Total liabilities and shareholders' equity Total Biogen Idec Inc. shareholders' equity Accumulated Other Comprehensive Income (Loss), Net of Tax Accumulated Other Comprehensive Income (Loss), Net of Tax, Ending Balance Accumulated Other Comprehensive Income (Loss), Net of Tax, Beginning Balance Accumulated other comprehensive income Accumulated other comprehensive income Retained earnings Treasury stock, at cost Additional paid-in capital Preferred stock, par value $0.001 per share Common stock, par value $0.0005 per share Other long-term liabilities Total liabilities Long-term Notes Payable Notes payable and line of credit Notes payable and line of credit Long-term deferred tax liability Current liabilities: Total current liabilities Current portion of notes payable and line of credit Accrued expenses and other Total assets Marketable securities Total Intangible assets, net Goodwill Investments and other assets Total current assets Total inventory Other current assets Accounts receivable, net Marketable securities Cash and cash equivalents, beginning of the period Preferred stock, par value Common stock, par value ASSETS Current assets: LIABILITIES AND SHAREHOLDERS' EQUITY Litigation [Abstract] Litigation [Abstract] Litigation. Segment Information [Abstract] Segment Information [Abstract] Segment Information. New Accounting Pronouncements Inventory Intangible Assets and Goodwill Fair Value Measurements Income Taxes Segment Information Basic earnings per share attributable to Biogen Idec Inc. Basic earnings per share attributable to Biogen Idec Inc. Weighted average number of common shares outstanding Basic earnings per share attributable to Biogen Idec Inc. Diluted earnings per share attributable to Biogen Idec Inc. Diluted earnings per share attributable to Biogen Idec Inc. Basic earnings per share attributable to Biogen Idec Inc. Earnings Per Share, Basic Basic earnings per share attributable to Biogen Idec Inc. Net income attributable to Biogen Idec Inc. Income tax expense Income before income tax expense Other Nonoperating Income (Expense) Other income (expense), net Total Other income (expense), net Income from operations Costs and expenses: Total costs and expenses Research And Development In Process IPR&D charge recorded related to collaboration agreement Acquired in-process research and development Acquired in-process research and development (Note 2) Amortization of acquired intangible assets Collaboration profit sharing Collaboration profit sharing Payments are made to Elan for their share of rest of world net operating profits to effect an equal sharing of collaboration operating profit. These payments include the reimbursement of our portion of third-party royalties that Elan pays on behalf of the collaboration, relating to sales outside of the U.S. These amounts are reflected in the collaboration profit sharing line in our consolidated statement of income. As sales of Tysabri outside the U.S. increase, our collaboration profit sharing expense will increase. Selling, general and administrative Research and development Cost of sales, excluding amortization of acquired intangible assets Revenues: Total revenues Other revenues Other Other revenues Product Cash and cash equivalents, end of the period Cash and cash equivalents, beginning of the period Effect of exchange rate changes on cash and cash equivalents Net increase (decrease) in cash and cash equivalents Cash flows from financing activities: Net Cash Provided by (Used in) Financing Activities Net cash flows used in financing activities Obligation under securities lending Obligation under securities lending This is the liability that is maintain on the Company ledger to offset the fact we loan certain securities from our portfolio to other institutions. As such securities transaction are classified as loaned securities on the accompanying consolidated cash flow. Collateral for the loaned securities, consisting of cash or other securities is maintained at a rate of approximately 102% of the market value of each loaned security. We have a current obligation to return the collateral which is reflected as collateral received on loaned securities on the accompanying consolidated cash (out) flow. Repayment of borrowings Change in cash overdraft Proceeds from issuance of stock for share-based compensation arrangements Purchases of treasury stock Purchase of treasury stock Repurchase of Common Stock Value Purchase of treasury stock Cash