Customer [Domain] Customer [Domain] Total payment to enter collaboration agreement Total payment to enter collaboration agreement Total payment to enter collaboration agreement Unrealized Gains (Losses) on Securities Available for Sale, Net of Tax Gains (losses) on securities available for sale AOCI, Accumulated Gain (Loss), Debt Securities, Available-for-sale, Parent [Member] Intangible assets Intangible Assets Excluding Goodwill [Table Text Block] Intangible assets excluding goodwill. 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Pre-tax charge Pre-tax Charge, Payment of Early Call Premium and Write off of Remaining Unamortized Debt Issuance Costs, Net of Derivative Gain Pre-tax Charge, Payment of Early Call Premium and Write off of Remaining Unamortized Debt Issuance Costs, Net of Derivative Gain Security Exchange Name Security Exchange Name Fair Value Equity Securities, FV-NI, Current Forfeited (in dollars per share) Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Forfeitures, Weighted Average Grant Date Fair Value Contingent payment Payment for Contingent Consideration Liability, Financing Activities Other Foreign Earnings Other Foreign Earnings [Member] Other Foreign Earnings [Member] Weighted average remaining lease term in years Operating Lease, Weighted Average Remaining Lease Term Translation of Foreign Currencies Foreign Currency Transactions and Translations Policy [Policy Text Block] Ionis Pharmaceuticals Ionis Pharmaceuticals [Member] Ionis Pharmaceuticals [Member] Denali Therapeutics Denali Therapeutics Inc. [Member] Denali Therapeutics Inc. Royalty percentage to be received Future Royalties Percentage To Be Received On Sale Of Product Future royalties percentage to be received on sale of product. Foreign earnings type [Domain] Foreign earnings type [Domain] [Domain] for Foreign earnings type [Axis] Schedule of Share-based Compensation Arrangements by Share-based Payment Award [Table] Schedule of Share-based Compensation Arrangements by Share-based Payment Award [Table] Operating lease supplemental cash flow disclosure Operating lease supplemental cash flow disclosure [Table Text Block] Operating lease supplemental cash flow disclosure [Table Text Block] Investment, Name [Axis] Equity Method Investee, Name [Axis] Investment, Name [Axis] Performance stock units settled in cash activity Performance Stock Units Settled in Cash [Table Text Block] Performance Stock Units Settled in Cash [Table Text Block] Derivative Instruments and Hedging Activities Derivatives, Policy [Policy Text Block] Total deferred charges and prepaid taxes Total deferred charges and prepaid taxes Total deferred charges and prepaid taxes Capital contribution from noncontrolling interest Noncontrolling Interest, Increase (Decrease), Other Noncontrolling Interest, Increase (Decrease), Other SPINRAZA SPINRAZA [Member] SPINRAZA [Member] Inventories Inventories [Member] Range of expected stock price volatility Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions, Expected Volatility Rate Liabilities associated with uncertain tax positions Liability for Uncertainty in Income Taxes, Current 2023 Long-Term Debt, Maturity, Year Two Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward] Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward] Purchased intangible assets Deferred Tax Liabilities, Intangible Assets Income tax assets and liabilities Increase (Decrease) in Income Taxes Payable Total intrinsic value of options exercised Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercises in Period, Intrinsic Value (Gain) loss on fair value remeasurement of contingent consideration Changes in fair value Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Gain (Loss) Included in Earnings Derivative contracts Foreign Currency Contracts, Liability, Fair Value Disclosure Reclassification out of Accumulated Other Comprehensive Income Reclassification out of Accumulated Other Comprehensive Income [Table Text Block] Tax Credit Carryforward [Line Items] Tax Credit Carryforward [Line Items] (Gain) loss on divestiture of Hillerød, Denmark manufacturing operations (Gain) loss on divestiture of Hillerød, Denmark manufacturing operations Disposal Group, Not Discontinued Operation, Loss (Gain) on Write-down Subsequent Event Type [Domain] Subsequent Event Type [Domain] Amortization of acquired intangible assets Amortization of Intangible Assets Remaining authorized amount of share repurchases Stock Repurchase Program, Remaining Authorized Repurchase Amount Additions for tax positions of prior periods Unrecognized Tax Benefits, Increase Resulting from Prior Period Tax Positions Deferred tax asset, net Deferred Tax Assets, Net Weighted average discount rate Operating Lease, Weighted Average Discount Rate, Percent Components of inventory Schedule of Inventory, Current [Table Text Block] Research and development Research and development Research and Development Expense [Member] UCB Pharma S.A. UCB Pharma S.A. [Member] UCB Pharma S.A. Schedule of Business Acquisitions by Acquisition, Contingent Consideration [Table] Schedule of Business Acquisitions by Acquisition, Contingent Consideration [Table] Total deferred tax assets Deferred Tax Assets, Net of Valuation Allowance 3.150% Senior Notes due May 1, 2050 3.15% Senior Notes due May 1, 2050 [Member] 3.15% Senior Notes due May 1, 2050 Major Distributor [Axis] Major Distributor [Axis] Major Distributor. Business Acquisition [Axis] Business Acquisition [Axis] Revenue from External Customer [Line Items] Revenue from External Customer [Line Items] Cash flow hedge Cash Flow Hedging [Member] Future contingent payment for annual worldwide net sales up to threshold, percentage 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 Award Type [Domain] Award Type [Domain] Local Phone Number Local Phone Number Gain (loss) on investments, net Gain (Loss) on Sale of Investments Total number of shares of common stock for issuance (in shares) Number Of Shares Of Common Stock For Issuance Total number of shares of common stock for issuance. Gains (Losses) on Net Investment Hedge, Net of Tax Gains (losses) on net investment hedge Accumulated Other Comprehensive Income (Loss), Derivative Qualifying as Hedge, Excluded Component, Including Portion Attributable to Noncontrolling Interest [Member] Distributor Two Distributor Two [Member] Distributor two. 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. Summary of contractual maturities: available-for-sale securities Investments Classified by Contractual Maturity Date [Table Text Block] Decrease in unrecognized tax benefits is reasonably possible Decrease in Unrecognized Tax Benefits is Reasonably Possible Preferred stock, par value $0.001 per share Preferred Stock, Value, Issued Acquired intangible asset Finite-lived Intangible Assets Acquired Distribution to noncontrolling interest Noncontrolling Interest, Decrease from Distributions to Noncontrolling Interest Holders Royalties payable period, post FDA approval Royalties Payable Period, Post FDA Approval Royalties Payable Period, Post FDA Approval Currency [Axis] Currency [Axis] Market stock units Market Stock Units (MSUs) Market Stock Units [Member] Market stock units. Retained earnings Retained Earnings [Member] 3.625% Senior Notes due September 15, 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] Fair-value post-combination equity compensation NST stock options Business Combination, Separately Recognized Transactions, Expenses and Losses Recognized Short-term debt securities Debt Securities [Member] Income before income taxes (benefit): Income (Loss) from Continuing Operations before Equity Method Investments, Income Taxes, Noncontrolling Interest [Abstract] Schedule of Antidilutive Securities Excluded from Computation of Earnings Per Share [Table] Schedule of Antidilutive Securities Excluded from Computation of Earnings Per Share [Table] Investments, All Other Investments [Abstract] Investments, All Other Investments [Abstract] Accounts receivable Increase (Decrease) in Accounts Receivable IMRALDI IMRALDI [Member] IMRALDI [Member] Germany GERMANY Derivative Instruments and Hedging Activities Disclosure [Abstract] Derivative Instruments and Hedging Activities Disclosure [Abstract] Property, Plant and Equipment [Line Items] Property, Plant and Equipment [Line Items] Entity Well-known Seasoned Issuer Entity Well-known Seasoned Issuer Comprehensive income Comprehensive Income (Loss), Net of Tax, Including Portion Attributable to Noncontrolling Interest Percentage of revenue from major distributors Entity Wide Percentage Of Revenue From Major Distributors Entity wide percentage of revenue from major distributors. Due to related parties Due to Related Parties Accounting for Share-Based Compensation Compensation Related Costs, Policy [Policy Text Block] Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items] Share-based Payment Arrangement, Expensed and Capitalized, Amount [Line Items] Other (income) expense, net Other Nonoperating Income (Expense) [Member] Basic earnings per share attributable to Biogen Inc. (in dollars per share) Earnings Per Share, Basic Less: Net gains (losses) realized during the period and on