion of Convertible Securities Issuance of Series C stock as a result of warrant exercises (in shares) Stock Issued During Period, Shares, Period Increase (Decrease) Collaborative Arrangement Disclosure [Text Block] License and Collaboration Agreements Costs and Expenses [Abstract] Operating expenses Costs and Expenses Total operating expenses Contingent consideration Business Combination, Contingent Consideration Arrangements, Change in Amount of Contingent Consideration, Liability Earnings Per Share [Text Block] Net Loss per Common Share Net Income (Loss), Including Portion Attributable to Noncontrolling Interest Net loss Net loss Net Income (Loss) Attributable to Noncontrolling Interest Less net loss attributable to non-controlling interest Loss attributable to non-controlling interest Depreciation and amortization Depreciation, Depletion and Amortization Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest Balance Balance Total stockholders' deficit Non-controlling interest Noncontrolling Interest [Member] Commitments and contingencies (Note 10) Commitments and Contingencies. Adjustments to reconcile net loss to net cash used in operating activities Adjustments to Reconcile Net Income (Loss) to Cash Provided by (Used in) Operating Activities [Abstract] Accounts payable Accounts Payable, Current Accrued dividends Dividends Payable, Current Stock-based compensation Adjustments to Additional Paid in Capital, Share-based Compensation, Requisite Service Period Recognition Prepaid expenses and other current assets Prepaid Expense and Other Assets, Current Noncontrolling Interest, Increase from Business Combination Ownership change in non-controlling interest Business Acquisition, Contingent Consideration, at Fair Value, Noncurrent Contingent consideration Net Cash Provided by (Used in) Continuing Operations Net increase (decrease) in cash and cash equivalents Net increase (decrease) in cash and cash equivalents Cash equivalents Accrued Expenses and other Nature of the Business Preferred Stock [Text Block] Convertible preferred stock Consolidated subsidiaries Commitments and Contingencies Income taxes Goodwill and intangible assets, net Fair Value of Financial Instruments Subsequent Events [Text Block] Subsequent Events Debt Retirement plan Retirement plan Compensation and Employee Benefit Plans [Text Block] Stock-Based Compensation Deferred tax incentives Deferred Credits and Other Liabilities, Current Deferred tax incentives Deferred Credits and Other Liabilities, Noncurrent Summary of Significant Accounting Policies Subsequent Events Fair value of liabilities assumed in acquisition Liabilities Assumed Stock Issued Fair value of equity issued in acquisition Non cash investing and financing activities Noncash Investing and Financing Items [Abstract] Related Party Transactions Disclosure [Text Block] Related Party Transactions Related Party Transactions Public Utilities Disclosure [Text Block] Initial Public Offering Initial Public Offering Initial Public Offering Disclosure [Text Block] Initial Public Offering The entire disclosure of initial public offering made by company. Net Income (Loss) Attributable to Parent Net loss attributable to Merrimack Pharmaceuticals, Inc. Fair Value Of Assets Acquired Included In Accrued Expenses Property and equipment included in ending accrued expenses The fair value of assets acquired in noncash investing or financing activities that were included in ending accrued expenses as of the balance sheet date. Fair Value Of Assets Acquired Included In Accounts Payable The fair value of assets acquired in noncash investing or financing activities that were included in ending accounts payable as of the balance sheet date. Property and equipment included in ending accounts payable Accrued expenses and other Increase (Decrease) in Accrued Liabilities and Other Operating Liabilities Deferred financing costs Payments of Stock Issuance Costs Deferred financing costs included in ending accrued expenses The amount of stock issuance costs capitalized which were included in accounts payable or accrued and other expenses. Stock Issuance Costs Incurred During Non cash or Partial Non cash Transaction Nature of the Business Nature of Operations [Text Block] Summary of Significant Accounting Policies Organization, Consolidation, Basis of Presentation, Business Description and Accounting Policies [Text Block] Other Liabilities, Current Accrued expenses and other Earnings Per Share, Basic Net loss per share available to common stockholders-basic (in dollars per share) Earnings Per Share, Diluted Net loss per share available to common stockholders-diluted (in dollars per share)