flows from investing activities: Net Cash Provided by (Used in) Investing Activities Net cash flows (used in) provided by investing activities Net cash flows provided by (used in) investing activities Collateral received under securities lending Collateral received under securities lending We loan certain securities from our portfolio to other institutions. As such securities transactions are classified as loaned securities on the accompanying consolidated cash flow. Collateral for the loaned securities, consisting of cash or other securities is maintained at a rate of approximately 102% of the market value of each loaned security. The cash collateral is recorded as collateral received for loaned securities as cash (in) flows. Purchases of other investments Payments to Acquire Businesses, Net of Cash Acquired Acquisitions (Note 2) Acquisitions, net of cash acquired Acquisitions, net of cash acquired Acquisitions, net of cash acquired Purchases of property, plant and equipment Proceeds from Sale and Maturity of Marketable Securities Proceeds from maturities and sales Proceeds from sales and maturities of marketable securities Proceeds from sales and maturities of marketable securities Purchases of marketable securities Other liabilities and taxes payable Accrued expenses and other current liabilities Other assets Other assets Other assets Increase (Decrease) in Inventories Inventory Inventory Inventory Accounts receivable Accounts receivable Accounts receivable Gain (Loss) on Investments Realized gain on sale of marketable securities and strategic investments Realized loss (gain) on sale of marketable securities and strategic investments Realized gain on sale of marketable securities and strategic investments Deferred income taxes Non-cash interest (income) expense and foreign exchange remeasurement loss (gain), net Share-based compensation Depreciation and amortization of fixed and intangible assets Cash flows from operating activities: Ending Balance Beginning Balance Litigation [Text Block] Litigation Litigation. Net cash flows provided by operating activities Net income attributable to Biogen Idec Inc. Marketable securities Marketable Securities, Noncurrent Collaboration profit sharing Collateral received under securities leading Obligation under securities lending Litigation [Abstract] Segment Information [Abstract] Unconsolidated joint business Unconsolidated joint business Revenues from unconsolidated joint business consists of our share of pretax co-promotion profits in the U.S.; reimbursement of selling and development expense in the U.S.; and revenue on sales of RITUXAN outside the U.S., which consist of our share of pretax co-promotion profits in Canada. Due from unconsolidated joint business Due from unconsolidated joint business Receivable from unconsolidated joint business for our share of the pretax co-promotion profits generated from our co-promotion arrangement for sales of RITUXAN. Impairment of investments Impairments of investments (Note 7) Treasury stock, at cost; 0 and 9,207 shares at December 31, 2009 and 2008, respectively Purchases of marketable securities Purchases of property, plant and equipment Purchases of other investments Non-cash interest (income) expense and foreign exchange remeasurement loss (gain), net Product Sales Revenue, Goods, Net Cost of sales, excluding amortization of acquired intangible assets Cost of Revenue Research and development Research and Development Expense Selling, general and administrative Selling, General and Administrative Expense Income tax expense Income Tax Expense (Benefit) Cash and cash equivalents Cash and Cash Equivalents, at Carrying Valu Marketable securities Marketable Securities, Current Accounts receivable, net Accounts Receivable, Net, Current Inventory Inventory, Net Property, plant and equipment, net Property, Plant and Equipment, Net, Beginning Balance Property, Plant and Equipment, Net, Ending Balance Property, Plant and Equipment, Net Intangible assets, net Intangible Assets, Net (Excluding Goodwill) Current portion of notes payable and line of credit Notes Payable, Current Additional paid-in capital Additional Paid in Capital, Beginning Balance Additional Paid in Capital, Ending Balance Additional Paid in Capital Retained earnings Retained Earnings (Accumulated Deficit), Beginning Balance Retained Earnings (Accumulated Deficit), Ending Balance Retained Earnings (Accumulated Deficit) Net income attributable to Biogen Idec