equity securities Equity Securities, FV-NI, Realized Gain (Loss) Operating loss carryforward, subject to expiration Deferred Tax Assets, Operating Loss Carryforwards, Subject to Expiration Investments in Variable Interest Entities [Abstract] Investments in Variable Interest Entities [Abstract] Investments In Variable Interest Entities. Revenues Revenue from Contract with Customer [Text Block] Sublease income Sublease Income 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. Beginning balance, unvested (in shares) Ending balance, unvested shares (in shares) Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number Other research and discovery Other research and discovery [Member] Other research and discovery [Member] 2022 Long-Term Debt, Maturity, Year One Due after one year through five years Debt Securities, Available-for-Sale, Amortized Cost, Maturity, Allocated and Single Maturity Date, after Year One Through Five Amortized Cost Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, Amortized Cost [Abstract] Foreign sales required to trigger milestone Foreign Sales Required To Trigger Milestone Foreign sales required to trigger milestone. Switzerland SWITZERLAND Machinery and Equipment Machinery and Equipment [Member] Fumarate Fumarate [Member] Fumarate [Member] Commercial paper Commercial Paper [Member] Total deferred tax liabilities Deferred Tax Liabilities, Net Other Other Noncash Income (Expense) 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] Asset derivative instruments Derivative Asset, Fair Value, Gross Asset Disposal Groups, Including Discontinued Operations [Table] Disposal Groups, Including Discontinued Operations [Table] Summary of the effect of derivatives designated as net investment hedging instruments on our consolidated statement of income Schedule of Net Investment Hedges in Accumulated Other Comprehensive Income (Loss) [Table Text Block] Entity Voluntary Filers Entity Voluntary Filers Gain (loss) on investments Gain (Loss) on Securities [Table Text Block] Plan Name [Axis] Plan Name [Axis] Assets: Assets, Fair Value Disclosure [Abstract] 4.050% Senior Notes due September 15, 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] Notes payable Long-term Debt Quoted Prices in Active Markets (Level 1) Fair Value, Inputs, Level 1 [Member] Cash flow from investing activities: Net Cash Provided by (Used in) Investing Activities [Abstract] March 2019 Share Repurchase Program Share Repurchase Program 3 [Member] Share Repurchase Program 3 Income Tax Disclosure [Abstract] Income Tax Disclosure [Abstract] Entity Small Business Entity Small Business 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. Additions based on tax positions related to the current period Unrecognized Tax Benefits, Increase Resulting from Current Period Tax Positions Fumapharm AG Fumapharm AG [Member] Fumapharm AG [Member] Finite-Lived Intangible Assets [Line Items] Finite-Lived Intangible Assets [Line Items] Scenario [Axis] Scenario [Axis] Minimum qualifying age for employee benefit plan Minimum Qualifying Age For Employee Benefit Plan Minimum qualifying age for employee benefit plan. Contingent consideration paid related to Fumapharm AG acquisition Payments to Acquire Other Productive Assets Additional paid-in capital Additional Paid in Capital Impairment of intangible assets Impairment of Intangible Assets, Indefinite-lived (Excluding Goodwill) LIABILITIES AND EQUITY Liabilities and Equity [Abstract] Unrecognized tax benefit Unrecognized Tax Benefits that Would Impact Effective Tax Rate Hedging Relationship [Domain] Hedging Relationship [Domain] Interest rate swap Interest Rate Swap [Member] Additions Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Purchases SEC Schedule, 12-09, Valuation and Qualifying Accounts Disclosure [Table] SEC Schedule, 12-09, Valuation and Qualifying Accounts Disclosure [Table] Research and development asset [Domain] Research and development asset [Domain] Research and development asset [Domain] Derivative liabilities Derivative Liability, Current Net gains (losses) recognized during the period on equity securities Equity Securities, FV-NI, Gain (Loss) Entity Interactive Data Current Entity Interactive Data Current 2027 and thereafter Long-Term Debt, Maturity, after Year Five Cash settlement of awards upon vesting Share-based Payment Arrangement, Cash Used to Settle Award 2025 Finite-Lived Intangible Asset, Expected Amortization, Year Four Other Consolidated Financial Statement Detail Additional Financial Information Disclosure [Text Block] Settlement payment Unrecognized Tax Benefits, Decrease Resulting from Settlements with Taxing Authorities FUMADERM FUMADERM [Member] FUMADERM [Member] Realized gains Debt Securities, Available-for-sale, Realized Gain Net Gains/(Losses) Recognized in Other Comprehensive Income (Effective Portion) (in millions) Derivatives used in Net Investment Hedge, Net of Tax, Period Increase (Decrease) Facility Location [Domain] Facility Location [Domain] -- None. No documentation exists for this element. -- Option exercise fee Research And Development Expense, Option Exercise Fee Research And Development Expense, Option Exercise Fee Balance Sheet Location [Domain] Balance Sheet Location [Domain] Comprehensive income (loss) attributable to noncontrolling interests, net of tax Comprehensive Income (Loss), Net of Tax, Attributable to Noncontrolling Interest Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items] Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items] Beginning balance Ending balance Unrecognized Tax Benefits 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. SOD1 SOD1 [Member] SOD1 [Member] Deferred tax asset, gross Deferred Tax Assets, Gross Income Tax Authority [Domain] Income Tax Authority [Domain] Entity Common Stock, Shares Outstanding Entity Common Stock, Shares Outstanding Gross Unrealized Gains Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax Performance units Performance Units [Member] Performance Units [Member] Entity Address, State or Province Entity Address, State or Province Claims asserted Loss Contingency, Damages Sought, Value Redemption percentage par value of senior notes Percentage Par Value Of Senior Notes Percentage Par Value Of Senior Notes Deferred tax asset Deferred Income Tax Assets, Net Current liabilities: Liabilities, Current [Abstract] 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. Facility Type [Domain] Facility Type [Domain] Facility Type BENEPALI BENEPALI [Member] BENEPALI [Member] Inventory Inventory Disclosure [Text Block] Accounting Policies [Abstract] Accounting Policies [Abstract] Derivative Instrument [Axis] Derivative Instrument [Axis] Divestiture of Denmark manufacturing operations Effective Income Tax Rate Reconciliation, Disposition of Asset, Percent Retirement Plan Sponsor Location [Axis] Retirement Plan Sponsor Location [Axis] Financial Instruments Cash, Cash Equivalents, and Marketable Securities [Text Block] Federal Current Federal Tax Expense (Benefit) Present value of operating lease liabilities Operating Lease, Liability Document Transition Report Document Transition Report Percentage of minimum investment return Percentage Of Minimum Investment Return Percentage of minimum investment return. Gross Unrealized Gains Equity Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax Equity Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax Post-acquisition equity compensation Post-acquisition equity compensation [Member] Post-acquisition equity compensation [Member] Common stock, par value $0.0005 per share Common Stock, Value, Issued Total reserves included in consolidated balance sheet Summary Of Product Revenue Reserves Included In Consolidated Balance Sheets [Table Text Block] Summary of total product revenue reserves included in consolidated balance sheets. Beginning balance, unvested (in dollars per share) Ending balance, unvested (in dollars per share) Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Weighted Average Grant Date Fair Value Total inventory Inventory Inventory, Net Current and Noncurrent Inventory, Net Current and Noncurrent Net increase (decrease) in cash and cash equivalents Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents, Period Increase (Decrease), Excluding Exchange Rate Effect Leases Lessee, Operating Leases [Text Block] Summary of fair and carrying value of debt instruments Fair Value, by Balance Sheet Grouping [Table Text Block] 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. Unfunded net pension Defined Benefit Plan, Funded (Unfunded) Status of Plan Option exercise fee Contract Option Exercise Fee Contract Option Exercise Fee Acquisitions Business Combination Disclosure [Text Block] Due from joint venture Payables To Divestiture Of Interest In Joint Venture Payables To Divestiture Of Interest In Joint Venture Reclassification out of Accumulated Other Comprehensive Income [Axis] Reclassification out of Accumulated Other Comprehensive Income [Axis] ICFR Auditor Attestation Flag ICFR Auditor Attestation Flag 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. Depreciation, amortization and other Deferred Tax Liabilities Depreciation Amortization Other