Disclosure of Compensation Related Costs, Share-based Payments [Abstract] Share-based Payments Disclosure of Compensation Related Costs, Share-based Payments [Text Block] 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 Available-for-sale Securities, Current Accounts receivable, net Accounts Receivable, Net, Current Due from anti-CD20 therapeutic programs Due from unconsolidated joint business Receivable from unconsolidated joint business for our share of the pre-tax co-promotion profits generated from our co-promotion arrangement for sales of RITUXAN. Inventory Inventory, Net Other current assets Other Assets, Current Total current assets Assets, Current Marketable securities Available-for-sale Securities, Noncurrent Property, plant and equipment, net Property, Plant and Equipment, Net Intangible assets, net Intangible Assets, Net (Excluding Goodwill) Goodwill Goodwill Investments and other assets Other Assets, Noncurrent Total assets Assets LIABILITIES AND EQUITY Liabilities and Equity [Abstract] Current liabilities: Liabilities, Current [Abstract] Current portion of notes payable and other financing arrangements Debt, Current Taxes payable Taxes Payable, Current Accounts payable Accounts Payable, Current Accrued expenses and other Other Liabilities, Current Total current liabilities Liabilities, Current Notes payable and other financing arrangements Long-term Debt Deferred tax liability Deferred Tax Liabilities, Net, Noncurrent Other long-term liabilities Other Liabilities, Noncurrent Total liabilities Liabilities Commitments and contingencies Commitments and Contingencies Equity: Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest [Abstract] Biogen Idec Inc. shareholders' equity Stockholders' Equity Attributable to Parent [Abstract] Preferred stock, par value $0.001 per share Preferred Stock, Value, Issued Common stock, par value $0.0005 per share Common Stock, Value, Issued Additional paid-in capital Additional Paid in Capital Accumulated other comprehensive loss Accumulated Other Comprehensive Income (Loss), Net of Tax Retained earnings Retained Earnings (Accumulated Deficit) Treasury stock, at cost; 23.8 million and 22.6 million shares, respectively Treasury Stock, Value Total Biogen Inc. shareholders’ equity Stockholders' Equity Attributable to Parent Noncontrolling interests Stockholders' Equity Attributable to Noncontrolling Interest Total equity Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest Total liabilities and equity Liabilities and Equity Investments, All Other Investments [Abstract] Financial Instruments (Textual) Financial Instruments (Textual) [Abstract] Financial Instruments. Original maturities of commercial paper and short-term debt securities Debt Instrument, Maturity Date, Description Average maturity of marketable securities, months Average Maturity Of Marketable Securities Average remaining time to maturity of marketable debt securities, available-for-sale securities. Business Combinations [Abstract] Schedule of Business Acquisitions, by Acquisition [Table] Schedule of Business Acquisitions, by Acquisition [Table] Products and Services [Axis] Products and Services [Axis] Products and Services [Domain] Products and Services [Domain] BIIB074 BIIB074 [Member] BIIB074 [Member] Business Acquisition [Axis] Business Acquisition [Axis] Business Acquisition, Acquiree [Domain] Business Acquisition, Acquiree [Domain] Remedy Pharmaceutical [Member] Remedy Pharmaceutical [Member] Remedy Pharmaceutical [Member] Convergence Pharmaceuticals Convergence Pharmaceuticals [Member] Convergence Pharmaceuticals [Member] Business Acquisition [Line Items] Business Acquisition [Line Items] Acquired in-process research and development Research and Development in Process Cash portion of consideration Payments to Acquire Businesses, Gross Potential future milestone payments Potential Future Milestone Payments Commitment To Third Party Approximately Potential future milestone payments commitment to third party approximately. Contingent consideration obligation Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Purchases Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Net Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Net In-process research and development Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Indefinite-Lived Intangible Assets Goodwill Commitments and Contingencies Disclosure [Abstract] Minimum rental commitments under non-cancelable leases Minimum Rental Commitments Under Noncancellable Leases [Abstract] Minimum rental commitments under noncancellable leases. Minimum lease payments, 2018 Operating Leases, Future Minimum Payments Due, Next Twelve Months Minimum lease payments, 2019 Operating Leases, Future Minimum Payments, Due in Two Years Minimum lease payments, 2020 Operating Leases, Future Minimum Payments, Due in Three Years Minimum lease payments, 2021 Operating Leases, Future Minimum Payments, Due in Four Years Minimum lease payments, 2022 Operating Leases, Future Minimum Payments, Due in Five Years Minimum lease payments, Thereafter Operating Leases, Future Minimum Payments, Due Thereafter Minimum lease payments, Total Operating Leases, Future Minimum Payments Due Less: income from subleases, 2018 Future Minimum Sublease Rentals, Sale Leaseback Transactions, Next Twelve Months Less: income from subleases, 2019 Future Minimum Sublease Rentals, Sale Leaseback Transactions, within Two Years Less: income from subleases, 2020 Future Minimum Sublease Rentals, Sale Leaseback Transactions, within Three Years Less: income from subleases, 2021 Future Minimum Sublease Rentals, Sale Leaseback Transactions, within Four Years Less: income from subleases, 2022 Future Minimum Sublease Rentals, Sale Leaseback Transactions, within Five Years Less: income from subleases, Thereafter Future Minimum Sublease Rentals, Sale Leaseback Transactions, Thereafter Less: income from subleases, Total Future Minimum Sublease Rentals, Sale Leaseback Transactions Net minimum lease payments, 2018 Net Minimum Lease Payments With In One Year Net minimum lease payments with in one year. Net minimum lease payments, 2019 Net Minimum Lease Payments With In Two Year Net minimum lease payments with in two year. Net minimum lease payments, 2020 Net Minimum Lease Payments With In Three Year Net minimum lease payments with in three year. Net minimum lease payments, 2021 Net Minimum Lease Payments With In Four Year Net minimum lease payments with in four year. Net minimum lease payments, 2022 Net Minimum Lease Payments With In Five Year Net minimum lease payments with in five year. Net minimum lease payments, Thereafter Net Minimum Lease Payments Thereafter Net minimum lease payments thereafter. Net minimum lease payments, Total Net Minimum Lease Payments Net minimum lease payments total. Debt Disclosure [Abstract] Schedule of Long-term Debt Instruments [Table] Schedule of Long-term Debt Instruments [Table] Eisai Eisai [Member] Eisai [Member] Derivative Instrument Risk [Axis] Derivative Instrument [Axis] Derivative Contract Type [Domain] Derivative Contract [Domain] Interest rate swap Interest Rate Swap [Member] Debt Instrument [Axis] Debt Instrument [Axis] Debt Instrument, Name [Domain] Debt Instrument, Name [Domain] 6.875% Senior Notes due 2018 Senior Notes Six Point Eight Seven Five Percent Due Twenty Eighteen [Member] Senior Notes Six Point Eight Seven Five Percent Due Twenty Eighteen. 2.90% Senior Notes due 2020 Senior Notes Two Point Nine Percent Due Twenty Twenty [Member] Senior Notes Two Point Nine Percent Due Twenty Twenty [Member] 3.625% Senior Notes due 2022 Senior Notes Three Point Six Two Five Percent Due Twenty Twenty Two [Member] Senior Notes Three Point Six Two Five Percent Due Twenty Twenty Two [Member] 4.05% Senior Notes due 2025 Senior Notes Four Point Zero Five Percent Due Twenty Twenty Five [Member] Senior Notes Four Point Zero Five Percent Due Twenty Twenty Five [Member] 5.20% Senior Notes due 2045 Senior Notes Five Point Two Percent Due Twenty Forty Five [Member] Senior Notes Five Point Two Percent Due Twenty Forty Five [Member] Note payable to Fumedica Notes Payable, Other Payables [Member] Debt Instrument Debt Instrument [Line Items] Interest rate on senior notes Debt Instrument, Interest Rate, Stated Percentage Redemption percentage par value of senior notes Percentage Par Value Of Senior Notes Percentage Par Value Of Senior Notes Increased carrying amount of interest rate swap Increased Carrying Amount Of Senior Notes Increased carrying amount of senior notes. Net charge upon extinguishment of debt Extinguishment of Debt, Gain (Loss), Net of Tax Payment for debt extinguishment Payment for Debt Extinguishment or Debt Prepayment Cost Senior Notes aggregate principal amount Debt Instrument, Face Amount Costs associated with the Senior Notes offerings Payments of Financing Costs Redemptions percentage of 2015 notes Debt Instrument, Redemption Price, Percentage Redemption percentage for change in control provision on the 2015 Senior Notes Debt Instrument, Make Whole Provision Redemption Price, Percentage Debt Instrument, Make Whole Provision Redemption Price, Percentage Fair value of the interest rate swaps Derivative Asset, Fair Value, Gross Asset Notes payable and other financing arrangements Notes Payable, Noncurrent Par value of notes payable Par Value Of Notes Payable Par value of notes payable. Senior unsecured revolving credit facility maximum borrowing capacity Line of Credit Facility, Maximum Borrowing Capacity Term of credit facility Line of Credit Facility, Expiration Period Amount outstanding under the credit facility Line of Credit Facility, Fair Value of Amount Outstanding Facility purchase Property, Plant and Equipment, Additions Retirement Benefits [Abstract] Schedule of Defined Benefit Plans Disclosures [Table] Schedule of Defined Benefit Plans Disclosures [Table] Retirement Plan Type [Axis] Retirement Plan Type [Axis] Retirement Plan Type [Domain] Retirement Plan Type [Domain] Foreign Plan Foreign Plan [Member] Geographical [Axis] Geographical [Axis] Geographical [Domain] Geographical [Domain] SWITZERLAND SWITZERLAND GERMANY GERMANY Defined Benefit Plan Disclosure [Line Items] Defined Benefit Plan Disclosure [Line Items] Employee Benefit Plans (Textual) [Abstract] Employee Benefit Plans (Textual) [Abstract] Employee benefit plans. Percentage of minimum investment return Percentage Of Minimum Investment Return Percentage of minimum investment return. Unfunded net pension Defined Benefit Plan, Funded (Unfunded) Status of Plan Employee Benefit Plan obligations Defined Benefit Plan, Benefit Obligation Pension Cost (Reversal of Cost) Pension Cost (Reversal of Cost) Periodic pension cost Defined Benefit Plan, Net Periodic Benefit Cost (Credit) Equity [Abstract] Statement [Table] Statement [Table] Equity Components [Axis] Equity Components [Axis] Equity Component [Domain] Equity Component [Domain] Unrealized Gains (Losses) on Securities Available for Sale Accumulated Net Investment Gain (Loss) Attributable to Parent [Member] Unrealized Gains (Losses) on Cash Flow Hedges Accumulated Net Gain (Loss) from Cash Flow Hedges Attributable to Parent [Member] Reclassification out of Accumulated Other Comprehensive Income [Axis] Reclassification out of Accumulated Other Comprehensive Income [Axis] Reclassification out of Accumulated Other Comprehensive Income [Domain] Reclassification out of Accumulated Other Comprehensive Income [Domain] Reclassification out of Accumulated Other Comprehensive Income [Member] Reclassification out of Accumulated Other Comprehensive Income [Member] Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items] Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items] Other income (expense) Other Nonoperating Income (Expense) Income tax benefit (expense) Income Tax Expense (Benefit) Product, net Sales Revenue, Goods, Net Operating Expenses Operating Expenses Nonoperating Income (Expense) Nonoperating Income (Expense) Net income attributable to Biogen Inc. Net Income (Loss) Attributable to Parent Sales Discounts, Returns and Allowances, Goods [Abstract] Balance Sheet Location [Axis] Balance Sheet Location [Axis] Balance Sheet Location [Domain] Balance Sheet Location [Domain] Reduction of accounts receivable Accounts Receivable [Member] Component of accrued expenses and other Other Current Liabilities [Member] Statement [Line Items] Statement [Line Items] Total reserves Valuation Allowances and Reserves, Balance Investments in Variable Interest Entities [Abstract] Investments In Variable Interest Entities. Investments in Variable Interest Entities Variable Interest Entity Disclosure [Text Block] Organization, Consolidation and Presentation of Financial Statements [Abstract] Co-promotion profit sharing formula Co Promotion Profit Sharing Formula [Table Text Block] Co-promotion profit sharing formula. Pretax profit