Inc. Depreciation and amortization of property, plant and equipment and intangible assets Depreciation and Amortization Share-based compensation Share-based Compensation Accrued expenses and other current liabilities Increase (Decrease) in Accrued Liabilities Repayment of borrowings Repayments of Short-term Debt Effect of exchange rate changes on cash and cash equivalents Effect of Exchange Rate on Cash and Cash Equivalents Revenues: Costs and expenses: Amortization of acquired intangible assets Amortization of Intangible Assets, Total Shares used in calculating diluted earnings per share Diluted earnings per share attributable to Biogen Idec Inc. Earnings Per Share, Diluted ASSETS Current assets: Other current assets Goodwill Beginning balance Ending balance Investments and other assets Investments and other assets Accrued expenses and other Other Liabilities, Current, Total Long-term deferred tax liability Other long-term liabilities Other long-term liabilities Preferred stock, par value $0.001 per share Preferred Stock, Value, Total Preferred Stock, Value, Beginning Balance Preferred Stock, Value, Ending Balance Common stock, par value $0.0005 per share Common Stock, Value, Total Common Stock, Value, Beginning Balance Common Stock, Value, Ending Balance Current liabilities: LIABILITIES AND SHAREHOLDERS' EQUITY Preferred stock, par value Common stock, par value Cash flows from operating activities: Non-cash interest (income) expense and foreign exchange remeasurement loss (gain), net Deferred income taxes Deferred Income Tax Expense (Benefit), Total Other liabilities and taxes payable Cash flows from investing activities: Purchases of marketable securities Payments to Acquire Marketable Securities Purchases of property, plant and equipment Payments to Acquire Property, Plant, and Equipment Purchases of other investments Cash flows from financing activities: Proceeds from issuance of stock for share-based compensation arrangements Change in cash overdraft Inventory Intangible Assets and Goodwill Fair Value Measurements Income Taxes New Accounting Pronouncements Segment Information Income from operations Operating Income (Loss) Income before income tax expense Income (Loss) from Continuing Operations before Income Taxes, Minority Interest, and Income (Loss) from Equity Method Investments Diluted earnings per share attributable to Biogen Idec Inc. Other current assets Other liabilities and taxes payable Net increase in cash and cash equivalents Cash and Cash Equivalents, Period Increase (Decrease) Treasury Stock, Value, Beginning Balance Treasury Stock, Value, Ending Balance Treasury stock, at cost Stockholders' Equity Treasury stock, at cost Liabilities and Stockholders' Equity Net Cash Provided by (Used in) Operating Activities Excess Tax Benefit from Share-based Compensation, Operating Activities Excess tax benefit from share-based compensation Excess tax benefit from share-based compensation Excess Tax Benefit from Share-based Compensation, Financing Activities Excess tax benefit from share-based compensation Entity Registrant Name Entity Central Index Key Document Type Document Period End Date Amendment Flag Current Fiscal Year End Date Entity Well-known Seasoned Issuer Entity Voluntary Filers Entity Current Reporting Status Entity Filer Category Entity Public Float Entity Common Stock, Shares Outstanding Revenues Costs and Expenses Assets, Current Assets Liabilities, Current Liabilities Accounts payable Accounts Payable, Current, Total Accounts Payable, Current Commitments and contingencies (Notes 14, 16 and 17) Commitments and Contingencies Taxes payable Taxes payable Taxes Payable, Current Write-down of inventory to net realizable value Inventory Write-down Adjustments to reconcile net income to net cash flows from operating activities: Adjustments to Reconcile Net Income (Loss) to Cash Provided by (Used in) Operating Activities [Abstract] Total shareholders' equity Beginning Balance Ending Balance Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest Noncontrolling interest, end of period Noncontrolling interest, beginning of period Stockholders' Equity Attributable to Noncontrolling Interest Noncontrolling interest Total Biogen Idec Inc. shareholders' equity Net Income (Loss) Attributable to Noncontrolling Interest Net income attributable to noncontrolling interest, net of tax Net income attributable to noncontrolling interest Net income Net income Net Income (Loss), Including Portion