Deferred tax liabilities depreciation amortization other. BG00011 BG00011 [Member] BG00011 Summary of unrealized gain (loss) on derivative instruments that are included in AOCI Derivative Instruments, Unrealized Gain (Loss) [Table Text Block] Derivative Instruments, Unrealized Gain (Loss) Antidilutive Securities, Name [Domain] Antidilutive Securities, Name [Domain] Foreign Deferred Foreign Income Tax Expense (Benefit) Legal Entity [Axis] Legal Entity [Axis] Dilutive securities (in shares) Incremental Common Shares Attributable to Dilutive Effect of Share-based Payment Arrangements Goodwill [Line Items] Goodwill [Line Items] Class of Stock [Axis] Class of Stock [Axis] Stromedix, Inc. Stromedix [Member] Stromedix. Collaborative Arrangement and Arrangement Other than Collaborative [Axis] Collaborative Arrangement and Arrangement Other than Collaborative [Axis] British pound United Kingdom, Pounds Other Segment Geographical Groups Of Countries Group Other [Member] Segment geographical groups of countries group other. Income Tax Authority [Axis] Income Tax Authority [Axis] Work in process Inventory, Work in Process, Net of Reserves Payment Due At First Anniversary Payment Due At First Anniversary [Member] Payment Due At First Anniversary Administrative Space Administrative Space [Member] Administrative Space (Gain) loss on strategic investments Increase (Decrease) in Equity Securities, FV-NI Entity [Domain] Entity [Domain] Selling, general and administrative Selling, general and administrative Selling, General and Administrative Expenses [Member] Summary of activity related to the UCB collaboration Summary of Activity Related to the UCB Collaboration [Table Text Block] Summary of Activity Related to the UCB Collaboration Share-based compensation Deferred Tax Assets, Tax Deferred Expense, Compensation and Benefits, Share-based Compensation Cost Finished goods Inventory, Finished Goods, Net of Reserves Purchase of common stock Purchase of common stock Purchase of common stock Share-based Payment Arrangement, Option Stock Options Share-based Payment Arrangement, Option [Member] Equity [Abstract] Equity [Abstract] Fair Value Hierarchy and NAV [Axis] Fair Value Hierarchy and NAV [Axis] Tax credits Effective Income Tax Rate Reconciliation, Tax Credit, Percent Interferon Interferon [Member] Interferon Measurement Input Type [Domain] Measurement Input Type [Domain] Segment Information Segment Reporting Disclosure [Text Block] Canadian dollar Canada, Dollars Property, Plant and Equipment [Table] Property, Plant and Equipment [Table] Research and development costs, percentage Research and Development Costs, Percentage Research and Development Costs, Percentage Short-term derivative Short-term derivative [Member] Short-term derivative [Member] Shares issued under employee stock purchase plan Shares Issued Under Employee Stock Purchase Plan [Table Text Block] Shares issued under employee stock purchase plan. Debt Securities, Available-for-sale [Line Items] Debt Securities, Available-for-sale [Line Items] Due after one year through five years Debt Securities, Available-for-Sale, Fair Value, Maturity, Allocated and Single Maturity Date, after Year One Through Five Issuance of common stock under stock option and stock purchase plans Stock Issued During Period, Value, Treasury Stock Reissued Increase (Decrease) in Stockholders' Equity [Roll Forward] Increase (Decrease) in Stockholders' Equity [Roll Forward] Document Fiscal Year Focus Document Fiscal Year Focus Property, Plant and Equipment [Abstract] Property, Plant and Equipment [Abstract] Additional paid-in capital Additional Paid-in Capital [Member] Repayments of borrowings Aggregate cash payments Repayments of Debt Long-term Debt, Type [Domain] Long-term Debt, Type [Domain] Twelve months Twelve months [Member] Twelve months [Member] Performance stock units settled in stock activity Performance Stock Units Settled in Stock [Table Text Block] Performance Stock Units Settled in Stock [Table Text Block] Equity Method of Accounting Equity Method Investments [Policy Text Block] Other, net Other Nonoperating Income (Expense) Guarantees Guarantees [Text Block] Other changes in operating assets and liabilities, net Increase (Decrease) in Other Operating Liabilities Retirement of common stock pursuant to the share repurchase program, at cost (in shares) Stock Repurchased and Retired During Period, Shares Other Payments for (Proceeds from) Other Investing Activities Acquired In-process Research and Development (IPR&D) In Process Research and Development, Policy [Policy Text Block] Interest in subsidiary (less than given percentage) Interest In Subsidiary Interest in subsidiary. Summary of activity related to BAN2401 and Elenbecestat collaboration 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] Current portion: Long-term Debt, Current Maturities [Abstract] Business Combinations [Abstract] Business Combinations [Abstract] Maximum Maximum Maximum [Member] Share-based Payment Arrangement [Abstract] Share-based Payment Arrangement [Abstract] Neurimmune Neurimmune Neurimmune [Member] Neurimmune. Expected Payment Timeline of Milestone [Axis] Expected Payment Timeline of Milestone [Axis] Expected Payment Timeline of Milestone [Axis] Raw materials Inventory, Raw Materials, Net of Reserves E2609 and BAN2401 E2609 and BAN2401 [Member] E2609 and BAN2401 [Member] Computer software and hardware Computer software and hardware Computer software and hardware JAPAN JAPAN Accrued expense and other Schedule of Accrued Liabilities [Table Text Block] Award Type [Axis] Award Type [Axis] Cash flow from financing activities: Net Cash Provided by (Used in) Financing Activities [Abstract] Operating lease liability maturity Lessee, Operating Lease, Liability, Maturity [Table Text Block] Total Biogen Inc. shareholders’ equity Stockholders' Equity Attributable to Parent City Area Code City Area Code Supplemental cash flow information Schedule of Cash Flow, Supplemental Disclosures [Table Text Block] Designated as Hedging Instrument Designated as Hedging Instrument [Member] Collaboration profit sharing Collaboration Profit Sharing [Member] Collaboration Profit Sharing Goodwill and Intangible Assets Disclosure [Abstract] Goodwill and Intangible Assets Disclosure [Abstract] Collaboration expense Collaboration expenses accrual Collaboration expenses accrual Purchase of treasury stock Payments for repurchase of common stock Payments for Repurchase of Common Stock Property, plant and equipment, useful life Property, Plant and Equipment, Useful Life Price per share Business Acquisition, Share Price Estimated Fair Value Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, Fair Value [Abstract] Other revenues Other revenues [Table Text Block] Other revenues [Table Text Block] Biogen Inc. shareholders’ equity Stockholders' Equity Attributable to Parent [Abstract] Employee stock purchase plan Employee Stock Purchase Plan [Member] Employee stock purchase plan. Concentration of Credit Risk Concentration Risk, Credit Risk, Policy [Policy Text Block] Land Land Long-term operating lease liabilities Operating Lease, Liability, Noncurrent Twenty billion Twenty billion [Member] Twenty billion [Member] Neurimmune's tax basis in ADUHELM Deferred Tax Assets, Counterparty Tax Basis in Asset Deferred Tax Assets, Counterparty Tax Basis in Asset Loss Contingency, Information about Litigation Matters [Abstract] Loss Contingency, Information about Litigation Matters [Abstract] Total Current Income Tax Expense (Benefit) Aggregate principal amount Senior notes aggregate principal amount Debt Instrument, Face Amount Periodic pension cost Defined Benefit Plan, Net Periodic Benefit Cost (Credit) Analysis of 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. 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 Income tax effect Share-based Payment Arrangement, Expense, Tax Benefit Proceeds from marketable securities, excluding strategic investments Realized Gain (Loss) on Investments [Table Text Block] Equity in (income) loss of investee, net of tax Income (Loss) from Equity Method Investments, Net of Tax Income (Loss) from Equity Method Investments, Net of Tax Research and Development Expense Research, Development, and Computer Software, Policy [Policy Text Block] Net change in acquired and in-licensed rights and patents Finite-Lived Intangible Assets, Period Increase (Decrease) Operating lease, contract term Lessee, Operating Lease, Term of Contract Derivatives, Fair Value [Line Items] Derivatives, Fair Value [Line Items] Selling, General and Administrative Expense Selling, General and Administrative Expenses, Policy [Policy Text Block] Business Acquisition [Line Items] Business Acquisition [Line Items] Share-based Payment Arrangement, Tranche Two Share-based Payment Arrangement, Tranche Two [Member] Share Repurchase Program [Axis] Share Repurchase Program [Axis] Investment in Samsung Bioepis Equity Method Investments Income Statement [Abstract] Income Statement [Abstract] Entity Registrant Name Entity Registrant Name Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, by Asset Class Asset Class [Domain] Prepaid research and discovery services Prepaid