Entity Central Index Key Document And Entity Information [Abstract] Document And Entity Information [Abstract] Document Type Document Type Amendment Flag Amendment Flag Document Period End Date Document Period End Date Document Fiscal Year Focus Document Fiscal Year Focus Document Fiscal Period Focus Document Fiscal Period Focus Trading Symbol Trading Symbol Entity Registrant Name Entity Registrant Name Entity Central Index Key Current Fiscal Year End Date Current Fiscal Year End Date Entity Filer Category Entity Filer Category Entity Common Stock, Shares Outstanding Entity Common Stock Shares Outstanding Statement Of Financial Position [Abstract] Assets Assets [Abstract] Current assets: Assets Current [Abstract] Cash and cash equivalents Cash And Cash Equivalents At Carrying Value Marketable securities Marketable Securities Current Prepaid expenses and other current assets Prepaid Expense And Other Assets Current Total current assets Assets Current Restricted cash Restricted Cash And Cash Equivalents Noncurrent Property and equipment, net Property Plant And Equipment Net Equity method investment Equity Method Investments Other assets Other Assets Noncurrent Total assets Assets Liabilities and stockholders’ equity Liabilities And Stockholders Equity [Abstract] Current liabilities: Liabilities Current [Abstract] Accounts payable, accrued expenses and other Accounts Payable And Accrued Liabilities Current Deferred rent Deferred Rent Credit Current Total current liabilities Liabilities Current Deferred rent, net of current portion Deferred Rent Credit Noncurrent Other long-term liabilities Other Liabilities Noncurrent Total liabilities Liabilities Commitments and contingencies Commitments And Contingencies Stockholders’ equity: Stockholders Equity [Abstract] Preferred stock, $0.01 par value: 10,000 shares authorized at March 31, 2018 and December 31, 2017; no shares issued or outstanding at March 31, 2018 or December 31, 2017 Preferred Stock Value Common stock, $0.01 par value: 20,000 shares authorized at March 31, 2018 and December 31, 2017; 13,343 shares issued and outstanding at March 31, 2018 and December 31, 2017 Common Stock Value Additional paid-in capital Additional Paid In Capital Common Stock Accumulated deficit Retained Earnings Accumulated Deficit Accumulated other comprehensive loss Accumulated Other Comprehensive Income Loss Net Of Tax Total stockholders’ equity Stockholders Equity Total liabilities and stockholders’ equity Liabilities And Stockholders Equity Preferred stock, par value Preferred Stock Par Or Stated Value Per Share Preferred stock, shares authorized Preferred Stock Shares Authorized Preferred stock, shares issued Preferred Stock Shares Issued Preferred stock, shares outstanding Preferred Stock Shares Outstanding Common stock, par value Common Stock Par Or Stated Value Per Share Common stock, shares authorized Common Stock Shares Authorized Common stock, shares issued Common Stock Shares Issued Common stock, shares outstanding Common Stock Shares Outstanding Statement Of Income And Comprehensive Income [Abstract] Operating expenses: Operating Expenses [Abstract] Research and development expenses Research And Development Expense General and administrative expenses General And Administrative Expense Total operating expenses Operating Expenses Loss from continuing operations Operating Income Loss Other income and expenses: Nonoperating Income Expense [Abstract] Interest income Investment Income Interest Interest expense Interest Expense Other income (expense), net Other Nonoperating Income Expense Total other income and expenses Nonoperating Income Expense Net loss from continuing operations Income Loss From Continuing Operations Including Portion Attributable To Noncontrolling Interest Discontinued operations: Income Loss From Discontinued Operations Net Of Tax [Abstract] Loss from discontinued operations, net of tax Income Loss From Discontinued Operations Net Of Tax Net loss Profit Loss Net loss attributable to non-controlling interest Net Income Loss Attributable To Noncontrolling Interest Net loss attributable to Merrimack Pharmaceuticals, Inc. Net