sharing formula Pretax Profit Sharing Formula [Table Text Block] Pretax Profit Sharing Formula [Table Text Block] Revenues from anti-CD20 therapeutic programs Revenues from anti-CD20 therapeutic programs [Table Text Block] Revenues from anti-CD20 therapeutic programs Summary of activity related to collaboration with AbbVie Inc. Summary Of Activity Related To Collaboration With Company Seven [Table Text Block] Summary of activity related to collaboration with company seven. Summary of activity related to collaboration with Eisai Summary Of Activity Related To Collaboration With Company Eight [Table Text Block] [Table Text Block] Summary Of Activity Related To Collaboration With Company Eight [Table Text Block] [Table Text Block] Schedule of Cash and Cash Equivalents [Table] Schedule of Cash and Cash Equivalents [Table] Investment Type [Axis] Investment Type [Axis] Investment Type Categorization [Domain] Investments [Domain] Commercial Paper Commercial Paper [Member] Overnight Reverse Repurchase Agreements Repurchase Agreements [Member] Money Market Funds Money Market Funds [Member] Short-term Debt Securities Debt Securities [Member] Cash and Cash Equivalents [Line Items] Cash and Cash Equivalents [Line Items] Summary of financial assets with original maturities of less than 90 days included within cash and cash equivalents Cash Equivalents, at Carrying Value [Abstract] Cash equivalents Cash Equivalents, at Carrying Value Schedule of Share-based Compensation Arrangements by Share-based Payment Award [Table] Schedule of Share-based Compensation Arrangements by Share-based Payment Award [Table] Schedule of Share-based Compensation Arrangement by Share-based Payment Award, Award Type and Plan Name [Axis] Award Type [Axis] Share-based Compensation Arrangements by Share-based Payment Award, Award Type and Plan Name [Domain] Equity Award [Domain] Market stock units Market Stock Units [Member] Market stock units. Range [Axis] Range [Axis] Range [Domain] Range [Domain] Minimum Minimum [Member] Maximum Maximum [Member] Share-based Compensation Arrangement by Share-based Payment Award Share-based Compensation Arrangement by Share-based Payment Award [Line Items] Assumptions used in valuation of market based stock units Valuation of Granted Stock Units [Abstract] Valuation of granted stock units. Expected dividend yield Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions, Expected Dividend Rate Range of expected stock price volatility Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions, Expected Volatility Rate Minimum range of risk-free interest rates Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions, Risk Free Interest Rate, Minimum Maximum range of risk-free interest rates Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions, Risk Free Interest Rate, Maximum Average stock price on grant date minimum Average Stock Price On Grant Date In Dollars Minimum Average stock price on grant date in dollars minimum. Average stock price on grant date maximum Average Stock Price On Grant Date In Dollars Maximum Average stock price on grant date in dollars maximum. Weighted average grant date fair value Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period, Weighted Average Grant Date Fair Value Market stock units activity Market Stock Units Activity [Abstract] Market stock units activity. Beginning Balance, Unvested, Shares Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number Beginning Balance, Weighted Average Grant Date Fair Value Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Weighted Average Grant Date Fair Value Shares, Hemophilia spin-off adjustment Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Period Increase (Decrease) Weighted Average Exercise Price, Hemophilia spin-off adjustment Share-based Compensation Arrangements by Share-based Payment Award, Options, Other Share Increase (Decrease) in Period, Weighted Average Exercise Price Shares, Granted Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period Weighted Average Grant Date Fair Value, Granted Shares, Vested Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Vested in Period Weighted Average Grant Date Fair Value, Vested Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Vested in Period, Weighted Average Grant Date Fair Value Shares, Forfeited Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Forfeited in Period Weighted Average Grant Date Fair Value, Forfeited Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Forfeitures, Weighted Average Grant Date Fair Value Ending Balance, Unvested, Shares Ending Balance, Weighted Average Grant Date Fair Value Supplemental cash flow information Schedule of Cash Flow, Supplemental Disclosures [Table Text Block] Other income (expense), net Schedule of Other Nonoperating Income (Expense) [Table Text Block] Accrued expenses and other Schedule of Accrued Liabilities [Table Text Block] Share-based compensation expense included in consolidated statements of income Schedule of Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Table Text Block] Summary of share-based compensation expense associated with each of our share-based compensating programs Schedule of Compensation Cost for Share-based Payment Arrangements, Allocation of Share-based Compensation Costs by Plan [Table Text Block] Stock option activity Share-based Compensation, Stock Options, Activity [Table Text Block] Tax benefit and cash received from stock option exercises Cash Proceeds Received and Tax Benefit from Share-based Payment Awards [Table Text Block] Market stock units activity Schedule Of Market Stock Units Activity [Table Text Block] Schedule Of Market Stock Units Activity. Assumptions used in valuation of market based stock units Assumptions Used In Valuation Of Market Based Stock Units [Table Text Block] Assumptions used in valuation of market based stock units. Cash settled performance shares activity Schedule Of Cash Settled Performance Shares Activity [Table Text Block] Schedule Of Cash Settled Performance Shares Activity. Performance units activity Performance units activity [Table Text Block] Performance units activity [Table Text Block] Time-vested restricted stock units activity Time Vested Restricted Stock Units Activity [Table Text Block] Time Vested Restricted Stock Units Activity. Shares issued under employee stock purchase plan Shares Issued Under Employee Stock Purchase Plan [Table Text Block] Shares issued under employee stock purchase plan. Fair Value Disclosures [Abstract] Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Table] Fair Value Measurements, Recurring and Nonrecurring [Table] Fair Value, Hierarchy [Axis] Fair Value, Hierarchy [Axis] Fair Value, Measurements, Fair Value Hierarchy [Domain] Fair Value Hierarchy [Domain] Quoted Prices in Active Markets (Level 1) Fair Value, Inputs, Level 1 [Member] Significant Other Observable Inputs (Level 2) Fair Value, Inputs, Level 2 [Member] Significant Unobservable Inputs (Level 3) Fair Value, Inputs, Level 3 [Member] Fair Value by Measurement Frequency [Axis] Measurement Frequency [Axis] Fair Value, Measurement Frequency [Domain] Fair Value, Measurement Frequency [Domain] Fair Value, Measurements, Recurring Fair Value, Measurements, Recurring [Member] Fair Value by Asset Class [Axis] Asset Class [Axis] Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, by Asset Class [Domain] Asset Class [Domain] Corporate debt securities Corporate Debt Securities [Member] Government securities US Treasury and Government [Member] Mortgage and other asset backed securities Asset-backed Securities [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] Summary of assets and liabilities recorded at fair value Fair Value, Assets, Liabilities and Stockholders' Equity Measured on Recurring Basis [Abstract] Assets: Assets, Fair Value Disclosure [Abstract] Cash equivalents Cash and Cash Equivalents, Fair Value Disclosure Marketable debt securities Available-for-sale Securities Marketable equity securities Investments, Fair Value Disclosure Derivative contracts Foreign Currency Contract, Asset, Fair Value Disclosure Plan assets for deferred compensation Plan Assets For Deferred Compensation Fair Value Disclosure Fair values as of the balance sheet date of all assets of deferred compensation plans. Total Assets, Fair Value Disclosure Liabilities: Liabilities, Fair Value Disclosure [Abstract] Derivative contracts Foreign Currency Contracts, Liability, Fair Value Disclosure Contingent consideration obligations Business Combination, Contingent Consideration, Liability Total Financial and Nonfinancial Liabilities, Fair Value Disclosure Loss Contingency, Information about Litigation Matters [Abstract] Litigation Legal Matters and Contingencies [Text Block] Earnings Per Share [Abstract] Earnings per Share Earnings Per Share [Text Block] Quarterly Financial Information Disclosure [Abstract] Schedule Of Development Milestone And Collaboration [Table] Schedule Of Development Milestone And Collaboration [Table] Schedule of development milestone and collaboration. TECFIDERA TECFIDERA [Member] TECFIDERA [Member] SPINRAZA SPINRAZA [Member] SPINRAZA [Member] Facility Location [Axis] Facility Location [Axis] -- None. No documentation exists for this element. -- Facility Location [Domain] Facility Location [Domain] -- None. No documentation exists for this element. -- Cambridge, MA Cambridge, MA [Member] Cambridge, MA [Member] Variable Interest Entities [Axis] Variable Interest Entities [Axis] Variable Interest Entity, Classification [Domain] Variable Interest Entity, Classification [Domain] Neurimmune Neurimmune [Member] Neurimmune. Collaborative Arrangements and Non-collaborative Arrangements [Axis] Type of Arrangement and Non-arrangement Transactions [Axis] Collaborative Arrangements and Non-collaborative Arrangement Transactions Arrangements and Non-arrangement Transactions [Domain] Bristol-Myers Squibb [Member] Bristol-Myers Squibb [Member] Bristol-Myers Squibb [Member] iPerian iPerian [Member] iPerian [Member] Alkermes Alkermes [Member] Alkermes [Member] Ionis Pharmaceuticals, Inc. Ionis Pharmaceuticals [Member] Ionis Pharmaceuticals [Member] Schedule Of Development Milestone And Collaboration [Line Items] Schedule Of Development Milestone And Collaboration [Line Items] Schedule Of Development Milestone And Collaboration. Amortization of Intangible Assets Amortization of Intangible Assets Research and development Research and Development Expense (Excluding Acquired in Process Cost) Developmental Milestone Payment Developmental Milestone Payment Developmental Milestone Payment Net income (loss) attributable to noncontrolling interests, net of tax Net Income (Loss) Attributable to Noncontrolling Interest Reduction in royalty rate payable on commercial sales Reduction in royalty rate payable on commercial sales Reduction in royalty rate payable on commercial sales Upfront And Milestone Payments Made To Collaborative Partner Upfront And Milestone Payments Made To Collaborative Partner Total upfront and milestone payments made to collaborative partner. Accrued income tax liability under the Transition Toll Tax Deferred Tax Assets, Gross, Noncurrent Accelerated depreciation Restructuring and Related Cost, Accelerated Depreciation License Fee License Fee License Fee TECFIDERA litigation settlement charge Litigation Settlement, Expense Time-vested restricted stock units Time Vested Restricted Stock Units [Member] Time vested restricted stock units. Time-vested restricted stock units Restricted Stock Units Activity [Abstract] Restricted stock units activity. Schedule of accumulated other comprehensive income (loss) Schedule of Accumulated Other Comprehensive Income (Loss) [Table Text Block] Reclassification out of accumulated other comprehensive income Reclassification out of Accumulated Other Comprehensive Income [Table Text Block] Goodwill and Intangible Assets Disclosure [Abstract] Schedule of Goodwill [Table] Schedule of Goodwill [Table] Disposal Group Name [Axis] Disposal Group Name [Axis] Disposal Group Name [Domain] Disposal Group Name [Domain] Bioverativ Bioverativ [Member] Bioverativ [Member] Fumapharm AG Fumapharm AG [Member] Fumapharm AG [Member] Goodwill [Line Items] Goodwill [Line Items] Elimination of goodwill allocated to our hemophilia business Disposal Group, Including Discontinued Operation, Goodwill Accumulated impairment losses related to goodwill Goodwill, Impaired, Accumulated Impairment Loss Summary of roll forward of the changes in goodwill Goodwill [Roll Forward] Goodwill, beginning of period Increase to goodwill Goodwill, Acquired During Period Other Goodwill, Other Increase (Decrease) Goodwill, end of period Basic and diluted earnings per share Schedule of Earnings Per Share, Basic and Diluted [Table Text Block] Schedule of Employee Stock Ownership Plan (ESOP) Disclosures [Table] Schedule of Employee Stock Ownership