Expense and Other Assets Notes payable, fair value Notes Payable, Fair Value Disclosure Income tax (benefit) expense Income tax benefit (expense) Total income tax expense Income Tax Expense (Benefit) Internal reorganization of certain intellectual property rights Effective Income Tax Rate Reconciliation, Intellectual Property Rights Reorganization Effective Income Tax Rate Reconciliation, Intellectual Property Rights Reorganization Other Goodwill, Other Increase (Decrease) Cumulative sales level [Axis] Cumulative sales level [Axis] Cumulative sales level [Axis] Accounts receivable, net Accounts Receivable, after Allowance for Credit Loss, Current Inventory, other reserves and accruals Deferred Tax Assets, Inventory 2.250% Senior Notes due May 1, 2030 2.25% Senior Notes due May 1, 2030 [Member] 2.25% Senior Notes due May 1, 2030 Accumulated Other Comprehensive Income (Loss) [Table] Accumulated Other Comprehensive Income (Loss) [Table] Entity Tax Identification Number Entity Tax Identification Number Investment Type [Axis] Investment Type [Axis] Document Fiscal Period Focus Document Fiscal Period Focus Total current assets Assets, Current 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. Net (distribution) contribution to noncontrolling interest Proceeds from (Payments to) Noncontrolling Interests Sangamo Therapeutics, Inc. Agreement Sangamo Therapeutics, Inc. Agreement [Member] Sangamo Therapeutics, Inc. Agreement Total lease payments Lessee, Operating Lease, Liability, to be Paid Reduction of accounts receivable Accounts Receivable [Member] Forfeited (in shares) Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Forfeited in Period Denmark Manufacturing Operations Denmark Manufacturing Operations [Member] Denmark Manufacturing Operations [Member] Schedule of Stock by Class [Table] Schedule of Stock by Class [Table] Payment terms of accounts receivable arising from product sales Payment Terms Of Accounts Receivable Payment terms of accounts receivable. Schedule of Variable Interest Entities [Table] Schedule of Variable Interest Entities [Table] Statement of Financial Position [Abstract] Statement of Financial Position [Abstract] Long term debt, exchanged amount Long Term Debt, Exchanged, Amount Long Term Debt, Exchanged, Amount Property, Plant and Equipment, Type [Domain] Long-Lived Tangible Asset [Domain] Derivative Instruments, Gain (Loss) [Line Items] Derivative Instruments, Gain (Loss) [Line Items] 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] Global licensing collaboration, payment, selection of targets Global Licensing Collaboration Agreement, Payment, Selection of Targets Global Licensing Collaboration Agreement, Payment, Selection of Targets After Second GAZYVA Threshold Date After Second GAZYVA 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. Sublease Sublease [Member] Sublease [Member] Equity Component [Domain] Equity Component [Domain] Loss on divestiture of Hillerød, Denmark manufacturing operations, net of tax Discontinued Operation, Provision for Loss (Gain) on Disposal, Net of Tax Other Stockholders' Equity, Other 2022 Lessee, Operating Lease, Liability, to be Paid, Year One Disposal Group Name [Domain] Disposal Group Name [Domain] Summary of activity related to Aducanumab collaboration Summary of Activity Related to Aducanumab Collaboration [Table Text Block] [Table Text Block] for Summary of Activity Related to Aducanumab Collaboration [Table] Variable Interest Entity [Line Items] Variable Interest Entity [Line Items] Deferred tax assets: Components of Deferred Tax Assets [Abstract] Domestic Plan Domestic Plan [Member] Remaining performance obligation, amount Revenue, Remaining Performance Obligation, Amount Marketable securities Debt Securities, Available-for-sale, Noncurrent Cumulative sales level Cumulative sales level Cumulative sales level Solothurn, Switzerland Solothurn, Switzerland [Member] Solothurn [Member] Cancellable future commitments Other Commitment Summary of share-based compensation expense associated with each of our share-based compensating programs Share-based Payment Arrangement, Cost by Plan [Table Text Block] Facility Type [Axis] Facility Type [Axis] Facility Type Japanese yen Japan, Yen Retirement of common stock pursuant to the share repurchase program, at cost Treasury Stock, Retired, Cost Method, Amount 2025 Lessee, Operating Lease, Liability, to be Paid, Year Four Total intangible assets Intangible Assets, Gross (Excluding Goodwill) Income tax expense, reduction in deferred tax assets and liabilities, net Income Tax Expense, Reduction in Deferred Tax Assets and Liabilities, Net Income Tax Expense, Reduction in Deferred Tax Assets and Liabilities, Net Weighted Average Weighted Average [Member] Government securities Non-current Government Securities Non-current [Member] Government Securities Non Current. Significant Other Observable Inputs (Level 2) Fair Value, Inputs, Level 2 [Member] Number of ophthalmolgy biosimilar products Number of Ophthalmolgy Biosimilar Products, Exclusive Rights Number of Ophthalmolgy Biosimilar Products, Exclusive Rights Auditor Location Auditor Location Segment Reporting [Abstract] Segment Reporting [Abstract] Numerator: Numerator [Abstract] Numerator. Amortized Cost Debt Securities, Available-for-sale, Amortized Cost Revenue from External Customers by Products and Services [Table] Revenue from External Customers by Products and Services [Table] Investment in common stock, shares purchased (in shares) Investment in common stock, shares purchased Investment in common stock, shares purchased Estimated life (in years) Finite-Lived Intangible Asset, Useful Life GAZYVA GAZYVA [Member] GAZYVA [Member] Use of Estimates Use of Estimates, Policy [Policy Text Block] Basic and diluted earnings per share Schedule of Earnings Per Share, Basic and Diluted [Table Text Block] Total other comprehensive income (loss), net of tax Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Parent Title of 12(b) Security Title of 12(b) Security Investment, Name [Domain] Equity Method Investee, Name [Domain] Investment, Name [Domain] Payment to Neurimmune Payments to Noncontrolling Interests Common stock authorized (in shares) Common Stock, Shares Authorized Performance stock units settled in cash PSUs Settled in Cash Performance Stock Units Settled in Cash [Member] Performance Stock Units Settled in Cash [Member] Sangamo Therapeutics Sangamo Common Stock [Member] Sangamo Common Stock Payment Due At Second Anniversary Payment Due At Second Anniversary [Member] Payment Due At Second Anniversary Title of Individual [Axis] Title of Individual [Axis] Document Type Document Type Product and Service [Domain] Product and Service [Domain] Percentage of earnings of the target number of units granted based on actual stock performance 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. Operating lease weighted average remaining term and discount rate Operating lease weighted average remaining term and discount rate [Table Text Block] Operating lease weighted average remaining term and discount rate [Table Text Block] Interest rate on senior notes Interest rate on senior notes Debt Instrument, Interest Rate, Stated Percentage Operating lease, renewal term Lessee, Operating Lease, Renewal Term Net income (loss) attributable to noncontrolling interests, net of tax Net Income (Loss) Attributable to Noncontrolling Interest Contingent consideration obligations Fair Value Fair value, beginning of year Fair value, end of year Business Combination, Contingent Consideration, Liability Increase in unrecognized tax benefit Unrecognized Tax Benefits, Period Increase (Decrease) Share-based Compensation Arrangement by Share-based Payment Award Share-based Compensation Arrangement by Share-based Payment Award [Line Items] Cash flows, revenue Cash flows, revenue [Member] Cash flows, revenue [Member] Amortized Cost Equity Securities, FV-NI, Cost Revolving Credit Facility Revolving Credit Facility [Member] Expense reflected within statements of income Expense reflected within statements of income Expense reflected within statements of income Overnight reverse repurchase agreements Repurchase Agreements [Member] Transition toll tax liabilities Deferred Tax Liabilities, Undistributed Foreign Earnings Measurement Frequency [Axis] Measurement Frequency [Axis] Operating lease costs Lease, Cost [Table Text Block] Geographical [Axis] Geographical [Axis] Fair Values Derivatives, Balance Sheet Location, by Derivative Contract Type [Table] Fair Values Derivatives, Balance Sheet Location, by Derivative Contract Type [Table] State and Local Jurisdiction State and Local Jurisdiction [Member] Vesting [Domain] Vesting [Domain] Product, net Total product revenue Product [Member] Net investment hedge Net Investment Hedging [Member] Hedging Relationship [Axis] Hedging Relationship [Axis] 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. Interest Expense Interest Expense [Member] Issuance of common stock under stock option and stock purchase plans (in shares) Stock Issued During Period, Shares, Treasury Stock Reissued Pretax profit sharing formula Pretax Profit Sharing Formula [Table Text Block] Pretax Profit