Attributable to Noncontrolling Interest Increase Decrease In Due From Unconsolidated Joint Business Due from unconsolidated joint business Due from unconsolidated joint business Increase Decrease In Due From Unconsolidated Joint Business. Earnings per Share Earnings Per Share [Text Block] Treasury stock, at cost Shareholders' equity: Share-based Payments Disclosure of Compensation Related Costs, Share-based Payments [Text Block] Purchases of marketable securities Purchases of property, plant and equipment Repayment of borrowings Obligation under securities lending Statements of Income [Abstract] Balance Sheets [Abstract] Statements of Cash Flows [Abstract] Components of inventories [Abstract] Inventory [Abstract] Earnings per Share [Abstract] Net income per share: New Accounting Pronouncements [Abstract] New Accounting Pronouncements [Abstract] Income Taxes [Abstract] Share-based Payments [Abstract] Derivative Instruments [Abstract] Derivative Instruments [Abstract] Income Taxes [Abstract] Balance Sheets [Abstract] Statements of Cash Flows [Abstract] Share-Based Payments [Abstract] Overview [Abstract] Business Overview [Abstract] Acquired in-process research and development Acquired in-process research and development The amount of purchased research and development assets that are acquired in a business combination have no alternative future use and are therefore written off in the period of acquisition. Changes in operating assets and liabilities, net: Increase (Decrease) in Operating Capital [Abstract] Derivative Instruments [Text Block] Derivative Instruments This note represents our disclosures related to: our hedging activities and use of derivative instruments; which includes disclosure related to our use of forward currency contracts to mitigate the risk of change in exchange rates and our use of interest rates swaps to manage our exposure to changes in interest rates and each of their respective notational amounts. Disclosures may also include the effect, if any, on our consolidated financial position and results of operations related to derivatives designated as hedging instruments. Financial Instruments [Text Block] Financial Instruments This note represents our disclosures related to: (1) Marketable Securities, Non-Marketable Securities and Other Investments which may consist of all investments in certain debt and equity securities (and other assets); including: securities held-to-maturity, trading securities, securities available-for-sale, and non-marketable securities; and (2) any unrealized gains and losses recognized on our investments due to changes in fair value as well as amounts charged to income if a decline in fair value of our investments is deemed to be other than temporary. Fair Value Measurements [Abstract] Fair Value Measurements Abstract. Intangible Assets and Goodwill [Abstract] Intangible Assets And Goodwill Abstract. Financial Instruments [Abstract] Financial Instruments [Abstract] FinancialInstrumentsAbstract. Total current liabilities Overview Organization, Consolidation and Presentation of Financial Statements Disclosure [Text Block] Revenue Recognition [Abstract] Revenue Recognition. Property, Plant and Equipment Property, Plant and Equipment Disclosure [Text Block] Other Income (Expense), Net Other Income And Other Expense Disclosure Text Block Other Income (Expense), Net [Abstract] Consolidated Variable Interest Entities Schedule of Variable Interest Entities [Text Block] Acquired in-process research and development Weighted-average shares used in calculating: Weighted Average Number of Shares Outstanding, Diluted [Abstract] Shareholders' equity: Property, Plant and Equipment [Abstract] Inventory (Tables) [Abstract] Inventory. Inventory (Details) [Abstract] Inventory. Raw materials Inventory, Raw Materials, Total Inventory, Raw Materials Work in process Inventory, Work in Process Finished goods Inventory, Finished Goods Property, Plant and Equipment (Details) [Abstract] Property, Plant and Equipment. Accumulated depreciation Accumulated Depreciation, Depletion and Amortization, Property, Plant, and Equipment, Total Accumulated Depreciation, Depletion and Amortization, Property, Plant, and Equipment, Beginning Balance Accumulated Depreciation, Depletion and Amortization, Property, Plant, and Equipment, Ending Balance Accumulated Depreciation, Depletion and Amortization, Property, Plant, and Equipment New Accounting Pronouncements (Policies) [Abstract] New