Plan (ESOP) Disclosures [Table] Plan Name [Axis] Plan Name [Axis] Plan Name [Domain] Plan Name [Domain] 2015 ESPP 2015 ESPP [Member] 2015 ESPP [Member] 1995 ESPP 1995 ESPP [Member] 1995 ESPP [Member] Employee Stock Ownership Plan (ESOP) Disclosures [Line Items] Employee Stock Ownership Plan (ESOP) Disclosures [Line Items] Shares issued under ESPP Stock Issued During Period, Shares, Employee Stock Purchase Plans Cash received under ESPP Stock Issued During Period, Value, Employee Stock Purchase Plan Fair Value Measurements Fair Value Disclosures [Text Block] Subsequent Events [Abstract] Subsequent Event [Table] Subsequent Event [Table] Karyopharm Karyopharm [Member] Karyopharm [Member] Subsequent Event Type [Axis] Subsequent Event Type [Axis] Subsequent Event Type [Domain] Subsequent Event Type [Domain] Subsequent Event Subsequent Event [Member] Subsequent Event [Line Items] Subsequent Event [Line Items] Estimated Additional Payments Upon Achievement Of Development And Commercial Milestones Estimated Additional Payments Upon Achievement Of Development And Commercial Milestones Estimated additional payments upon achievement of development and commercial milestones. Quarterly Financial Data (Unaudited) Quarterly Financial Information [Text Block] Schedule of Collaborative Arrangements and Non-collaborative Arrangement Transactions [Table] Schedule of Collaborative Arrangements and Non-collaborative Arrangement Transactions [Table] AbbVie AbbVie [Member] AbbVie Collaborative arrangements and non-collaborative arrangement transactions Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items] Product revenues, net Revenues Costs and expenses Costs and Expenses Co-promotion profits and losses Co-promotion profits and losses Co-promotion profits and losses Biogen's share of co-promotion losses Share Of Co Promotion Profits Share of co promotion profits. Schedule of Business Acquisitions by Acquisition, Contingent Consideration [Table] Schedule of Business Acquisitions by Acquisition, Contingent Consideration [Table] Contingent Consideration by Type [Axis] Contingent Consideration by Type [Axis] Contingent Consideration Type [Domain] Contingent Consideration Type [Domain] Other long-term liabilities Other Noncurrent Liabilities [Member] Accrued expenses and other Accrued Liabilities [Member] 6.875% Senior Notes due 2018 Stromedix, Inc. Stromedix [Member] Stromedix. Biogen Idec International Neuroscience GmbH Biogen Idec International Neuroscience Member Biogen idec international neuroscience. Business Acquisition, Contingent Consideration [Line Items] Business Acquisition, Contingent Consideration [Line Items] Interest rate on senior notes 6.875% Senior Notes aggregate principal amount Contingent consideration obligations Business Combination, Contingent Consideration, Liability, Noncurrent Milestone converted to short-term obligation Other Liabilities Fair value measurements, changes in valuation techniques Fair Value Measurements, Changes in Valuation Techniques Additions Contingent consideration obligations Maximum contingent consideration in the form of development and approval milestones Business Combination, Contingent Consideration Arrangements, Range of Outcomes, Value, High Discount rate used for net cash outflow projections for fair value measurement Fair Value Inputs, Discount Rate Milestone payments made during period Milestone Payments Made During Period Milestone Payments Made During Period. Income Tax Disclosure [Abstract] Reconciliation between the U.S. federal statutory tax rate and effective tax rate Effective Income Tax Rate Reconciliation, Percent [Abstract] Statutory rate Effective Income Tax Rate Reconciliation, at Federal Statutory Income Tax Rate, Percent State taxes Effective Income Tax Rate Reconciliation, State and Local Income Taxes, Percent Taxes on foreign earnings Effective Income Tax Rate Reconciliation, Foreign Income Tax Rate Differential, Percent Credits and net operating loss utilization Effective Income Tax Rate Reconciliation, Tax Credit, Percent Purchased intangible assets Effective Income Tax Rate Reconciliation, Nondeductible Expense, Depreciation and Amortization, Percent Manufacturing deduction Effective income tax rate reconciliation manufacturing deduction Effective income tax rate reconciliation manufacturing deduction 2017 Tax Act Effective Income Tax Rate Reconciliation, Change in Enacted Tax Rate, Percent Impairment of ZINBRYTA related tax assets Effective Income Tax Rate Reconciliation, Nondeductible Expense, Impairment Losses, Percent Permanent items Effective Income Tax Rate Reconciliation, Nondeductible Expense, Other, Percent Other Effective Income Tax Rate Reconciliation, Other Adjustments, Percent Effective tax rate Effective Income Tax Rate Reconciliation, Percent Collaborative and Other Relationships Collaborative Arrangement Disclosure [Text Block] Property, Plant and Equipment [Abstract] Components of property, plant and equipment, net Property, Plant and Equipment [Table Text Block] Indebtedness Debt Disclosure [Text Block] Quarterly Financial Data (Unaudited) Quarterly Financial Information [Table Text Block] Intangible Assets and Goodwill Goodwill and Intangible Assets Disclosure [Text Block] Restructuring and Related Activities [Abstract] Charges and spending related to our 2015 restructuring program Schedule of Restructuring Reserve by Type of Cost [Table Text Block] Notes Payable to Fumedica Notes Payable To Fumedica [Member] Notes payable. Notes payable, fair value Notes Payable, Fair Value Disclosure Total fair value Debt Instrument, Fair Value Disclosure Income (Loss) from Continuing Operations, Including Portion Attributable to Noncontrolling Interest [Abstract] Income (Loss) from Continuing Operations, Net of Tax, Including Portion Attributable to Noncontrolling Interest [Abstract] Income before income taxes (benefit): Income (Loss) from Continuing Operations before Equity Method Investments, Income Taxes, Noncontrolling Interest [Abstract] Domestic Income (Loss) from Continuing Operations before Income Taxes, Domestic Foreign Income (Loss) from Continuing Operations before Income Taxes, Foreign Income before income tax expense and equity in loss of investee, net of tax Income (Loss) from Continuing Operations before Equity Method Investments, Income Taxes, Noncontrolling Interest Income tax expense (benefit): Income Tax Expense (Benefit), Continuing Operations [Abstract] Current Current Income Tax Expense (Benefit), Continuing Operations [Abstract] Federal Current Federal Tax Expense (Benefit) State Current State and Local Tax Expense (Benefit) Foreign Current Foreign Tax Expense (Benefit) Total Current Income Tax Expense (Benefit) Deferred Deferred Income Tax Expense (Benefit), Continuing Operations [Abstract] Federal Deferred Federal Income Tax Expense (Benefit) State Deferred State and Local Income Tax Expense (Benefit) Foreign Deferred Foreign Income Tax Expense (Benefit) Total Deferred Income Tax Expense (Benefit) Total income tax expense Accounting Policies [Abstract] Property, plant and equipment estimated useful lives Schedule Of Property Plant And Equipment Useful Lives [Table Text Block] Schedule of property plant and equipment useful lives. Total debt maturities Long-term Debt, Fiscal Year Maturity [Abstract] 2018 Long-term Debt, Maturities, Repayments of Principal in Next Twelve Months 2019 Long-term Debt, Maturities, Repayments of Principal in Year Two 2020 Long-term Debt, Maturities, Repayments of Principal in Year Three 2021 Long-term Debt, Maturities, Repayments of Principal in Year Four 2022 Long-term Debt, Maturities, Repayments of Principal in Year Five 2023 and thereafter Long-term Debt, Maturities, Repayments of Principal after Year Five Total Selected Quarterly Financial Information [Abstract] Selected Quarterly Financial Information [Abstract] Product, net Revenues from anti-CD20 therapeutic programs Revenues from anti-CD20 therapeutic programs Revenues from anti-CD20 therapeutic programs consists of our share of pre-tax 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 pre-tax co-promotion profits in Canada. Other Other Revenue, Net Total revenues Gross Profit Gross Profit Net income Net Income (Loss), Including Portion Attributable to Noncontrolling Interest Net income attributable to Biogen Idec Inc Basic earnings per share attributable to Biogen Inc. Earnings Per Share, Basic Diluted earnings per share attributable to Biogen Inc. Earnings Per Share, Diluted Basic earnings per share attributable to Biogen Inc. Weighted Average Number of Shares Outstanding, Basic Diluted earnings per share attributable to Biogen Inc. Weighted Average Number of Shares Outstanding, Diluted Income before income tax provision and the income tax expense Schedule of Components of Income Tax Expense (Benefit) [Table Text Block] Components of deferred tax assets and liabilities Schedule of Deferred Tax Assets and Liabilities [Table Text Block] Reconciliation between the U.S. federal statutory tax rate and effective tax rate Schedule of Effective Income Tax Rate Reconciliation [Table Text Block] Reconciliation of beginning and ending amount of unrecognized tax benefits Schedule of Unrecognized Tax Benefits Roll Forward [Table Text Block] Schedule of Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Table] Schedule of Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Table] Income Statement Location [Axis] Income Statement Location [Axis] Income Statement Location [Domain] Income Statement Location [Domain] Research and development Research and Development Expense [Member] Selling, general and administrative Selling, General and Administrative Expenses [Member] Restructuring charges Restructuring Charges [Member] Total share-based compensation expense, net of tax Parent [Member] Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items] Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items] Share-based Compensation Expense included in consolidated statements of income Share-based Compensation, Allocation and Classification in Financial Statements [Abstract] Share-based compensation expense Allocated Share-based Compensation Expense Subtotal Allocated Share-based Compensation Expense, Net of Tax Capitalized share-based compensation costs Employee Service Share-based Compensation, Allocation of Recognized Period Costs, Capitalized Amount Share-based compensation expense included in total costs and expenses Share Based Compensation Expense Included In Costs And Expenses Share based compensation expense included in costs and expenses. Income tax effect Employee Service Share-based Compensation, Tax Benefit from Compensation Expense Reconciliation of the beginning and ending of unrecognized tax benefits Unrecognized Tax Benefits, Income Tax Penalties and Interest Expense [Abstract] Balance at January 1 Unrecognized Tax Benefits Additions based on tax positions related to the current period Unrecognized Tax Benefits, Increase Resulting from Current Period Tax Positions Additions for tax positions of prior periods Unrecognized Tax Benefits, Increase Resulting from Prior Period Tax Positions Reductions for tax positions of prior periods Unrecognized Tax Benefits, Decrease Resulting from Prior Period Tax Positions Statute expirations Unrecognized Tax Benefits, Reduction Resulting from Lapse of Applicable Statute of Limitations Settlements Unrecognized Tax Benefits, Decrease Resulting from Settlements with Taxing Authorities Balance at December 31 Segment Reporting [Abstract] Segment Information Segment Reporting Disclosure [Text Block] Cash settled performance units Cash settled performance shares [Member] Cash settled performance shares [Member] Performance Units [Member] Performance Units [Member] Performance Units [Member] Employee stock purchase plan Employee Stock Purchase Plan [Member] Employee stock purchase plan. Summary of share based compensation expense associated with different programs Summary of share based compensation expense associated with different programs [Abstract] Summary of share-based compensation expense associated with different programs. Share-based compensation expense Summary of Significant Accounting Policies Significant Accounting Policies [Text Block] Schedule of Restructuring and Related Costs [Table] Schedule of Restructuring and Related Costs [Table] Restructuring Plan [Axis] Restructuring Plan [Axis] Restructuring Plan [Domain] Restructuring Plan [Domain] 2017 Restructuring Charges [Member] 2017 Restructuring Charges [Member] 2017 Restructuring Charges [Member] 2016 Restructuring Charges 2016 Restructuring Charges [Member] 2016 Restructuring Charges [Member] 2015 Restructuring Charges 2015 Restructuring Charges [Member] 2015 Restructuring Charges [Member] Restructuring Cost and Reserve [Line Items] Restructuring Cost and Reserve [Line Items] Total non-recurring