Sharing Formula [Table Text Block] Future contingent payment threshold Future contingent payment threshold Future contingent payment threshold Marketable Securities and Other Investments Investment, Policy [Policy Text Block] Net cash flow used in financing activities Net Cash Provided by (Used in) Financing Activities Treasury stock, at cost; 23.8 million and 23.8 million shares, respectively Treasury Stock, Value Weighted-average shares used in calculating: Weighted Average Number of Shares Outstanding, Diluted [Abstract] RITUXAN RITUXAN [Member] RITUXAN. Warehouse, Utilities and Support Space Warehouse, Utilities and Support Space [Member] Warehouse, Utilities and Support Space Vesting period Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period Net interest expense Unrecognized Tax Benefits, Income Tax Penalties and Interest Expense (Gain) loss on equity method investment Income (loss) from equity method investments Biogen's share of reimbursement from Eisai of ADUHELM milestone payments reflected in collaboration profit (loss) sharing in our consolidated statements of income Income (Loss) from Equity Method Investments Fair Value Measurements Fair Value Disclosures [Text Block] Restructuring charges Restructuring Charges 2024 Finite-Lived Intangible Asset, Expected Amortization, Year Three Foreign Tax Authority Foreign Tax Authority [Member] Contingent Consideration Business Combinations Policy [Policy Text Block] Royalties and licensing fees Accrued Royalties, Current Due after five years 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. Reduction in royalty rate payable on commercial sales Reduction in royalty rate payable on commercial sales Reduction in royalty rate payable on commercial sales Goodwill Goodwill and Intangible Assets, Goodwill, Policy [Policy Text Block] Document Period End Date Document Period End Date 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. Corporate debt securities Current Corporate Debt Securities Current [Member] Corporate debt securities Current. Acquisitions of intangible assets Payments to Acquire Intangible Assets Property, Plant and Equipment, Type [Axis] Lease Contractual Term [Axis] Unrealized gains (losses) on securities available for sale, net of tax OCI, Debt Securities, Available-for-Sale, Gain (Loss), after Adjustment and Tax After First GAZYVA Threshold Date until Second GAZYVA Threshold Date After First GAZYVA Threshold Date until Second GAZYVA 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. Consolidated Entities [Domain] Consolidated Entities [Domain] Business Overview Business Overview [Policy Text Block] Describes an overview of the company and its operations. New Anti Cd20 New Anti Cd20 [Member] New Anti-CD20. State taxes Effective Income Tax Rate Reconciliation, State and Local Income Taxes, Percent State Deferred State and Local Income Tax Expense (Benefit) Returns Sales Returns and Allowances [Member] Components of property, plant and equipment, net Property, Plant and Equipment [Table Text Block] Debt instrument, term Debt Instrument, Term Income Statement Location [Domain] Income Statement Location [Domain] Income Statement Location [Domain] Contingent Consideration Type [Domain] Contingent Consideration Type [Domain] Marketable equity securities Investments, Fair Value Disclosure ADUHELM ADUHELM [Member] ADUHELM Neurimmune tax impacts Effective Income Tax Rate Reconciliation, Nondeductible Expense, Research and Development, Percent Property, plant and equipment, net Total property, plant and equipment, net Property, Plant and Equipment, Net Operating expense Operating Expenses Due after five years 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. Accrued milestone payment Accrued Milestone Payments Accrued Milestone Payments Total development expense incurred by the collaboration related to the advancement of lecanemab and elenbecestat Expense Incurred By Collaboration Total expense incurred by collaboration. Estimated future amortization of intangible assets Schedule of Finite-Lived Intangible Assets, Future Amortization Expense [Table Text Block] Acorda Acorda [Member] Acorda. Net income per share: Earnings Per Share [Abstract] Earnings Per Share [Abstract] Collaborative Arrangement Collaborative Arrangement [Member] 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. Total cost Property, plant and equipment, gross Property, Plant and Equipment, Gross Unrealized gains (losses) recognized during the period on equity securities Equity Securities, FV-NI, Unrealized Gain (Loss) Revenues Total product revenue Total revenue from anti-CD20 therapeutic programs Revenues Revenue Revenues Amounts reclassified from accumulated other comprehensive income (loss) Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax GILTI Deferred tax liabilities, GILTI tax calculation Deferred tax liabilities, GILTI tax calculation Foreign exchange contract Foreign Exchange Contract [Member] Amortization and impairment of acquired intangible assets Amortization and Impairment of Acquired Intangible Assets Amortization and Impairment of Acquired Intangible Assets Proceeds from divestiture of interest in joint venture Proceeds from Divestiture of Interest in Joint Venture Cost and expense: Costs and Expenses [Abstract] Collaboration agreement term Period Of Collaboration Agreement Period Of Collaboration Agreement Sales trigger gross sales threshold Sales Trigger Gross Sales Threshold Sales trigger gross sales threshold. FLIXABI FLIXABI [Member] FLIXABI [Member] Entity Current Reporting Status Entity Current Reporting Status 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 [Abstract] Effective tax rate Effective Income Tax Rate Reconciliation, Percent Loss contingency Loss Contingency, Estimate of Possible Loss Series A Series A Preferred Stock [Member] Current portion of notes payable Debt, Current General Business General Business Tax Credit Carryforward [Member] Beginning balance (in shares) Ending balance (in shares) Shares, Issued Global licensing collaboration, payment, sales based milestone Global Licensing Collaboration Agreement, Payment, Achievement of Specific Sales Based Milestones Global Licensing Collaboration Agreement, Payment, Achievement of Specific Sales Based Milestones Tax Credit Carryforward, Name [Domain] Tax Credit Carryforward, Name [Domain] United States U.S. UNITED STATES Discount rate Measurement Input, Discount Rate [Member] Investments in Variable Interest Entities Variable Interest Entity Disclosure [Text Block] Taxes payable Taxes Payable, Current Transition toll tax repatriation tax rate Transition Toll Tax Repatriation Tax Rate Transition Toll Tax Repatriation Tax Rate Leases [Abstract] Leases [Abstract] Income Taxes Income Tax, Policy [Policy Text Block] Intangible Assets Goodwill and Intangible Assets, Intangible Assets, Policy [Policy Text Block] Accumulated Other Comprehensive Income (Loss) [Line Items] Accumulated Other Comprehensive Income (Loss) [Line Items] Entity Address, Postal Zip Code Entity Address, Postal Zip Code Diluted earnings per share attributable to Biogen Inc. (in shares) Shares used in calculating diluted earnings per share (in shares) Weighted Average Number of Shares Outstanding, Diluted Domestic Income (Loss) from Continuing Operations before Income Taxes, Domestic Amortization Amortization 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. Acquired in-process research and development Payments to Acquire in Process Research and Development Income Taxes Income Tax Disclosure [Text Block] Long-term Debt, Type [Axis] Long-term Debt, Type [Axis] Auditor Information [Abstract] Auditor Information [Abstract] Development of product candidate expense Research and Development Expense, Development Of Product Candidate Research and Development Expense, Development Of Product Candidate Expected Payment Timeline of Milestone [Domain] Expected Payment Timeline of Milestone [Domain] [Domain] for Expected Payment Timeline of Milestone [Axis] Subtotal: MS MS Product Revenues [Member] MS Product Revenues [Member] Tax credits Deferred Tax Liabilities, Tax Credit Carryforwards Deferred Tax Liabilities, Tax Credit Carryforwards Statement of Cash Flows [Abstract] Statement of Cash Flows [Abstract] Class of Stock [Line Items] Class of Stock [Line Items] 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. Credit Facility [Axis] Credit Facility [Axis] Not designated as hedging instrument Not Designated as Hedging Instrument [Member] Finite-Lived Intangible Assets, Major Class Name [Domain] Finite-Lived Intangible Assets, Major Class Name [Domain] Entity Address, Address Line One Entity Address, Address Line One Retirement Plan Sponsor Location [Domain] Retirement Plan Sponsor Location [Domain] Cash received under the 2015 ESPP Stock Issued During Period, Value, Employee Stock Purchase Plan Less: accumulated depreciation Accumulated Depreciation, Depletion and Amortization, Property, Plant, and Equipment Cash flow from operating activities: Net Cash Provided by (Used in) Operating Activities [Abstract] Consolidation Consolidation, Policy [Policy Text Block] Entity Shell Company Entity Shell Company Future minimum batch