Income Loss Other comprehensive loss: Other Comprehensive Income Loss Net Of Tax Portion Attributable To Parent [Abstract] Unrealized loss on marketable securities Other Comprehensive Income Availableforsale Securities Adjustment Net Of Tax Portion Attributable To Parent Other comprehensive loss Other Comprehensive Income Loss Net Of Tax Portion Attributable To Parent Comprehensive loss Comprehensive Income Net Of Tax Amounts attributable to Merrimack Pharmaceuticals, Inc.: Income Amounts Attributable To Reporting Entity Disclosures [Abstract] Net loss from continuing operations Income Loss From Continuing Operations Loss from discontinued operations, net of tax Income Loss From Discontinued Operations Net Of Tax Attributable To Reporting Entity Basic and dilutive net loss per common share Earnings Per Share Basic And Diluted [Abstract] Net loss from continuing operations Income Loss From Continuing Operations Per Basic And Diluted Share Net loss from discontinued operations, net of tax Income Loss From Discontinued Operations Net Of Tax Per Basic And Diluted Share Net loss per share Earnings Per Share Basic And Diluted Weighted-average common shares used per share calculations—basic and diluted Weighted Average Number Of Share Outstanding Basic And Diluted Premiums paid on marketable securities. Amortization and accretion on marketable securities. Represents the expense recognized during the period arising from equity-based compensation arrangements (for example, shares of stock, unit, stock options or other equity instruments) with employees, directors and nonemployees. Increase decrease in deferred rent. Statement Of Cash Flows [Abstract] Statement [Table] Statement [Table] Legal Entity Legal Entity [Axis] Entity Entity [Domain] Subsidiary of the Company. Silver Creek Pharmaceuticals Inc [Member] Silver Creek Pharmaceuticals Inc [Member] Statement [Line Items] Statement [Line Items] Cash flows from operating activities Net Cash Provided By Used In Operating Activities [Abstract] Loss from continuing operations 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] Non-cash interest expense Paid In Kind Interest Depreciation and amortization expense Depreciation Depletion And Amortization Premiums paid on marketable securities Premiums Paid On Marketable Securities Amortization and accretion on marketable securities Amortization And Accretion On Marketable Securities Stock-based compensation expense Allocated Share Based Compensation Expense Including Non Employees Loss on equity method investment Gain Loss On Sale Of Equity Investments Changes in operating assets and liabilities: Increase Decrease In Operating Capital [Abstract] Prepaid expenses and other current assets Increase Decrease In Prepaid Deferred Expense And Other Assets Accounts payable, accrued expenses and other Increase Decrease In Accounts Payable And Accrued Liabilities Deferred rent Increase Decrease In Deferred Rent Net cash used in continuing operations for operating activities Net Cash Provided By Used In Operating Activities Continuing Operations Net cash provided by discontinuing operations for operating activities Cash Provided By Used In Operating Activities Discontinued Operations Net cash used in operating activities Net Cash Provided By Used In Operating Activities Cash flows from investing activities Net Cash Provided By Used In Investing Activities [Abstract] Purchase of property and equipment Payments To Acquire Property Plant And Equipment Purchases of marketable securities Payments To Acquire Marketable Securities Net cash used in investing activities Net Cash Provided By Used In Investing Activities Cash flows from financing activities Net Cash Provided By Used In Financing Activities [Abstract] Proceeds from exercise of options to purchase common stock Proceeds From Stock Options Exercised Proceeds from issuance of Series C preferred stock by Silver Creek Pharmaceuticals, Inc., net of issuance costs Proceeds From Issuance Of Preferred Stock And Preference Stock Net cash provided by financing activities Net Cash Provided By Used In Financing Activities Continuing