operation and capital expenditures Other Nonrecurring Expense Redirection of resources Resources Redirected through Restructuring Program Resources Redirected through Restructuring Program Restructuring charges Restructuring Charges Restructuring and related cost, expected percentage of positions eliminated Restructuring and related cost, expected percentage of positions eliminated Restructuring and related cost, expected percentage of positions eliminated Previously accrued incentive compensation Restructuring and Related Cost, Expected Cash Payments Restructuring and Related Cost, Expected Cash Payments Adjustments to previous estimates, net Restructuring Reserve, Accrual Adjustment Total restructuring charges expected Restructuring and Related Cost, Expected Cost Valuation and Qualifying Accounts Disclosure [Table] Valuation and Qualifying Accounts Disclosure [Table] Valuation Allowances and Reserves Type [Axis] Valuation Allowances and Reserves Type [Axis] Valuation Allowances and Reserves [Domain] Valuation Allowances and Reserves [Domain] Discounts Reserve for Cash Discount [Member] Contractual Adjustments Contractual Adjustments [Member] Contractual adjustments. Returns Sales Returns and Allowances [Member] Valuation and Qualifying Accounts Disclosure [Line Items] Valuation and Qualifying Accounts Disclosure [Line Items] Analysis of the amount of, and change in, reserves Analysis Of Amount Of And Change In Product Revenue Reserves [Abstract] Analysis of the amount of, and change in product revenue reserves. Beginning balance Current provisions relating to sales in current year Current Provisions To Product Revenue Reserves Relating To Sales In Current Year Current provisions to product revenue reserves relating to sales in current year. Adjustments relating to prior years Adjustments To Product Revenue Reserves Relating To Sales In Prior Years Adjustments to product revenue reserves relating to sales in prior years. Payments/returns relating to sales in current year Payments Returns Applied Against Product Revenue Reserves Relating To Current Year Payments/ returns applied against product revenue reserves relating to current year. Payments/returns relating to sales in prior years Payments Returns Applied Against Product Revenue Reserves Relating To Prior Year Payments/ returns applied against product revenue reserves relating to prior year. Ending balance Major Types of Debt and Equity Securities [Axis] Major Types of Debt and Equity Securities [Axis] Major Types of Debt and Equity Securities [Domain] Major Types of Debt and Equity Securities [Domain] Strategic Investments Strategic Investments [Member] Strategic investments. Strategic investment portfolio Strategic Investment Portfolio Strategic investment portfolio. Total expense incurred by collaboration Expense Incurred By Collaboration Total expense incurred by collaboration. Biogen's share of expense reflected within our consolidation statements of income Expense Reflected Within Statements Of Income Biogen Idec's share of expense reflected within our consolidated statements of income. Inventory Disclosure [Abstract] Inventory Inventory Disclosure [Text Block] Schedule of Revenue by Major Customers, by Reporting Segments [Table] Schedule of Revenue by Major Customers, by Reporting Segments [Table] U.S UNITED STATES Europe Europe [Member] Asia Asia [Member] Other Segment Geographical Groups Of Countries Group Other [Member] Segment geographical groups of countries group other. Revenue, Major Customer [Line Items] Revenue, Major Customer [Line Items] Geographic information Segments, Geographical Areas [Abstract] Other revenues Long-lived assets Long-Lived Assets Analysis of the change in reserves Analysis Of Amount Of And Change In Product Revenue Reserves [Table Text Block] Analysis of amount of and change in product revenue reserves. Total reserves, included in consolidated balance sheets Summary Of Product Revenue Reserves Included In Consolidated Balance Sheets [Table Text Block] Summary of total product revenue reserves included in consolidated balance sheets. Schedule of Available-for-sale Securities [Table] Schedule of Available-for-sale Securities [Table] Corporate debt securities Current Corporate Debt Securities Current [Member] Corporate debt securities Current. Corporate debt securities Non-current Corporate Debt Securities Non Current [Member] Corporate Debt Securities Non Current. Government securities Current Government Securities Current [Member] Government securities Current. Government securities Non-current Government Securities Non-current [Member] Government Securities Non Current. Mortgage and other asset backed securities Current Mortgage And Other Asset Backed Securities Current [Member] Mortgage and other asset backed securities Current. Mortgage and other asset backed securities Non-current Mortgage And Other Asset Backed Securities Non Current [Member] Mortgage And Other Asset Backed Securities Non Current. Marketable equity securities Equity Securities [Member] Schedule of Available-for-sale Securities [Line Items] Schedule of Available-for-sale Securities [Line Items] Marketable Debt and Equity Securities Available-for-sale Securities [Abstract] Fair Value Gross Unrealized Gains Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax Gross Unrealized Losses Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax Amortized Cost Available-for-sale Securities, Amortized Cost Basis Proceeds from Marketable Debt Securities Proceeds from Marketable Debt Securities [Abstract] Proceeds from marketable debt securities. Proceeds from maturities and sales Proceeds from Sale and Maturity of Available-for-sale Securities Realized gains Available-for-sale Securities, Gross Realized Gains Realized losses Available-for-sale Securities, Gross Realized Losses Equity Components [Axis] Unfunded Status of Postretirement Benefit Plans Accumulated Defined Benefit Plans Adjustment Attributable to Parent [Member] Translation Adjustments Accumulated Foreign Currency Adjustment Attributable to Parent [Member] Accumulated Other Comprehensive Income (Loss) [Line Items] Accumulated Other Comprehensive Income (Loss) [Line Items] Accumulated other comprehensive income (loss), net of tax beginning balance Other comprehensive income (loss), before reclassifications Other Comprehensive Income (Loss), before Reclassifications, Net of Tax Amounts reclassified from accumulated other comprehensive income (loss) Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax Net current period other comprehensive income (loss) Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Parent Accumulated other comprehensive income (loss), net of tax ending balance Schedule of Finite-Lived Intangible Assets [Table] Schedule of Finite-Lived Intangible Assets [Table] AVONEX AVONEX [Member] AVONEX TYSABRI TYSABRI product [Member] TYSABRI product [Member] Biosimilars Biosimilars [Member] Biosimilars ZINBRYTA ZINBRYTA [Member] ZINBRYTA [Member] Indefinite-lived Intangible Assets by Major Class [Axis] Indefinite-lived Intangible Assets [Axis] Indefinite-lived Intangible Assets, Major Class Name [Domain] Indefinite-lived Intangible Assets, Major Class Name [Domain] In-process research and development In Process Research and Development [Member] Trademarks and trade names Trademarks and Trade Names [Member] Finite-Lived Intangible Assets by Major Class [Axis] Finite-Lived Intangible Assets by Major Class [Axis] Finite-Lived Intangible Assets, Major Class Name [Domain] Finite-Lived Intangible Assets, Major Class Name [Domain] Out-licensed patents Out Licensed Patents [Member] Out-licensed patents. Developed technology Developed Technology Rights [Member] Acquired and in-licensed rights and patents In Licensed Patents [Member] In-licensed patents. Finite-Lived Intangible Assets [Line Items] Finite-Lived Intangible Assets [Line Items] Intangible assets Intangible Assets, Net (Excluding Goodwill) [Abstract] Estimated life, (In Years) Finite-Lived Intangible Asset, Useful Life Indefinite lived intangible assets useful life Indefinite Lived Intangible Assets Useful Life Indefinite lived intangible assets useful life. Cost Finite-Lived Intangible Assets, Gross Cost and net Indefinite-Lived Intangible Assets (Excluding Goodwill) Total intangible assets, gross Intangible Assets, Gross (Excluding Goodwill) Accumulated Amortization Finite-Lived Intangible Assets, Accumulated Amortization Net Finite-Lived Intangible Assets, Net Intangible assets, net Intangible Assets and Goodwill (Additional Textual) Intangible Assets And Goodwill (Additional Textual) [Abstract] Intangible assets and goodwill additional. Expected future amortization expense, 2018 Finite-Lived Intangible Assets, Amortization Expense, Next Twelve Months Expected future amortization expense, 2019 Finite-Lived Intangible Assets, Amortization Expense, Year Two Expected future amortization expense, 2020 Finite-Lived Intangible Assets, Amortization Expense, Year Three Expected future amortization expense, 2021 Finite-Lived Intangible Assets, Amortization Expense, Year Four Expected future amortization expense, 2022 Finite-Lived Intangible Assets, Amortization Expense, Year Five Amortization of acquired intangible assets Impairment of out-licensed patent Impairment of Intangible Assets, Finite-lived Net change in acquired and in-licensed rights and patents Finite-Lived Intangible Assets, Period Increase (Decrease) Payment made to Forward Pharma Litigation Settlement, Amount Awarded to Other Party Summary of common stock Summary Of Common Stock [Table Text Block] Summary of common stock. Summary of Contractual Maturities: Available-for-Sale Securities Available-for-sale Securities, Debt Maturities [Abstract] Due in one year or less, Estimated Fair Value Available-for-sale Securities, Debt Maturities, Next Twelve Months, Fair Value Due in one year or less, Amortized Cost Available-for-sale Securities, Debt Maturities, Next Twelve Months, Amortized Cost Basis Due after one year through five years, Estimated Fair Value Available-for-sale Securities, Debt Maturities, Year Two Through Five, Fair Value Due after one year through five years, Amortized Cost Available-for-sale Securities, Debt Maturities, Year Two Through Five, Amortized Cost Basis Due after five years, Estimated Fair Value Available For Sale Securities Debt Maturities After Five Years Fair Value This item represents the fair value of debt securities, which are expected to mature after five years from the balance sheet date and which are categorized neither as held-to-maturity nor trading securities. Due after five years, Amortized Cost Available For Sale Securities Debt Maturities After Five Years Amortized Cost This item represents debt securities, at cost, net of adjustments made for accretion, amortization, other-than-temporary impairments, and hedging, if any, which are expected to mature after five years from the balance sheet date and which are categorized neither as held-to-maturity nor trading securities. Total available-for-sale securities, Fair Value Total available-for-sale securities, Amortized Cost Available-for-sale Debt Securities, Amortized Cost Basis Performance units Performance Shares [Member] Employee Benefit Plans Pension and Other Postretirement Benefits Disclosure [Text Block] GAZYVA GAZYVA [Member] GAZYVA [Member] RITUXAN RITUXAN [Member] RITUXAN. Roche Group - Genentech Roche Group Genentech Member Roche group Genentech. Co-promotion profit sharing formula Co Promotion Profit Sharing Formula [Abstract] Co-promotion profit sharing formula. Until GAZYVA First Non-CLL FDA Approval Percentage Of Co Promotion Operating Profits Greater Than First Fifty Million Option One Percentage of co promotion operating profits greater than first fifty million option one. After First GA101 Threshold Date Percentage Of Co Promotion Operating Profits Greater Than First Fifty Million Option Two [Abstract] Percentage of co promotion operating profits greater than first fifty million option two. Until First GAZYVA Threshold Date Percentage Of Co Promotion Operating Profits Greater Than First Fifty Million Option Two Sub Option One Percentage of co promotion operating profits greater than first fifty million option two sub option one. After First Threshold Date and until Second Threshold Date Percentage Of Co Promotion Operating Profits Greater Than First Fifty Million Option Two Sub Option Two Percentage of co promotion operating profits greater than first fifty million option two sub option two. After Second Threshold Date Percentage Of Co Promotion Operating Profits Greater Than First Fifty Million Option Two Sub Option Three Percentage of co promotion operating profits greater than first fifty million option two sub option three. Revenues from unconsolidated joint business Revenues From Unconsolidated Joint Business [Abstract] Revenues