production for Denmark manufacturing operations Purchase Commitment, Remaining Minimum Amount Committed Total current liabilities Liabilities, Current Payments related to issuance of stock for share-based compensation arrangements, net Payment, Tax Withholding, Share-based Payment Arrangement Reduction in contingent consideration Business Combination, Contingent Consideration Arrangements, Change in Amount of Contingent Consideration, Asset Foreign Exchange Forward Foreign Exchange Forward [Member] Money market funds Money Market Funds [Member] Reclassification out of Accumulated Other Comprehensive Income Reclassification out of Accumulated Other Comprehensive Income [Member] Asia Asia [Member] Current Fiscal Year End Date Current Fiscal Year End Date Derivative Instruments, Gain (Loss) by Hedging Relationship, by Income Statement Location, by Derivative Instrument Risk [Table] Derivative Instruments, Gain (Loss) [Table] Eisai Eisai [Member] Eisai [Member] Co-promotion operating profit threshold for Rituxan in US 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. Statement [Table] Statement [Table] Collaborative Arrangement and Arrangement Other than Collaborative [Table] Collaborative Arrangement and Arrangement Other than Collaborative [Table] Distributor One Distributor One [Member] Distributor one. New Accounting Pronouncements New Accounting Pronouncements, Policy [Policy Text Block] Other Deferred Tax Assets, Other Unrecognized compensation cost related to unvested share-based compensation Share-based Payment Arrangement, Nonvested Award, Cost Not yet Recognized, Amount 2015 Senior Notes 2015 Senior Notes [Member] 2015 Senior Notes Statistical Measurement [Axis] Statistical Measurement [Axis] Subsequent Event Subsequent Event [Member] Net Finite-Lived Intangible Assets, Net Goodwill Goodwill, beginning of year Goodwill, end of year Goodwill Noncontrolling interests Stockholders' Equity Attributable to Noncontrolling Interest Taxes on foreign earnings Effective Income Tax Rate Reconciliation, Foreign Income Tax Rate Differential, Percent Summary of Activity Related to Sage Therapuetics Summary of Activity Related to Sage Therapeutics [Table Text Block] Summary of Activity Related to Sage Therapeutics Scenario [Domain] Scenario [Domain] Summary of roll forward of the changes in goodwill Schedule of Goodwill [Table Text Block] Adjustments to reconcile net income to net cash flow from operating activities: Adjustments to Reconcile Net Income (Loss) to Cash Provided by (Used in) Operating Activities [Abstract] Marketable equity securities, non-current Equity Securities, Non-Current [Member] Equity Securities, Non-Current Biogen Idec International Neuroscience GmbH Biogen Idec International Neuroscience Member Biogen idec international neuroscience. Subsequent Event Type [Axis] Subsequent Event Type [Axis] Summary of the fair value for our outstanding derivatives Schedule of Derivative Instruments in Statement of Financial Position, Fair Value [Table Text Block] Net profits on investment Income (Loss) from Equity Method Investments, Net of Dividends or Distributions Operating lease cost Operating Lease, Cost 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 Denali, Ionis, Sage, and Sangamo Denali, Ionis, Sage, and Sangamo [Member] Denali, Ionis, Sage, and Sangamo Unfunded Status of Postretirement Benefit Plans, Net of Tax Accumulated Defined Benefit Plans Adjustment Attributable to Parent [Member] Net Gains/(Losses) Recognized in Other Comprehensive Income (Amounts Excluded from Effectiveness Testing) (in millions) Derivatives used in net investment hedge, net of tax, excluded component increase (decrease) Derivatives used in net investment hedge, net of tax, excluded component increase (decrease) Inventory [Line Items] Inventory [Line Items] Collaborative arrangements and non-collaborative arrangement transactions Collaborative Arrangement and Arrangement Other than Collaborative [Line Items] Deferred income taxes Total Deferred Income Tax Expense (Benefit) Expense incurred by the collaboration Expense incurred by the collaboration Expense incurred by the collaboration Expense incurred by the collaboration 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] Authorized amount of share repurchases Stock Repurchase Program, Authorized Amount Hedging Designation [Domain] Hedging Designation [Domain] Statutory rate Effective Income Tax Rate Reconciliation, at Federal Statutory Income Tax Rate, Percent Earnings per Share Earnings Per Share, Policy [Policy Text Block] License fee License Fee License Fee Trademarks and trade names Trademarks and Trade Names [Member] Fair Value, Measurements, Fair Value Hierarchy [Domain] Fair Value Hierarchy and NAV [Domain] Net income attributable to Biogen Inc. Net income attributable to Biogen Inc. Net income attributable to Biogen Inc. Net Income (Loss) Attributable to Parent Accounts Receivable Receivable [Policy Text Block] Operating lease assets Operating Lease, Right-of-Use Asset Foreign currency forward contracts that were entered into to hedge forecasted revenue Schedule of Notional Amounts of Outstanding Derivative Positions [Table Text Block] Net operating loss Deferred Tax Assets, Operating Loss Carryforwards Employee compensation and benefits Employee-related Liabilities, Current Settlement refund Unrecognized Tax Benefits, Increase Resulting from Settlements with Taxing Authorities Income Statement Location [Axis] Income Statement Location [Axis] Income Statement Location [Axis] Accumulated Amortization Finite-Lived Intangible Assets, Accumulated Amortization Statistical Measurement [Domain] Statistical Measurement [Domain] Tax benefit and cash received from stock option exercises Cash Proceeds Received and Tax Benefit from Share-based Payment Awards [Table Text Block] Geographic information Schedule of Revenue from External Customers Attributed to Foreign Countries by Geographic Area [Table Text Block] Total debt maturities Schedule of Maturities of Long-term Debt [Table Text Block] Leasehold improvements Leasehold Improvements, Gross Payments/credits 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. Consideration received for sale of Denmark manufacturing operations Disposal Group, Including Discontinued Operation, Consideration Entity Incorporation, State or Country Code Entity Incorporation, State or Country Code Proceeds from divestiture of Hillerød, Denmark manufacturing operations Proceeds from Sale of Property, Plant, and Equipment Revenue from Contract with Customer [Abstract] Revenue from Contract with Customer [Abstract] Impairment of intangible assets Impairment of Intangible Assets (Excluding Goodwill) Investments and other assets Other Assets, Noncurrent 2020 Share Repurchase Program Share Repurchase Program 5 [Member] Share Repurchase Program 5 Deferred tax assets, increase in value Deferred Tax Assets, Increase (Decrease), Value Deferred Tax Assets, Increase (Decrease), Value Expected dividend yield Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions, Expected Dividend Rate Retirement Benefits [Abstract] Retirement Benefits [Abstract] In-process research and development In Process Research and Development [Member] Proceeds from sales of strategic investments Proceeds from Sale of Equity Method Investments Entity File Number Entity File Number Redemptions percentage of 2015 notes Debt Instrument, Redemption Price, Percentage Other income (expense), net Schedule of Other Nonoperating Income (Expense) [Table Text Block] TYSABRI TYSABRI product [Member] TYSABRI product [Member] Expected license fee and regulatory milestone payments Expected License Fee And Regulatory Milestone Payments Expected license fee and regulatory milestone payments. Deferred tax assets, value, reduction in value Deferred Tax Assets, Value, Reduction in Value Deferred Tax Assets, Value, Reduction in Value DPN DPN [Member] DPN Marketable debt and equity securities Debt Securities, Available-for-sale [Table Text Block] Biogen's share of pre-tax profits in the U.S. for RITUXAN and GAZYVA Biogen's share of pre-tax profits in the U.S. for RITUXAN and GAZYVA Share Of Co Promotion Profits Share of co promotion profits. Acquisition of Nightstar Therapeutics plc, net of cash acquired Payments to Acquire Business Two, Net of Cash Acquired Divestitures Disposal Groups, Including Discontinued Operations, Disclosure [Text Block] SEC Schedule, 12-09, Valuation and Qualifying Accounts Disclosure [Line Items] SEC Schedule, 12-09, Valuation and Qualifying Accounts Disclosure [Line Items] Investment in common stock, value Investment in common stock, value Investment in common stock, value Organization, Consolidation and Presentation of Financial Statements [Abstract] Organization, Consolidation and Presentation of Financial Statements [Abstract] Schedule of Revenue by Major Customers, by Reporting Segments [Table] Schedule of Revenue by Major Customers, by Reporting Segments [Table] Business Acquisition, Contingent Consideration [Line Items] Business Acquisition, Contingent Consideration [Line Items] Costs associated with the senior notes offerings Payments of Financing Costs Other current assets Other