Operations Net decrease in cash, cash equivalents and restricted cash Cash Cash Equivalents Restricted Cash And Restricted Cash Equivalents Period Increase Decrease Including Exchange Rate Effect Cash, cash equivalents and restricted cash, beginning of period Cash Cash Equivalents Restricted Cash And Restricted Cash Equivalents Cash, cash equivalents and restricted cash, end of period Non-cash investing and financing activities Noncash Investing And Financing Items [Abstract] Purchases of property and equipment in accounts payable, accrued expenses and other Capital Expenditures Incurred But Not Yet Paid Receivables related to property and equipment sale in other current assets Noncash Or Part Noncash Divestiture Amount Of Consideration Received1 Supplemental disclosure of cash flows Supplemental Cash Flow Information [Abstract] Cash paid for interest Interest Paid Organization Consolidation And Presentation Of Financial Statements [Abstract] Nature of the Business Nature Of Operations Accounting Policies [Abstract] Summary of Significant Accounting Policies Significant Accounting Policies [Text Block] Fair Value Disclosures [Abstract] Fair Value of Financial Instruments Fair Value Disclosures [Text Block] Investments Debt And Equity Securities [Abstract] Marketable Securities and Cash Equivalents Cash Cash Equivalents And Marketable Securities [Text Block] Payables And Accruals [Abstract] Accounts Payable, Accrued Expenses and Other Accounts Payable And Accrued Liabilities Disclosure [Text Block] Disclosure of Compensation Related Costs, Share-based Payments [Abstract] Stock-Based Compensation Disclosure Of Compensation Related Costs Share Based Payments [Text Block] Earnings Per Share [Abstract] Net Loss Per Common Share Earnings Per Share [Text Block] Discontinued Operations And Disposal Groups [Abstract] Discontinued Operations Disposal Groups Including Discontinued Operations Disclosure [Text Block] Noncontrolling Interest [Abstract] Investment in Silver Creek Minority Interest Disclosure [Text Block] Accounting Changes And Error Corrections [Abstract] Recent Accounting Pronouncements New Accounting Pronouncements And Changes In Accounting Principles [Text Block] Basis of Presentation Basis Of Accounting Policy Policy [Text Block] Consolidation Consolidation Policy [Text Block] Unaudited interim financial information. Unaudited Interim Financial Information Unaudited Interim Financial Information Policy [Text Block] Condensed consolidated statements of cash flows. Condensed Consolidated Statements of Cash Flows Condensed Consolidated Statements Of Cash Flows Policy [Text Block] Use of Estimates Use Of Estimates Marketable Securities Marketable Securities Available For Sale Securities Policy Recent Accounting Pronouncements New Accounting Pronouncements Policy Policy [Text Block] Schedule of cash, cash equivalents and restricted cash. Schedule of Reconciliation of Cash, Cash Equivalents, and Restricted Cash Schedule Of Cash Cash Equivalents And Restricted Cash Table [Text Block] Summary of Assets Measured at Fair Value on a Recurring Basis Fair Value Assets Measured On Recurring Basis [Text Block] Summary of Marketable Securities and Cash Equivalents Schedule Of Available For Sale Securities Reconciliation Table [Text Block] Schedule of Accounts Payable, Accrued Expenses and Other Schedule Of Accounts Payable And Accrued Liabilities Table [Text Block] Schedule of Assumptions Used to Calculate Fair Value of Options Granted to Employees Schedule Of Share Based Payment Award Stock Options Valuation Assumptions Table [Text Block] Tabular disclosure of employee and nonemployee service share based compensation allocation of recognized period costs. Disclosure Of Compensation Related Costs Sharebased Payments [Abstract] Schedule of Recognized Stock-Based Compensation Expense Schedule Of Employee And Nonemployee Service Share Based Compensation Allocation Of Recognized Period Costs Table [Text Block] Summary of Stock Option Activity Schedule Of Share Based Compensation Stock Options Activity Table [Text Block] Schedule of Outstanding Securities Excluded from Computation