from unconsolidated joint business. Biogen's share of pre-tax profits in the U.S. for RITUXAN and GAZYVA, including the reimbursement of selling and development expenses Revenue on sales in the rest of world for RITUXAN Total revenues from anti-CD20 therapeutic programs Derivative Instruments and Hedging Activities Disclosure [Abstract] Foreign currency forward contracts that were entered into to hedge forecasted revenue Schedule of Notional Amounts of Outstanding Derivative Positions [Table Text Block] Summary of the effect of derivatives designated as hedging instruments on our consolidated statements of income Derivative Instruments, Gain (Loss) [Table Text Block] Summary of the fair value for our outstanding derivatives Schedule of Derivative Instruments in Statement of Financial Position, Fair Value [Table Text Block] Fair Values Derivatives, Balance Sheet Location, by Derivative Contract Type [Table] Fair Values Derivatives, Balance Sheet Location, by Derivative Contract Type [Table] Currency [Axis] Currency [Axis] All Currencies [Domain] All Currencies [Domain] Euro Euro Member Countries, Euro British pound sterling United Kingdom, Pounds Swiss francs Switzerland, Francs Canadian dollar Canada, Dollars Foreign Exchange Contract Foreign Exchange Contract [Member] Other current assets Other Current Assets [Member] Investments and other assets Other Investments [Member] Hedging Designation [Axis] Hedging Designation [Axis] Hedging Designation [Domain] Hedging Designation [Domain] Designated as Hedging Instrument Designated as Hedging Instrument [Member] Not Designated as Hedging Instrument Not Designated as Hedging Instrument [Member] Derivatives, Fair Value [Line Items] Derivatives, Fair Value [Line Items] Aggregate notional amount Derivative, Notional Amount Summary of derivatives designated as hedging instruments Derivative Asset, Fair Value, Amount Not Offset Against Collateral [Abstract] Asset derivatives Liability derivatives Derivative Liability, Fair Value, Gross Liability Revenue from External Customers by Products and Services [Table] Revenue from External Customers by Products and Services [Table] Solothurn Solothurn [Member] Solothurn [Member] DENMARK DENMARK Major Distributor [Axis] Major Distributor [Axis] Major Distributor. Major Distributor [Domain] Major Distributor [Domain] Major Distributor. Distributor One Distributor One [Member] Distributor one. Distributor Two Distributor Two [Member] Distributor two. Revenue from External Customer [Line Items] Revenue from External Customer [Line Items] Revenues from anti-CD20 therapeutic programs in percentage Revenues In Percentage From Anti-CD20 Therapeutic Programs Revenues In Percentage From Anti-CD20 Therapeutic Programs Entity wide percentage of revenue from major distributors Entity Wide Percentage Of Revenue From Major Distributors Entity wide percentage of revenue from major distributors. Long-lived assets related to operations in Denmark Segment Information (Textual) [Abstract] Segment Information (Textual) [Abstract] Segment Information. Number of reportable segment Number of Reportable Segments Property, Plant and Equipment [Table] Property, Plant and Equipment [Table] Deferred Tax Asset Deferred Tax Asset [Domain] Retained Earnings Retained Earnings [Member] Distributor One Distributor Two Property, Plant and Equipment, Type [Axis] Property, Plant and Equipment, Type [Axis] Property, Plant and Equipment, Type [Domain] Property, Plant and Equipment, Type [Domain] Building Building [Member] Furniture and Fixtures Furniture and Fixtures [Member] Machinery and Equipment Machinery and Equipment [Member] Computer Equipment Computer Equipment [Member] Property, Plant and Equipment Property, Plant and Equipment [Line Items] Cumulative Effect of New Accounting Principle in Period of Adoption Cumulative Effect of New Accounting Principle in Period of Adoption Percentage receivables of wholesale distributor accounted in consolidated receivables Percentage Receivables Of Wholesale Distributor Accounted In Consolidated Receivables Percentage receivables of wholesale distributor accounted in consolidated receivables. Property, plant and equipment, useful life Property, Plant and Equipment, Useful Life Business (Textual) Business (Textual) [Abstract] Business. Interest in subsidiary (less than given percentage) Interest In Subsidiary Interest in subsidiary. Payment terms of accounts receivable arising from product sales Payment Terms Of Accounts Receivable Payment terms of accounts receivable. Estimated useful lives of leasehold improvements Estimated Useful Lives Of Leasehold Improvements Estimated useful lives of leasehold improvements. Purchase price of common stock under ESPP Purchase Price Of Common Stock Under ESPP Purchase Price Of Common Stock Under ESPP. Percentage of market value per share of common stock Percentage Of Market Value Per Share Of Common Stock Percentage of market value per share of common stock. Compensation expense over purchase period Compensation Expense Over Purchase Period Compensation expense over purchase period. Discount rate recognized in compensation expense Discount Rate Recognized In Compensation Expense Discount rate recognized in compensation expense. Advertising costs Advertising Expense Summary of assets and liabilities recorded at fair value Schedule of Fair Value, Assets and Liabilities Measured on Recurring Basis [Table Text Block] Summary of fair and carrying value of debt instruments Fair Value, by Balance Sheet Grouping [Table Text Block] Fair value of contingent consideration obligations Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Table Text Block] Stock Option Activity Stock Option Activity [Abstract] Stock option activity. Beginning Balance, Outstanding shares Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Number Beginning Balance, Weighted Average Exercise Price Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Exercise Price Shares, Hemophilia spin-off adjustment Share-based Compensation Arrangement by Share-based Payment Award, Other Share Increase (Decrease) Shares, Granted Share-based Compensation Arrangement by Share-based Payment Award, Options, Grants in Period, Net of Forfeitures Weighted Average Exercise Price, Granted Share-based Compensation Arrangements by Share-based Payment Award, Options, Grants in Period, Weighted Average Exercise Price Shares, Exercised Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercises in Period Weighted Average Exercise Price, Exercised Share-based Compensation Arrangements by Share-based Payment Award, Options, Exercises in Period, Weighted Average Exercise Price Shares, Cancelled Share-based Compensation Arrangement by Share-based Payment Award, Options, Forfeitures in Period Weighted Average Exercise Price, Cancelled Share-based Compensation Arrangements by Share-based Payment Award, Options, Forfeitures in Period, Weighted Average Exercise Price Ending Balance, Outstanding shares Ending Balance, Weighted Average Exercise Price Income Taxes Income Tax Disclosure [Text Block] Components of inventories Inventory, Net [Abstract] Raw materials Inventory, Raw Materials, Net of Reserves Work in process Inventory, Work in Process, Net of Reserves Finished goods Inventory, Finished Goods, Net of Reserves Total inventory Inventory, net current and noncurrent Inventory, net current and noncurrent Inventory, current Inventory, noncurrent Inventory, Noncurrent Land Land Buildings Buildings and Improvements, Gross Leasehold improvements Leasehold Improvements, Gross Machinery and equipment Machinery and Equipment, Gross Computer software and hardware Computer software and hardware Computer software and hardware Furniture and fixtures Furniture and Fixtures, Gross Construction in progress Construction in Progress, Gross Total cost Property, Plant and Equipment, Gross Less: accumulated depreciation Accumulated Depreciation, Depletion and Amortization, Property, Plant, and Equipment Total property, plant and equipment, net Scenario [Axis] Scenario [Axis] Scenario, Unspecified [Domain] Scenario, Unspecified [Domain] Percentage of development expenses to be reimbursed Scenario, Forecast [Member] U.S Outside the U.S Foreign Tax Authority [Member] European Union European Union [Member] JAPAN JAPAN Research and development Equity Method Investee, Name [Axis] Investment, Name [Axis] Equity Method Investee, Name [Domain] Investment, Name [Domain] Samsung Biosimilar Agreement Samsung Biosimilar Agreement [Member] Samsung bio-similar agreement. New Anti-CD20 New Anti Cd20 [Member] New Anti-CD20. OCREVUS OCREVUS [Member] OCREVUS SOD1 SOD1 [Member] SOD1 [Member] Lead programs Lead programs [Member] Lead programs [Member] Discovery programs Discovery programs [Member] Discovery programs [Member] Acorda Acorda [Member] Acorda. AGTC AGTC [Member] AGTC [Member] University of Pennsylvania University of Pennsylvania [Member] University of Pennsylvania [Member] Other research and discovery Other research and discovery [Member] Other research and discovery [Member] Collaborative and Other Relationships (Textual) Collaborations (Textual) [Abstract] Collaborations. Future royalties percentage to be received on sale of ocrelizumab Future Royalties Percentage To Be Received On Sale Of Product Future royalties percentage to be received on sale of product. Royalty operating profit threshold for highest royalty rate percentage Royalty operating profit threshold for highest royalty rate percentage Royalty operating profit threshold for highest royalty rate percentage Reduction in royalty rate Reduction in royalty rate Reduction in royalty rate Period of collaboration agreement Period Of Collaboration Agreement Period of collaboration agreement. Percentage of Co promotion Operating Profits first fifty million Percentage Of Co Promotion Operating Profits First Fifty Million Percentage of co promotion operating profits first fifty million. Co-promotion operating profit threshold for Rituxan in U S and Canada to determine share of co promotion operating profit prior to amendment Co Promotion Operating Profit Threshold To Determine Share Of Co Promotion Operating Profit Prior To Amendment Co promotion operating profit threshold to determine share of co promotion operating profit prior to amendment. Limit of gross sale of GAZYVA to be achieved in preceding 12 months under option one Threshold Of Gross Sales Of To Be Achieved In Preceding Twelve Consecutive Months Under Option One Threshold of gross sales of to be achieved in preceding twelve consecutive months under option one. Limit of gross sale of GAZYVA to be achieved in any 12 months under option one Threshold Of Gross Sales To Be Achieved In Any Twelve Consecutive Months Under Option One Threshold of gross sales to be achieved in any twelve consecutive months under option one. Sales Trigger Gross Sales Threshold Sales Trigger Gross Sales Threshold Sales trigger gross sales threshold. Future percentage of co-promotion operating profits Future percentage of co-promotion operating profits Future percentage of co-promotion operating profits Expected additional milestone payments when certain sales threshold is met Expected Additional Milestone Payments When Meeting Certain Sales Threshold Expected additional milestone payments when meeting certain sales threshold. Foreign sales required to trigger milestone Foreign Sales Required To Trigger Milestone Foreign sales required to trigger milestone. Research and development Estimated additional payments upon achievement of development and commercial milestones. Until First GAZYVA Threshold Date Until GAZYVA First Non-CLL FDA Approval After First Threshold Date and until Second Threshold Date Collaboration Expenses in Dispute Collaboration Expenses in Dispute Collaboration Expenses in Dispute Biogen share of co-promotion profits or losses Biogen share of co-promotion profits or losses Biogen share of co-promotion profits or losses Net impairment charges Asset Impairment Charges Unrecorded tax benefit Unrecognized Tax Benefits, Period Increase (Decrease) Collaboration profit (loss) 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. Percentage Of Royalties As Per Collaboration Percentage Of Royalties As Per Collaboration Percentage of royalties as per collaboration. Royalty cost of sales Direct Operating Cost, Royalty Expense Expected License Fee And Regulatory Milestone Payments Expected License Fee And Regulatory Milestone Payments Expected license fee and regulatory milestone payments. Additional milestone payment Additional Milestone Payment Additional milestone payment. Term of collaboration agreement Term of collaboration agreement Term of collaboration agreement Prepaid research and discovery services Prepaid Expense and Other Assets Additional milestone payments for product candidate using a different modality Additional