Current Assets [Member] Variable lease cost Variable Lease, Cost InnoCare Pharma Limited (InnoCare) Agreement InnoCare Pharma Limited (InnoCare) Agreement [Member] InnoCare Pharma Limited (InnoCare) Agreement BIIB111 and BIIB112 BIIB111 and BIIB112 [Member] BIIB111 and BIIB112 Buildings Building [Member] Repurchase of common stock (in shares) Weighted Average Number of Shares, Common Stock Subject to Repurchase or Cancellation Granted (in dollars per share) Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period, Weighted Average Grant Date Fair Value 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 Title of Individual [Domain] Title of Individual [Domain] Schedule of Operating Leased Assets [Table] Schedule of Operating Leased Assets [Table] Disposal Group Name [Axis] Disposal Group Name [Axis] Non-current portion of notes payable Total Notes Payable, Noncurrent Share Repurchase Program [Domain] Share Repurchase Program [Domain] Contractual Adjustments Contractual Adjustments [Member] Contractual adjustments. Credit Facility [Domain] Credit Facility [Domain] ASSETS Assets [Abstract] 2024 Long-Term Debt, Maturity, Year Three Accumulated impairment losses related to goodwill Goodwill, Impaired, Accumulated Impairment Loss Derivative Instruments Derivative Instruments and Hedging Activities Disclosure [Text Block] Ownership percentage before additional purchase transaction Equity Method Investment, Ownership Percentage Before Additional Purchase Transaction Equity Method Investment, Ownership Percentage Before Additional Purchase Transaction Granted (in shares) Number of shares granted (in shares) Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period Research and development Research and Development Expense (Excluding Acquired in Process Cost) Market stock units activity Schedule Of Market Stock Units Activity [Table Text Block] Schedule Of Market Stock Units Activity. Intangible Assets and Goodwill Goodwill and Intangible Assets Disclosure [Text Block] Property, Plant and Equipment Property, Plant and Equipment, Policy [Policy Text Block] 2015 ESPP 2015 ESPP [Member] 2015 ESPP [Member] Debt Instrument [Axis] Debt Instrument [Axis] Marketable debt securities: Debt Securities, Available-for-sale [Abstract] Mortgage and other asset backed securities Current Mortgage And Other Asset Backed Securities Current [Member] Mortgage and other asset backed securities Current. Commitments and Contingencies Commitments and Contingencies Disclosure [Text Block] Reduction in royalty rate Reduction in royalty rate Reduction in royalty rate Share-based compensation expense included in total cost and expense Share-based compensation expense included in total cost and expense Share Based Compensation Expense Included In Costs And Expenses Share based compensation expense included in costs and expenses. Unobservable Input Business Combination, Contingent Consideration, Liability, Measurement Input Inventory, Current [Table] Inventory, Current [Table] Tax expense on disposal group Discontinued Operation, Tax Effect of Discontinued Operation Extinguishment of Debt [Axis] Extinguishment of Debt [Axis] Operating lease assets obtained in exchange for lease obligations Right-of-Use Asset Obtained in Exchange for Operating Lease Liability Components of deferred tax assets and liabilities Schedule of Deferred Tax Assets and Liabilities [Table Text Block] Vesting percentage Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Rights, Percentage Commitments and Contingencies Disclosure [Abstract] Commitments and Contingencies Disclosure [Abstract] Capitalized share-based compensation costs Share-based Payment Arrangement, Amount Capitalized Tax Credit Carryforward [Table] Tax Credit Carryforward [Table] Schedule of Employee Stock Ownership Plan (ESOP) Disclosures [Table] Schedule of Employee Stock Ownership Plan (ESOP) Disclosures [Table] Operating expense Operating Expense [Member] Number of operating segments Number of Operating Segments Guarantees [Abstract] Guarantees [Abstract] Current portion of contingent consideration obligations Business Combination, Contingent Consideration, Liability, Current Indefinite-lived Intangible Assets, Major Class Name [Domain] Indefinite-lived Intangible Assets, Major Class Name [Domain] Unrealized gains (losses) on pension benefit obligation, net of tax Other Comprehensive (Income) Loss, Defined Benefit Plan, after Reclassification Adjustment, after Tax Government securities US Treasury and Government [Member] 2024 Lessee, Operating Lease, Liability, to be Paid, Year Three Non-current portion: Long-term Debt, Excluding Current Maturities [Abstract] SEC Schedule, 12-09, Valuation Allowances and Reserves Type [Axis] SEC Schedule, 12-09, Valuation Allowances and Reserves Type [Axis] Expected costs to sell disposal group Disposal Group, Including Discontinued Operation, Other Expense Contingencies Commitments and Contingencies, Policy [Policy Text Block] Deferred tax liabilities, value, reduction in value Deferred Tax Liabilities, Value, Reduction in Value Deferred Tax Liabilities, Value, Reduction in Value Revenue by product Disaggregation of Revenue [Table Text Block] 2026 Long-Term Debt, Maturity, Year Five Expenses related to savings plan Defined Contribution Plan, Cost Due from related parties Due from Related Parties Shares Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward] 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. Unrealized gain (loss) on derivative instruments Net unrealized gains (losses) Unrealized Gain (Loss) on Derivatives GILTI Effective Tax Rate Reconciliation, GILTI tax Effective Tax Rate Reconciliation, GILTI tax Machinery and equipment Machinery and Equipment, Gross Tax credits Deferred Tax Assets, Tax Credit Carryforwards 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. Omnibus Plan number of shares authorized (in shares) Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Authorized Redemption percentage for change in control provision Debt Instrument, Make Whole Provision Redemption Price, Percentage Debt Instrument, Make Whole Provision Redemption Price, Percentage Cash paid for amounts included in the measurement of lease liabilities Operating Lease, Payments Summary of Activity Related to Sangamo Therapeutics Summary of Activity Related to Sangamo Therapeutics [Table Text Block] Summary of Activity Related to Sangamo Therapeutics Premium on purchase of common stock Premium on purchase of common stock Premium on purchase of common stock Goodwill [Roll Forward] Goodwill [Roll Forward] Discounts Reserve for Cash Discounts [Member] Reserve for Cash Discounts [Member] Number of wholesalers Number of Wholesalers Number of Wholesalers Total Assets, Fair Value Disclosure Fair value, in-process research and development, discount rate Fair Value, Indefinite Lived Intangible Assets, Discount Rate Fair Value, Indefinite Lived Intangible Assets, Discount Rate Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward] Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward] Expected profit share percentage Equity Method Investments, Expected Profit Share Equity Method Investments, Expected Profit Share Percentage of royalties as per collaboration Percentage Of Royalties As Per Collaboration Percentage of royalties as per collaboration. 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] Income tax expense (benefit): Income Tax Expense (Benefit), Continuing Operations [Abstract] Total liabilities and equity Liabilities and Equity Other Other Accrued Liabilities, Current Other Effective Income Tax Rate Reconciliation, Other Adjustments, Percent Percentage of earnings of the target number of units granted based on actual stock performance 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. Share-based Payment Arrangement, Tranche One Share-based Payment Arrangement, Tranche One [Member] Intangible asset, fair value Finite-lived Intangible Assets, Fair Value Disclosure 2026 Lessee, Operating Lease, Liability, to be Paid, Year Five Proceeds from sales and maturities of marketable securities Proceeds from maturities and sales Proceeds from Sale and Maturity of Debt Securities, Available-for-sale Alkermes Alkermes [Member] Alkermes [Member] Cash payments on debt extinguishment, including accrued interest Payment for Debt Extinguishment or Debt Prepayment Cost, Including Accrued Interest Payment for Debt Extinguishment or Debt Prepayment Cost, Including Accrued Interest 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. Europe Europe, Excluding Germany [Member] Europe, Excluding Germany Divestitures [Abstract] Divestitures [Abstract] Divestitures [Abstract] Tax benefit realized for stock options Share-based Payment Arrangement, Exercise of Option, Tax Benefit Cash equivalents, total Cash Equivalents, at Carrying Value Deferred tax liability, increase in value Deferred Tax Liability, Increase (Decrease), Value Deferred Tax Liability, Increase (Decrease), Value Derivative contracts Foreign Currency Contract, Asset, Fair Value Disclosure Earnings per Share Earnings Per Share [Text Block] Repurchase of common stock pursuant to the share repurchase program, at cost Treasury Stock, Value, Acquired, Cost Method 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. Ownership percentage Equity Method Investment, Ownership Percentage Foreign Plan Foreign Plan [Member] 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. 