of Diluted Loss Per Share Schedule Of Antidilutive Securities Excluded From Computation Of Earnings Per Share [Text Block] Schedule of Discontinued Operations Schedule Of Disposal Groups Including Discontinued Operations Income Statement Balance Sheet And Additional Disclosures [Text Block] Number of development programs active. Number of development programs active Number Of Development Programs Active Accumulated deficit Net loss from continuing operations Cash in continuing operations for operating activities Cash and cash equivalents and marketable securities Cash Cash Equivalents And Short Term Investments Summary of significant accounting policies. Summary of significant accounting policies. Summary Of Significant Accounting Policies [Table] Summary Of Significant Accounting Policies [Table] Range Range [Axis] Range Range [Member] Maximum [Member] Maximum [Member] Minimum [Member] Minimum [Member] Silver Creek Pharmaceuticals Inc [Member] Summary Of Significant Accounting Policies [Line Items] Summary Of Significant Accounting Policies [Line Items] Ownership interest (as a percent) Minority Interest Ownership Percentage By Parent Discontinued operations Cash Cash Equivalents Restricted Cash And Restricted Cash Equivalents [Abstract] Cash, cash equivalents and restricted cash. Cash, cash equivalents and restricted cash. Cash Cash Equivalents And Restricted Cash [Table] Cash Cash Equivalents And Restricted Cash [Table] Balance Sheet Location Balance Sheet Location [Axis] Balance Sheet Location Balance Sheet Location [Domain] Prepaid Expenses and Other Current Assets [Member] Prepaid Expenses And Other Current Assets [Member] Cash Cash Equivalents And Restricted Cash [Line Items] Cash Cash Equivalents And Restricted Cash [Line Items] Restricted cash (short term) Restricted Cash Current Restricted cash (long term) Restricted Cash Noncurrent Total cash, cash equivalents and restricted cash shown in the condensed consolidated statement of cash flows Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Table] Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Table] Measurement Frequency Fair Value By Measurement Frequency [Axis] Fair Value, Measurement Frequency Fair Value Measurement Frequency [Domain] Recurring Basis [Member] Fair Value Measurements Recurring [Member] Level 2 [Member] Fair Value By Fair Value Hierarchy Level [Axis] Fair Value Hierarchy Fair Value Measurements Fair Value Hierarchy [Domain] Level 1 [Member] Fair Value Inputs Level1 [Member] Level 2 [Member] Fair Value Inputs Level2 [Member] Level 3 [Member] Fair Value Inputs Level3 [Member] Asset Class Fair Value By Asset Class [Axis] Asset Class Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation By Asset Class [Domain] Money Market Funds [Member] Money Market Funds [Member] Corporate Debt Securities [Member] Corporate Debt Securities [Member] Commercial Paper [Member] Commercial Paper [Member] Commercial Paper [Member] Commercial Paper Not Included With Cash And Cash Equivalents [Member] Government Securites [Member] U S Treasury And Government [Member] Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] Cash equivalents Cash And Cash Equivalents Fair Value Disclosure Marketable securities Available For Sale Securities Fair value transfers between Level 1 and Level 2 Fair Value Assets Level1 To Level2 Transfers Description Amortized Cost and Fair Value Debt Securities [Abstract] Amortized Cost And Fair Value Debt Securities [Abstract] Marketable securities Marketable Securities Schedule Of Available For Sale Securities [Table] Schedule Of Available For Sale Securities [Table] Debt Security Category Information By Category Of Debt Security [Axis] Other than Temporary Impairment, Credit Losses Recognized in Earnings, Categories of Investments Other Than Temporary Impairment Credit Losses Recognized In Earnings Categories Of Investments [Domain] Marketable Securities [Member] Marketable Securities [Member] Marketable Securities [Member] Cash and Cash Equivalents Cash And Cash Equivalents [Axis] Cash and Cash Equivalents Restricted Cash And Cash Equivalents Cash And Cash Equivalents [Member]