milestone payments for product candidate using a different modality Additional milestone payments for product candidate using a different modality Percentage of future development costs related to Eisai Percentage of future development costs related to Eisai Percentage of future development costs related to Eisai Potential future milestone payments commitment, approximately Purchase of common stock Purchase of common stock Purchase of common stock Total licensing and other fees License Costs Prepaid research and development Future research and development commitment Future research and development commitment Future research and development commitment Investments By Third Party In Joint Venture As Per Agreement. Investments By Third Party In Joint Venture As Per Agreement. Investments by third party in joint venture as per agreement Joint Venture Owner Ship Percentage By Third Party. Joint Venture Owner Ship Percentage By Third Party. Joint venture owner ship percentage by third party. Equity Method Investments Equity Method Investments Percentage of equity interest to the portion of total capital stock Equity Method Investment, Ownership Percentage Equity Method Investment Ownership Percentage Maximum Equity Method Investment Ownership Percentage Maximum Equity method investment ownership percentage maximum. Equity in loss of investee, net of tax Income (Loss) from Equity Method Investments Equity Method Investments, Expected Profit Share Equity Method Investments, Expected Profit Share Equity Method Investments, Expected Profit Share Statement of Cash Flows [Abstract] Cash flows from operating activities: Net Cash Provided by (Used in) Operating Activities [Abstract] 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] Depreciation and amortization Depreciation, Depletion and Amortization Share-based compensation Share-based Compensation Deferred income taxes Loss (gain) on fair value remeasurement of contingent consideration Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Gain (Loss) Included in Earnings Other Other Noncash Income (Expense) Changes in operating assets and liabilities, net: Increase (Decrease) in Operating Capital [Abstract] Accounts receivable Increase (Decrease) in Accounts Receivable Due from anti-CD20 therapeutic programs Increase (Decrease) in Other Operating Assets and Liabilities, Net Inventory Increase (Decrease) in Inventories Other assets Increase (Decrease) in Other Operating Assets Accrued expenses and other current liabilities Increase (Decrease) in Accrued Liabilities Income tax assets and liabilities Increase (Decrease) in Income Taxes Payable Other liabilities Increase (Decrease) in Other Operating Liabilities Net cash flows provided by operating activities Net Cash Provided by (Used in) Operating Activities Cash flows from investing activities: Net Cash Provided by (Used in) Investing Activities [Abstract] Proceeds from sales and maturities of marketable securities Purchases of marketable securities Payments to Acquire Available-for-sale Securities Contingent consideration related to Fumapharm AG acquisition Payments to Acquire Other Productive Assets Payments to Acquire in Process Research and Development Payments to Acquire in Process Research and Development Acquisitions of businesses, net of cash acquired Payments to Acquire Businesses, Net of Cash Acquired Purchases of property, plant and equipment Payments to Acquire Property, Plant, and Equipment Acquisitions of intangible assets Payments to Acquire Intangible Assets Other Payments for (Proceeds from) Other Investing Activities Net cash flows 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] Purchases of treasury stock Payments for Repurchase of Common Stock Payments related to issuance of stock for share-based compensation arrangements, net Payments Related to Tax Withholding for Share-based Compensation Net distribution to noncontrolling interest Payments to Noncontrolling Interests Proceeds from borrowings Proceeds from Notes Payable Repayments of borrowings Repayments of Debt Net cash contribution to Bioverativ, Inc. Payments of Capital Distribution Contingent consideration payments Payments to Acquire Productive Assets Other Proceeds from (Payments for) Other Financing Activities Net cash flows provided by (used in) financing activities Net Cash Provided by (Used in) Financing Activities Net increase in cash and cash equivalents Cash and Cash Equivalents, Period Increase (Decrease) Effect of exchange rate changes on cash and cash equivalents Effect of Exchange Rate on Cash and Cash Equivalents Cash and cash equivalents, beginning of the year Cash and cash equivalents, end of the year Preferred stock, par value Preferred Stock, Par or Stated Value Per Share Common stock, par value Common Stock, Par or Stated Value Per Share Treasury stock at cost, shares Treasury Stock, Shares Financial Instruments Cash, Cash Equivalents, and Marketable Securities [Text Block] Commitments and Contingencies Commitments and Contingencies Disclosure [Text Block] Restructuring Type [Axis] Restructuring Type [Axis] Type of Restructuring [Domain] Type of Restructuring [Domain] Workforce reduction Employee Severance [Member] Pipeline programs Contract Termination [Member] Restructuring reserve, beginning period Restructuring Reserve Expense Restructuring Costs Payments Payments for Restructuring Restructuring reserve, ending period Summary of financial assets with maturities of less than 90 days included within cash and cash equivalents Schedule of Cash and Cash Equivalents [Table Text Block] Marketable securities including strategic investments Available-for-sale Securities [Table Text Block] Summary of contractual maturities: Available-for-sale securities Investments Classified by Contractual Maturity Date [Table Text Block] Proceeds from marketable securities, excluding strategic investments Realized Gain (Loss) on Investments [Table Text Block] Defined Contribution Plan [Table] Defined Contribution Plan [Table] Domestic Plan Domestic Plan [Member] Deferred Compensation Plan Other Postretirement Benefits Plan [Member] Defined Contribution Plan Disclosure [Line Items] Defined Contribution Plan Disclosure [Line Items] Minimum Qualifying Age For Employee Benefit Plan Minimum Qualifying Age For Employee Benefit Plan Minimum qualifying age for employee benefit plan. Expenses related to savings plan Defined Contribution Plan, Cost Deferred compensation liability Deferred Compensation Liability, Classified, Noncurrent Document and Entity Information [Abstract] Document and Entity Information. Entity Registrant Name Entity Registrant Name Entity Central Index Key Entity Central Index Key Current Fiscal Year End Date Current Fiscal Year End Date Entity Filer Category Entity Filer Category Document Type Document Type Document Period End Date Document Period End Date Document Fiscal Year Focus Document Fiscal Year Focus Document Fiscal Period Focus Document Fiscal Period Focus Amendment Flag Amendment Flag Entity Common Stock, Shares Outstanding Entity Common Stock, Shares Outstanding Entity Well-known Seasoned Issuer Entity Well-known Seasoned Issuer Entity Voluntary Filers Entity Voluntary Filers Entity Current Reporting Status Entity Current Reporting Status Entity Public Float Entity Public Float Schedule of Variable Interest Entities [Table] Schedule of Variable Interest Entities [Table] Variable Interest Entities by Classification of Entity [Axis] Neurimmune Variable Interest Entity [Line Items] Variable Interest Entity [Line Items] Payment to Neurimmune Additional reduction in royalty rate payable on commercial sales Additional reduction in royalty rate payable on commercial sales Additional reduction in royalty rate payable on commercial sales Additional potential payment to Neurimmune Additional Potential Payments to Noncontrolling Interest Additional Potential Payments to Noncontrolling Interest Investment in Variable Interest Entities (Textual) Investment in Variable Interest Entities (Textual) [Abstract] Investment in Variable Interest Entities. Remaining potential development milestone payments and royalties on commercial sales under the terms of collaboration agreement Remaining Potential Development Milestone Payments And Royalties On Commercial Sales Under Terms Of Collaboration Agreement Expected total payments including upfront and milestone payments as well as the greater of an annual maintenance fee or usage-based royalty payments under the terms of collaboration agreement. Investment in biotechnology companies that are determined to be unconsolidated variable interest entities Investment in biotechnology companies that are determined to be unconsolidated variable interest entities Investment in biotechnology companies that are determined to be unconsolidated variable interest entities. Discontinued Operations and Disposal Groups [Abstract] Assets and liabilities charged against equity Disposal Groups, Including Discontinued Operations [Table Text Block] Cash settled performance shares Cash Settled Performance Shares [Abstract] Cash settled performance shares. Reserves for Discounts and Allowances Schedule of Valuation and Qualifying Accounts Disclosure [Text Block] Other Consolidated Financial Statement Detail Additional Financial Information Disclosure [Text Block] Revenue-related reserves for discounts and allowances Cash Paid During the Year Supplemental Cash Flow Information [Abstract] Interest Interest Paid, Net Income taxes Income Taxes Paid, Net Other Nonoperating Income (Expense) [Abstract] Interest income Investment Income, Interest Interest expense Interest Expense Gain (loss) on investments, net Gain (Loss) on Sale of Investments Foreign exchange gains (losses), net Foreign Currency Transaction Gain (Loss), before Tax Total other income (expense), net Accrued Liabilities, Current [Abstract] Current portion of contingent consideration obligations Business Combination, Contingent Consideration, Liability, Current Employee compensation and benefits Employee-related Liabilities, Current Royalties and licensing fees Accrued Royalties, Current Collaboration expenses Collaboration expenses accrual Collaboration expenses accrual Accrued construction in progress Construction Payable Accrued TECFIDERA litigation settlement and license charges Estimated Litigation Liability Other Other Accrued Liabilities, Current Accrued expenses and other Schedule of Stock by Class [Table] Schedule of Stock by Class [Table] Share Repurchase Program [Axis] Share Repurchase Program [Axis] Share Repurchase Program [Domain] Share Repurchase Program [Domain] 2016 Share Repurchase Program 2016 Share Repurchase Program Year To Date [Member] 2016 Share Repurchase Program Year To Date 2011 Share Repurchase Program 2011 Share Repurchase Program [Member] 2011 Share Repurchase Program Class of Stock [Axis] Class of Stock [Axis] Class of Stock [Domain] Class of Stock [Domain] Series A Series A Preferred Stock [Member] Series X junior participating Series X Preferred Stock [Member] Series X preferred stock. Undesignated Undesignated Preferred Stock [Member] Undesignated preferred stock. Class of Stock [Line Items] Class of Stock [Line Items] Preferred stock, shares authorized Preferred Stock, Shares Authorized Authorized amount of share repurchases Stock Repurchase Program, Authorized Amount Common stock shares authorized for repurchase Stock Repurchase Program, Number of Shares Authorized to be Repurchased Repurchase of common stock, at cost, shares Treasury Stock, Shares, Acquired Payments for repurchase of common stock Fair Value, Measurements, Recurring Fair value, beginning of period Loss (gain) on fair value remeasurement of contingent consideration Payments Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Settlements Fair value, end of period Summary of Indebtedness Schedule of Debt [Table Text Block] Total debt maturities Schedule of Maturities of Long-term Debt [Table Text Block] Revenue by product Revenue from External Customers by Products and Services [Table Text Block] Geographic information Schedule of Revenue from External Customers Attributed to Foreign Countries by Geographic Area [Table Text Block] ITALY ITALY Cumulative sales level [Axis] Cumulative sales level [Axis] Cumulative sales level [Axis] Cumulative sales level [Domain] Cumulative sales level [Domain] [Domain] for Cumulative sales level [Axis] Fifteen billion Fifteen billion [Member] [Member] Fifteen billion [Member] [Member] Sixteen billion Sixteen billion [Member] Sixteen billion [Member] Increase to goodwill Cumulative sales level Cumulative sales level Cumulative sales level Debt Instrument, Interest Rate, Stated Percentage Prepaid taxes Prepaid Taxes Net impairment on certain tax assets Deferred Tax Assets, Tax Deferred Expense, Reserves and Accruals, Impairment Losses Proposed settlement Loss Contingency, Estimate