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Rest of World Non-US [Member] Long-term operating lease liabilities Long-term operating lease liabilities [Member] Long-term operating lease liabilities [Member] Intangible assets, net Total intangible assets Intangible Assets, Net (Excluding Goodwill) Comprehensive income attributable to Biogen Inc. 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Due in one year or less Debt Securities, Available-for-Sale, Amortized Cost, Maturity, Allocated and Single Maturity Date, Year One Furniture and fixtures Furniture and Fixtures, Gross Property, Plant and Equipment Property, Plant and Equipment Disclosure [Text Block] TGN TGN [Member] TGN Regulatory Milestones Regulatory Milestones [Member] Regulatory Milestones [Member] Other long-term liabilities Other Liabilities, Noncurrent Directors Plan Directors Plan [Member] Directors plan. Employee benefit plan obligations Defined Benefit Plan, Benefit Obligation Dividend yield percentage Dividend Yield Percentage Dividend Yield Percentage Repurchase of common stock pursuant to the share repurchase program, at cost (in shares) Repurchase of common stock, at cost (in shares) Treasury Stock, Shares, Acquired Share-based Payment Arrangement, Expensed and Capitalized, Amount [Table] Share-based Payment Arrangement, Expensed and Capitalized, Amount [Table] Debt Instrument Debt Instrument [Line Items] Trading Symbol Trading Symbol 2018 Share Repurchase Program Share Repurchase Program 2 [Member] Share Repurchase Program 2 Component of accrued expense and other Other Current Liabilities [Member] Share-based Payment Arrangement, Tranche Three Share-based Payment Arrangement, Tranche Three [Member] 3.250% Senior Notes due February 15, 2051 Three Point Two Five Percent Senior Notes Due February Twenty Fifty One [Member] Three Point Two Five Percent Senior Notes Due February Twenty Fifty One Reclassification out of Accumulated Other Comprehensive Income [Table] Reclassification out of Accumulated Other Comprehensive Income [Table] Denominator: Denominator [Abstract] Denominator. 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Research Research Tax Credit Carryforward [Member] BIIB112 BIIB112 [Member] BIIB112 [Member] Defined Contribution Plan Disclosure [Line Items] Defined Contribution Plan Disclosure [Line Items] Reclassification out of Accumulated Other Comprehensive Income [Domain] Reclassification out of Accumulated Other Comprehensive Income [Domain] Net income Net income Net Income (Loss), Including Portion Attributable to Noncontrolling Interest Investments [Domain] Investments [Domain] Cash and cash equivalents Cash and cash equivalents, beginning of the year Cash and cash equivalents, end of the year Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents Facility Location [Axis] Facility Location [Axis] -- None. No documentation exists for this element. -- BIIB054 BIIB054 [Member] BIIB054 Commercial Milestones Commercial Milestones [Member] Commercial Milestones [Member] Common stock, par value (in dollars per share) Common Stock, Par or Stated Value Per Share Maximum range of risk-free interest rates Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions, Risk Free Interest Rate, Maximum Current assets: Assets, Current [Abstract] Statement of Comprehensive Income [Abstract] Statement of Comprehensive Income [Abstract] Other Proceeds from (Payments for) Other Financing Activities Upfront payment for collaboration agreement Upfront Payment For Collaboration Agreement Upfront Payment For Collaboration Agreement Cost Finite-Lived Intangible Assets, Gross Impairment of Long-Lived Assets Impairment or Disposal of Long-Lived Assets, Policy [Policy Text Block] Fair value measurement and measurement inputs, recurring and nonrecurring Fair Value Measurement Inputs Fair Value Measurement Inputs Cash equivalents Cash and Cash Equivalents, Fair Value Disclosure Deferred tax liabilities, temporary differences, purchase accounting, foreign subsidiaries Deferred tax liabilities, temporary differences, purchase accounting, foreign subsidiaries Deferred tax liabilities, temporary differences, purchase accounting, foreign subsidiaries Other comprehensive income (loss), net of tax Other Comprehensive Income (Loss), Net of Tax Fair value, measurements, recurring Fair Value, Recurring Fair Value, Recurring [Member] Sage Therapeutics Sage Therapeutics Inc. [Member] Sage Therapeutics Inc. Equity Stockholders' Equity Note Disclosure [Text Block] Statute expirations Unrecognized Tax Benefits, Reduction Resulting from Lapse of Applicable Statute of Limitations Unrealized Gains (Losses) on Cash Flow Hedges, Net of Tax Gains (losses) on cash flow hedges Accumulated Gain (Loss), Net, Cash Flow Hedge, Parent [Member] Purchased intangible assets Effective Income Tax Rate Reconciliation, Nondeductible Expense, Depreciation and Amortization, Percent 2026 Finite-Lived Intangible Asset, Expected Amortization, Year Five Fair Value Measurements Fair Value of Financial Instruments, Policy [Policy Text Block] 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. Auditor Firm ID Auditor Firm ID Payments to increase investment in Samsung Bioepis Payments to Acquire Equity Method Investments Royalty Royalty [Member] Research and development asset [Axis] Research and development asset [Axis] Research and development asset [Axis] Net cash flow (used in) provided by investing activities Net Cash Provided by (Used in) Investing Activities Changes in operating assets and liabilities, net: Increase (Decrease) in Operating Capital [Abstract] Vested (in dollars per share) Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Vested in Period, Weighted Average Grant Date Fair Value Time-vested restricted stock units activity Time Vested Restricted Stock Units Activity [Table Text Block] Time Vested Restricted Stock Units Activity. Class of Stock [Domain] Class of Stock [Domain] Total transaction value Payments to Acquire Businesses, Gross Contract Manufacturing Customer Contract Manufacturing Customer [Member] Contract Manufacturing Customer Euro Euro Member Countries, Euro 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 FAMPYRA FAMPYRA [Member] FAMPYRA [Member] Defined Contribution Plan [Table] Defined Contribution Plan [Table] Senior unsecured revolving credit facility maximum borrowing capacity Line of Credit Facility, Maximum Borrowing Capacity Payments for pre-combination equity compensation Other Payments to Acquire Businesses 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. Global licensing collaboration, shares purchased (in shares) Global Licensing Collaboration Agreement, Shares, Purchased Global Licensing Collaboration Agreement, Shares, Purchased Buildings Buildings and Improvements, Gross Debt Instrument, Name [Domain] Debt Instrument, Name [Domain] Other long-lived assets Long-lived assets Long-Lived Assets Operating Leased Assets [Line Items] Operating Leased Assets [Line Items] Other current assets Other Assets, Current Aducanumab Aducanumab [Member] Aducanumab [Member] Mortgage and other asset backed securities Asset-backed Securities [Member] Advertising costs Advertising Expense Mosunetuzumab Mosunetuzumab [Member] Mosunetuzumab Effect of dilutive securities: Dilutive Securities, Effect on Basic Earnings Per Share [Abstract] Deferred tax liabilities, goodwill Deferred Tax Liabilities, Goodwill Liability derivative instruments Derivative Liability, Fair Value, Gross Liability Range of durations of foreign currency forward contracts Derivative, Remaining Maturity Other Relationships Other Relationships [Member] Other Relationships Summary of Activity Related to Denali Therapeutics Collaboration Summary of Activity Related to Denali Therapeutics Collaboration [Table Text Block] Summary of Activity Related to Denali Therapeutics Collaboration Aggregate principal amount, debt converted Extinguishment of Debt, Amount Net Gains/(Losses) Reclassified from AOCI into Operating Income (in millions) Other Comprehensive Income (Loss), Cash Flow Hedge, Gain (Loss), Reclassification, before Tax Summary of significant unobservable inputs Fair Value Measurement Inputs and Valuation Techniques [Table Text Block] Beginning balance Ending balance Revenue-related reserves for discounts and allowances SEC Schedule, 12-09, Valuation Allowances and Reserves, Amount Construction in progress Construction in Progress, Gross Inventory Inventory, Policy [Policy Text Block] Foreign Current Foreign Tax Expense (Benefit) Completed technology Developed Technology Rights [Member] Due in one year or less Debt Securities, Available-for-Sale, Fair Value, Maturity, Allocated and Single Maturity Date, Year One Schedule of Finite-Lived Intangible Assets [Table] Schedule of Finite-Lived Intangible Assets [Table] Revenue from anti-CD20 therapeutic programs Revenues from anti-cd20 therapeutic programs [Member] Revenues from anti-cd20 therapeutic programs