Accounting Pronouncements. Earnings per Share (Tables) [Abstract] Earnings per share. Basic and diluted earnings per share [Text Block] Basic and diluted earnings per share Basic and diluted earnings per share. Earnings per share Earnings per Share (Details) [Abstract] Earnings per share. Basic and diluted earnings per share [Abstract] Basic And Diluted Earnings Per Share Abstract Basic and diluted earnings per share. Anti dilutive securities excluded from computation of basic and dilutive earnings per share [Abstract] Anti dilutive Securities Excluded From Computation Of Basic And Dilutive Earnings Per Share Abstract Anti dilutive Securities Excluded from Computation of Basic And Dilutive Earnings Per Share. Numerator: Numerator Abstract Numerator. Denominator: Denominator Abstract Denominator. Net income used in calculating basic and diluted earnings per share Net income attributable to Biogen Idec Inc. Adjustment for net income allocable to preferred stock Preferred Stock Dividends and Other Adjustments Numerator: Numerator Of Anti dilutive Securities Numerator Of Anti dilutive Securities. Denominator: Denominator Of Anti dilutive Securities Denominator Of Anti dilutive Securities. Total Antidilutive Securities Excluded from Computation of Earnings Per Share, Amount Net income allocable to preferred stock Income Taxes (Tables) [Abstract] Income Taxes. Reconciliation Between The Federal Statutory Tax Rate And Effective Tax Rate [Text Block] Reconciliation between the U.S. federal statutory tax rate and effective tax rate Reconciliation between the federal statutory tax rate and effective tax rate. Income Taxes (Details) [Abstract] Income Taxes. Reconciliation between the U.S. federal statutory tax rate and effective tax rate [Abstract] Effective Income Tax Rate, Continuing Operations, Tax Rate Reconciliation [Abstract] Other Effective Income Tax Rate Reconciliation, Other Adjustments Effective Income Tax Rate, Continuing Operations Effective tax rate Effective tax rate Statutory rate Effective Income Tax Rate Reconciliation, at Federal Statutory Income Tax Rate State taxes Effective Income Tax Rate Reconciliation, State and Local Income Taxes Credits and net operating loss utilization Effective Income Tax Rate Reconciliation, Tax Credits, Total Effective Income Tax Rate Reconciliation, Tax Credits Assessment of Tax Including Penalties and Interest Notice of Assessment of corporate excise tax including penalties and interest The amount associated with the Notice of Assessement of corporate tax including penalties and interest. Other Income (Expense), Net (Tables) [Abstract] Other Income (Expense), Net. Other income (expense), net [Text Block] Other income (expense), net Other income (expense), net. Other Income (Expense), Net (Details) [Abstract] Other Income (Expense), Net. Other Income (Expense), Net [Abstract] Other Income Expense Net Abstract Other Income (Expense), Net. Interest income Investment Income, Interest Interest Expense Interest expense Interest Expense, Total Interest expense Foreign Currency Transaction Gain (Loss), before Tax Foreign Currency Transaction Gain (Loss), before Tax, Total Net gains (losses) on foreign currency transactions Revenue Recognition (Details) [Abstract] Revenue Recognition. Revenue Recognition (Tables) [Abstract] Revenue Recognition. Time Sharing Transactions, Allowance for Uncollectible Accounts [Roll Forward] Analysis of the amount of, and change in, product revenue reserves [Abstract] Statement [Table] Statement [Line Items] Dilutive Securities [Abstract] Incremental Common Shares Attributable to Share-based Payment Arrangements Incremental Common Shares Attributable to Share-based Payment Arrangements, Total Stock options and employee stock purchase plan Weighted Average Number Diluted Shares Outstanding Adjustment Components of inventories [Text Block] Components of inventories Components of inventories. Anti dilutive securities excluded from computation of basic and dilutive earnings per share Schedule of Antidilutive Securities Excluded from Computation of Earnings Per Share [Text Block] Effect of dilutive securities: Income Taxes (Textuals) [Abstract] Income Taxes Textuals Abstract Income Taxes Textuals. IPR&D Effective Income Tax Rate Reconciliation, Nondeductible Expense, Research and Development Document Fiscal Year Focus Net income used in calculating basic and diluted earnings per share Net Income (Loss) Available to Common Stockholders, Basic