of Possible Loss Reimbursement Limit Stated In Resolution Reimbursement Limit Stated In Resolution Reimbursement limit stated in the resolution. Loss contingency accrual Loss Contingency Accrual Loss contingency, adjustment related to claim amount, percent Loss Contingency, Adjustment Related To Claim Amount, Percent Loss Contingency, Adjustment Related To Claim Amount, Percent PLEGRIDY PLEGRIDY [Member] PLEGRIDY [Member] TYSABRI TYSABRI [Member] Tysabri. FAMPYRA FAMPYRA [Member] FAMPYRA [Member] ELOCTATE ELOCTATE [Member] ELOCTATE [Member] ALPROLIX ALPROLIX Product [Member] ALPROLIX Product [Member] FUMADERM FUMADERM [Member] FUMADERM [Member] BENEPALI BENEPALI [Member] BENEPALI [Member] FLIXABI FLIXABI [Member] FLIXABI [Member] Statement, Business Segments [Axis] Segments [Axis] Segment [Domain] Segments [Domain] Rest of world Rest Of World [Member] Rest of world. Revenue by product Sales Revenue, Goods, Net [Abstract] Subsequent Events Subsequent Events [Text Block] Disposal Groups, Including Discontinued Operations [Table] Disposal Groups, Including Discontinued Operations [Table] Hemophilia Products Hemophilia Products [Member] Hemophilia Products Assets and Liabilities Distributed to Bioverativ [Line Items] Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] Term of Bioverativ Transaction Services Agreement Term of Bioverativ Transaction Services Agreement Term of Bioverativ Transaction Services Agreement Term of Bioverativ Manufacturing and Supply Agreement Term of Bioverativ Manufacturing and Supply Agreement Term of Bioverativ Manufacturing and Supply Agreement Cash Disposal Group, Including Discontinued Operation, Cash and Cash Equivalents Cost of sales Cost of Goods Sold Statement of Comprehensive Income [Abstract] Other comprehensive income: Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Parent [Abstract] Unrealized gains (losses) recognized during the period, net of tax Other Comprehensive Income (Loss), Unrealized Holding Gain (Loss) on Securities Arising During Period, Net of Tax Less: reclassification adjustment for (gains) losses included in net income, net of tax Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI for Sale of Securities, Net of Tax Unrealized gains (losses) on securities available for sale, net of tax Other Comprehensive Income (Loss), Available-for-sale Securities Adjustment, Net of Tax Unrealized gains (losses) recognized during the period, net of tax Other Comprehensive Income (Loss), Unrealized Gain (Loss) on Derivatives Arising During Period, Net of Tax Less: reclassification adjustment for (gains) losses included in net income, net of tax Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI on Derivatives, Net of Tax Unrealized gains (losses) on cash flow hedges, net of tax Other Comprehensive Income (Loss), Derivatives Qualifying as Hedges, Net of Tax Unrealized gains (losses) on pension benefit obligation, net of tax Other Comprehensive (Income) Loss, Defined Benefit Plan, after Reclassification Adjustment, after Tax Currency translation adjustment Other Comprehensive Income (Loss), Foreign Currency Transaction and Translation Adjustment, Net of Tax Total other comprehensive income (loss), net of tax Comprehensive income attributable to Biogen Inc. Comprehensive Income (Loss), Net of Tax, Attributable to Parent Comprehensive income (loss) attributable to noncontrolling interests, net of tax Comprehensive Income (Loss), Net of Tax, Attributable to Noncontrolling Interest Comprehensive income Comprehensive Income (Loss), Net of Tax, Including Portion Attributable to Noncontrolling Interest Derivative Instruments, Gain (Loss) by Hedging Relationship, by Income Statement Location, by Derivative Instrument Risk [Table] Derivative Instruments, Gain (Loss) [Table] Hedging Relationship [Axis] Hedging Relationship [Axis] Hedging Relationship [Domain] Hedging Relationship [Domain] Cash flows, revenue Cash flows, revenue [Member] Cash flows, revenue [Member] Cash flows, operating expenses Cash flows, operating expenses [Member] Cash flows, operating expenses [Member] Income Statement Location [Axis] Income Statement Location [Domain] Revenue Sales [Member] Other income (expense) Other Nonoperating Income (Expense) [Member] Operating Expense Operating Expense [Member] Derivative Instruments, Gain (Loss) [Line Items] Derivative Instruments, Gain (Loss) [Line Items] Summary of the effect of derivatives designated as hedging instruments on the consolidated statements of income Derivative Instruments, Gain (Loss) Recognized in Other Comprehensive Income (Loss), Effective Portion, Net [Abstract] Amount Reclassified from Accumulated Other Comprehensive Income into Income Gain/(Loss) (Effective Portion) Derivative Instruments, Gain (Loss) Reclassified from Accumulated OCI into Income, Effective Portion, Net Net gains (losses) in earnings of foreign currency forward contracts due to hedge ineffectiveness Derivative Instruments, Gain (Loss) Recognized in Income, Ineffective Portion and Amount Excluded from Effectiveness Testing, Net Intangible assets Intangible Assets Excluding Goodwill [Table Text Block] Intangible assets excluding goodwill. Estimated future amortization of intangible assets Schedule of Finite-Lived Intangible Assets, Future Amortization Expense [Table Text Block] Summary of roll forward of the changes in goodwill Schedule of Goodwill [Table Text Block] Land Land [Member] Depreciation expense Depreciation Interest cost capitalization related to construction in progress Interest Costs Capitalized Term of original lease Lessee, Operating Lease, Term of Contract Term of amended lease Term Of Lease Term Of Lease. Capital lease obligation Capital Lease Obligations Hemophilia Spin-Off Disposal Groups, Including Discontinued Operations, Disclosure [Text Block] Products and Services [Table] Products and Services [Table] Inventory [Axis] Inventory [Axis] Inventory [Domain] Inventory [Domain] Work-in-process Work-in-process [Member] Work-in-process Finished Goods Finished Goods [Member] Finished Goods Inventory [Line Items] Inventory [Line Items] Inventory Disposal Group, Including Discontinued Operation, Inventory, Current Write-downs on excess, obsolete, unmarketable or other inventory Inventory Write-down Restructuring Restructuring and Related Activities Disclosure [Text Block] Tax benefit and cash received from stock option Tax Benefit And Cash Received From Stock Option [Abstract] Tax benefit and cash received from stock option. Tax benefit realized for stock options Employee Service Share-based Compensation, Tax Benefit from Exercise of Stock Options Cash received from the exercise of stock options Proceeds from Stock Options Exercised Property, Plant and Equipment Property, Plant and Equipment Disclosure [Text Block] Summary of common stock Common Stock, Number of Shares, Par Value and Other Disclosures [Abstract] Common stock, shares authorized Common Stock, Shares Authorized Common stock, shares issued Common Stock, Shares, Issued Common stock, shares outstanding Common Stock, Shares, Outstanding Current portion: Long-term Debt, Current Maturities [Abstract] Notes payable to Fumedica Notes Payable, Current Financing arrangement for the purchase of the RTP facility Long-term Debt and Capital Lease Obligations, Current Current portion of notes payable and other financing arrangements Non-current portion: Long-term Debt, Excluding Current Maturities [Abstract] Non-current notes payable Financing arrangement for the purchase of the RTP facility Long-term Debt and Capital Lease Obligations Non-current portion of notes payable and other financing arrangements Derivative Maturity [Axis] Derivative Maturity [Axis] Derivative Maturity [Axis] Derivative Maturity [Domain] Derivative Maturity [Domain] [Domain] for Derivative Maturity [Axis] Short-term derivative Short-term derivative [Member] Short-term derivative [Member] Interest rate lock Interest Rate Contract [Member] Designated as Hedging Instrument Gain/Loss on fair value of foreign currency forward contracts Unrealized Gain (Loss) on Foreign Currency Derivatives, Net, before Tax Range of durations of foreign currency forward contracts Derivative, Remaining Maturity Gain on treasury rate lock settlement Net gains (losses) of other income (expense) related to foreign currency forward contracts Gain (Loss) on Foreign Currency Derivative Instruments Not Designated as Hedging Instruments Guarantees [Abstract] Guarantees Guarantees [Text Block] Business Overview Business Overview [Policy Text Block] Describes an overview of the company and its operations. Consolidation Consolidation, Policy [Policy Text Block] Use of Estimates Use of Estimates, Policy [Policy Text Block] Revenue Recognition Revenue Recognition, Policy [Policy Text Block] Fair Value Measurements Fair Value of Financial Instruments, Policy [Policy Text Block] Cash and Cash Equivalents Cash and Cash Equivalents, Policy [Policy Text Block] Accounts Receivable Receivables, Policy [Policy Text Block] Concentration of Credit Risk Concentration Risk, Credit Risk, Policy [Policy Text Block] Marketable Securities and Other Investments Investment, Policy [Policy Text Block] Equity Method of Accounting Equity Method Investments [Policy Text Block] Inventory Inventory, Policy [Policy Text Block] Property, Plant and Equipment Property, Plant and Equipment, Policy [Policy Text Block] Intangible Assets Goodwill and Intangible Assets, Intangible Assets, Policy [Policy Text Block] Acquired In-process Research and Development (IPR&D) In Process Research and Development, Policy [Policy Text Block] Goodwill Goodwill and Intangible Assets, Goodwill, Policy [Policy Text Block] Impairment of Long-Lived Assets Impairment or Disposal of Long-Lived Assets, Policy [Policy Text Block] Contingent Consideration Business Combinations Policy [Policy Text Block] Derivative Instruments and Hedging Activities Derivatives, Policy [Policy Text Block] Translation of Foreign Currencies Foreign Currency Transactions and Translations Policy [Policy Text Block] Royalty Cost of Sales Royalty Cost of Sales [Policy Text Block] Royalty Cost of Sales [Policy Text Block] Accounting for Share-Based Compensation Compensation Related Costs, Policy [Policy Text Block] Research and Development Expenses Research, Development, and Computer Software, Policy [Policy Text Block] Selling, General and Administrative Expenses Selling, General and Administrative Expenses, Policy [Policy Text Block] Income Taxes Income Tax, Policy [Policy Text Block] Contingencies Commitments and Contingencies, Policy [Policy Text Block] Earnings per Share Earnings Per Share, Policy [Policy Text Block] New Accounting Pronouncements New Accounting Pronouncements, Policy [Policy Text Block] Components of deferred tax assets and liabilities Components of Deferred Tax Assets and Liabilities [Abstract] Tax credits Deferred Tax Assets, Tax Credit Carryforwards Inventory, other reserves, and accruals Deferred Tax Assets, Inventory Intangibles, net Deferred Tax Assets, Goodwill and Intangible Assets Net operating loss Deferred Tax Assets, Operating Loss Carryforwards Share-based compensation Deferred Tax Assets, Tax Deferred Expense, Compensation and Benefits, Share-based Compensation Cost Other Deferred Tax Assets, Other Valuation allowance Deferred Tax Assets, Valuation Allowance Total deferred tax assets Deferred Tax Assets, Net of Valuation Allowance Purchased intangible assets Deferred Tax Liabilities, Intangible Assets Depreciation, amortization and other Deferred Tax Liabilities Depreciation Amortization Other Deferred tax liabilities depreciation amortization other. Total deferred tax liabilities Deferred Tax Liabilities, Net Directors Plan Directors Plan [Member] Directors plan. 2017 Omnibus Equity Plan [Member] 2017 Omnibus Equity Plan [Member] 2017 Omnibus Equity Plan Attainment Of Performance Criteria in 2013 Attainment Of Performance Criteria in 2013 [Member] Attainment Of Performance Criteria in 2013 Vesting [Axis] Vesting [Axis] Vesting [Domain] Vesting [Domain] Settlement of PUs Share-based Compensation Award, Tranche One [Member] Title of Individual [Axis] Title of Individual [Axis] Relationship to Entity [Domain] Relationship to Entity [Domain] Director Director [Member] Total number of shares of common stock for issuance Number Of Shares Of Common Stock For Issuance Total number of shares of common stock for issuance. Ratio of total number of shares reserved under the plan Ratio Of Number Of Shares Reserved Under Plan Ratio of total number of shares reserved under the plan. Omnibus Plan number of shares authorized Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Authorized Number of equal annual increments Number Of Equal Annual Increments Number of equal annual increments. Percentage of earnings of the target number of units granted based on actual stock performance (Minimum) Minimum Target Number Of Market Stock Units Granted Based On Stock Performance In Percent Minimum target number of market stock units granted based on stock performance in percent. Percentage of earnings of the target number of units granted based on actual stock performance (Maximum) Maximum Target Number Of Market Stock Units Granted Based On Stock Performance In Percent Maximum target number of market stock units granted based on stock performance in percent. Number of shares granted Number of days for calculation of average closing stock price Number Of Days For Calculation Of Average Closing Stock Price Number of days for calculation of average closing stock price. Total fair value of vested awards Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Vested in Period, Fair Value Cash in settlement of CSPS awards upon vesting Employee Service Share-based Compensation, Cash Flow Effect, Cash Used to Settle Awards PUs converted to share settlements Employee Service Share-based Compensation, Shares Used to Settle Awards Employee Service Share-based Compensation, Shares Used to Settle Awards Share-Based Payments (Textual) Share-Based Payments (Textual) [Abstract] Share Based Payments. Unrecognized compensation cost related to unvested share-based compensation Employee Service Share-based Compensation, Nonvested Awards, Compensation Cost Not yet Recognized Weighted-average period to recognize the cost of unvested awards Weighted Average Period To Recognize Cost Of Nonvested Awards Weighted-average period to recognize the cost of nonvested awards. Number of share-based compensation plans pursuant to which awards are currently being made Number Of Share Based Compensation Plans Pursuant To Which Awards Are Currently Being Made Number of share-based compensation plans pursuant to which awards are currently being made. Number of shares granted under stock options Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Available for Grant Total intrinsic value of options exercised Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercises in Period, Intrinsic Value Aggregate intrinsic values of options outstanding Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Intrinsic Value Weighted average remaining contractual term for options outstanding Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Remaining Contractual Term Equity Stockholders' Equity Note Disclosure [Text Block] Accumulated Other Comprehensive Income (Loss) Comprehensive Income (Loss) Note [Text Block] Income Tax Authority [Axis] Income Tax Authority [Axis] Income Tax Authority [Domain] Income Tax Authority [Domain] 2017 Tax Act U.S. 2017 Tax Act [Member] U.S. 2017 Tax Act Future Contingent Milestone Types [Axis] Future Contingent Milestone Types [Axis] Future Contingent Milestone Types [Axis] Future Contingent Milestone Types [Domain] Future Contingent Milestone Types [Domain] [Domain] for Future Contingent Milestone Types [Axis] Development Milestones [Member] Development Milestones [Member] Development Milestones [Member] Regulatory Milestones [Member] Regulatory Milestones [Member] Regulatory Milestones [Member] Commercial Milestones [Member] Commercial Milestones [Member] Commercial Milestones [Member] Fourteen billion Fourteen billion [Member] Fourteen billion [Member] Eleven billion Eleven billion [Member] Eleven billion [Member] Twelve billion Twelve billion [Member] Twelve billion [Member] Thirteen billion Thirteen billion [Member] Thirteen billion [Member] One billion One billion [Member] One billion [Member] Each additional one billion up to twenty billion Each additional one billion up to twenty billion [Member] Each additional one billion up to twenty billion [Member] Three billion Three billion [Member] Three billion [Member] Future contingent payment for annual worldwide net sales up to $2.0 billion Future contingent payment for annual worldwide net sales up to $2.0 billion Future contingent payment for annual worldwide net sales up to $2.0 billion Future contingent payment threshold Future contingent payment threshold Future contingent payment threshold Future contingent payment for annual worldwide net sales that exceed $2.0 billion Future contingent payment for annual worldwide net sales that exceed $2.0 billion Future contingent payment for annual worldwide net sales that exceed $2.0 billion Amount paid in cash Cancellable future commitments Other Commitment Accrued income tax liability under the Transition Toll Tax, current Deferred Tax Liabilities, Gross, Current Commitments And Contingencies (Textual) Commitments And Contingencies (Textual) [Abstract] Commitments And Contingencies. Accrued expenses Accrued Expenses On Funding Commitment Related To Clinical Research Accrued expenses on funding commitment related to clinical research. Liabilities associated with uncertain tax positions Liability for Uncertainty in Income Taxes, Current Lease rent expense which terminates at various dates through 2028 Operating Leases, Rent Expense Income Statement [Abstract] Revenues: Revenues [Abstract] Total revenues Cost and expenses: Costs and Expenses [Abstract] Cost of sales, excluding amortization of acquired intangible assets Research and development Selling, general and administrative Selling, General and Administrative Expense Restructuring charges Total cost and expenses Income from operations Operating Income (Loss) Other income (expense), net Income tax expense Equity in loss of investee, net of tax Net income Net income attributable to Biogen Inc. Net income per share: Weighted-average shares used in calculating: Weighted Average Number of Shares Outstanding, Diluted [Abstract] Derivative Instruments Derivative Instruments and Hedging Activities Disclosure [Text Block] Tax Credit Carryforward [Table] Tax Credit Carryforward [Table] Foreign earnings type [Axis] Foreign earnings type [Axis] Foreign earnings type [Axis] Foreign earnings type [Domain] Foreign earnings type [Domain] [Domain] for Foreign earnings type [Axis] Earnings in the form of cash and cash equivalents Foreign earnings in the form of cash and cash equivalents [Member] Foreign earnings in the form of cash and cash equivalents [Member] Other Foreign Earnings Other Foreign Earnings [Member] Other Foreign Earnings [Member] Prepaid taxes Prepaid taxes [Member] Prepaid taxes [Member] Tax Credit Carryforward [Axis] Tax Credit Carryforward [Axis] Tax Credit Carryforward, Name [Domain] Tax Credit Carryforward, Name [Domain] General Business General Business Tax Credit Carryforward [Member] Research Research Tax Credit Carryforward [Member] Domestic Country Domestic Tax Authority [Member] State and Local Jurisdiction State and Local Jurisdiction [Member] Foreign Tax Authority [Member] Tax Credit Carryforward [Line Items] Tax Credit Carryforward [Line Items] Statutory rate Transition Toll Tax repatriation tax rate Transition Toll Tax Repatriation Tax Rate Transition Toll Tax Repatriation Tax Rate Income tax expense (benefit) Income Tax Expense (Benefit), Continuing Operations, Adjustment of Deferred Tax (Asset) Liability Operating Loss Carryforwards Operating Loss Carryforwards Tax Credit Carryforward, Amount Tax Credit Carryforward, Amount Deferred Tax Assets, Operating Loss Carryforwards, State and Local Deferred Tax Assets, Operating Loss Carryforwards, State and Local Notice of assessment of corporate withholding tax including penalties and interest Income Tax Examination, Penalties and Interest Accrued Income Taxes (Textual) Income Taxes (Textual) [Abstract] Income Taxes. Total deferred charges and prepaid taxes Total deferred charges and prepaid taxes Total deferred charges and prepaid taxes Unrecognized tax benefit Unrecognized Tax Benefits that Would Impact Effective Tax Rate Net interest expense Unrecognized Tax Benefits, Income Tax Penalties and Interest Expense Unrecognized Tax Benefits, Interest on Income Taxes Accrued Unrecognized Tax Benefits, Interest on Income Taxes Accrued Statement of Stockholders' Equity [Abstract] Statement, Equity Components [Axis] Preferred stock Preferred Stock [Member] Common stock Common Stock [Member] Additional paid-in capital Additional Paid-in Capital [Member] Accumulated other comprehensive income (loss) AOCI Attributable to Parent [Member] Retained earnings Treasury stock Treasury Stock [Member] Noncontrolling interests Noncontrolling Interest [Member] Parent Beginning Balance Beginning Balance Beginning Balance Beginning Balance, shares Shares, Issued Other comprehensive income (loss), net of tax Other comprehensive income (loss), net of tax Other Comprehensive Income (Loss), Net of Tax Noncontrolling interest, increase (decrease) other Noncontrolling interest, increase (decrease) other Noncontrolling interest, increase (decrease) other Acquisition of noncontrolling interest Noncontrolling Interest, Increase from Business Combination Distribution to noncontrolling interest Noncontrolling Interest, Decrease from Distributions to Noncontrolling Interest Holders Deconsolidation of noncontrolling interest Noncontrolling Interest, Decrease from Deconsolidation Repurchase of common stock, at cost Treasury Stock, Value, Acquired, Cost Method Repurchase of common stock, at cost, shares Retirement of common stock pursuant to the 2016 Share Repurchase Program, at cost Treasury Stock, Retired, Cost Method, Amount Retirement of common stock pursuant to the 2015 Share Repurchase Program, at cost, shares Stock Repurchased and Retired During Period, Shares Issuance of common stock under stock option and stock purchase plans Stock Issued During Period, Value, Treasury Stock Reissued Issuance of common stock under stock option and stock purchase plans, shares Stock Issued During Period, Shares, Treasury Stock Reissued Issuance of common stock under stock award plan Stock Issued During Period, Value, Stock Options Exercised Issuance of common stock under stock award plan Compensation related to share-based payments Hemophilia spin-off adjustment Stockholders' Equity Note, Spinoff Transaction Tax benefit from share-based payments Adjustments to Additional Paid in Capital, Income Tax Benefit from Share-based Compensation Tax benefit Stockholders' Equity, Other Ending Balance Ending Balance Ending Balance Ending Balance, shares Acquisitions Business Combination Disclosure [Text Block] Schedule of Antidilutive Securities Excluded from Computation of Earnings Per Share [Table] Schedule of Antidilutive Securities Excluded from Computation of Earnings Per Share [Table] Antidilutive Securities Excluded from Computation of Earnings Per Share, by Antidilutive Securities [Axis] Antidilutive Securities [Axis] Antidilutive Securities, Name [Domain] Antidilutive Securities, Name [Domain] Stock options and employee stock purchase plan Stock options and employee stock purchase plan [Member] Stock options and employee stock purchase plan. Antidilutive Securities Excluded from Computation of Earnings Per Share Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items] Basic and diluted earnings per share Basic And Diluted Earnings Per Share [Abstract] Basic and diluted earnings per share. Numerator: Numerator [Abstract] Numerator. Denominator: Denominator [Abstract] Denominator. Weighted average number of common shares outstanding Effect of dilutive securities: Dilutive Securities, Effect on Basic Earnings Per Share [Abstract] Stock options and employee stock purchase plan Incremental Common Shares Attributable to Dilutive Effect of Share-based Payment Arrangements Dilutive potential common shares Weighted Average Number Diluted Shares Outstanding Adjustment Shares used in calculating diluted earnings per share Earnings per share (Textual) Earnings Per Share (Textual) [Abstract] Earnings per share. Repurchase of common stock Weighted Average Number of Shares, Common Stock Subject to Repurchase or Cancellation Accounts Receivable Disposal Group, Including Discontinued Operation, Accounts, Notes and Loans Receivable, Net Property, Plant and Equipment, net Disposal Group, Including Discontinued Operation, Property, Plant and Equipment Intangible Assets, net Disposal Group, Including Discontinued Operation, Intangible Assets Goodwill Other, net Disposal Group, Including Discontinued Operation, Other Assets Assets Transfered, net Disposal Group, Including Discontinued Operation, Assets Accrued Expenses and Other Current Liabilities Disposal Group, Including Discontinued Operation, Accrued Liabilities, Current Other Long-Term Liabilities Disposal Group, Including Discontinued Operation, Other Liabilities, Noncurrent Liabilities Transferred, net Disposal Group, Including Discontinued Operation, Liabilities Components of inventory Schedule of Inventory, Current [Table Text Block] Minimum rental commitments under non-cancelable leases Schedule of Future Minimum Rental Payments for Operating Leases [Table Text Block]