Trading Arrangements, by Individual Trading Arrangements, by Individual [Table] Trading Arrangement: Trading Arrangement [Axis] Maximum Maximum [Member] Intangible assets Intangible Assets Excluding Goodwill [Table Text Block] Intangible assets excluding goodwill. Title of 12(b) Security Title of 12(b) Security Zorevunersen Zorevunersen [Member] Zorevunersen Adjustment to Compensation, Amount Adjustment to Compensation Amount Financial Instrument [Axis] Financial Instrument [Axis] Segment Reporting [Abstract] Estimated future amortization for acquired intangible assets Intangible Asset, Finite-Lived, and Capitalized Cost, Software to be Sold, Leased, or Marketed, Estimated Amortization Expense [Table Text Block] Restructuring Plan [Axis] Restructuring Plan [Axis] Employee compensation and benefits Employee-related Liabilities, Current Stoke Therapeutics Inc Stoke Therapeutics Inc [Member] Stoke Therapeutics Inc Derivative Instrument [Axis] Derivative Instrument [Axis] Issuance of common stock under stock award plan (in shares) Stock Issued During Period, Shares, Restricted Stock Award, Net of Forfeitures 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. Government securities US Treasury and Government [Member] SPINRAZA SPINRAZA [Member] SPINRAZA [Member] Entity Common Stock, Shares Outstanding Entity Common Stock, Shares Outstanding Inventory, noncurrent Inventory, Noncurrent Accrued expense and other Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Assets, Operating Lease Assets Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Assets, Operating Lease Assets Mortgage and other asset backed securities Asset-Backed Securities [Member] 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] UCB Pharma S.A. UCB Pharma S.A. [Member] UCB Pharma S.A. 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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. 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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 SEC Schedule, 12-09, Valuation Allowances and Reserves [Domain] SEC Schedule, 12-09, Valuation Allowances and Reserves [Domain] Document Period End Date Document Period End Date Expected additional milestone payments when meeting certain sales threshold Expected Additional Milestone Payments When Meeting Certain Sales Threshold Expected Additional Milestone Payments When Meeting Certain Sales Threshold Other research and discovery Other research and discovery [Member] Other research and discovery [Member] Debt instrument, face amount Debt Instrument, Face Amount Debt instrument, term Debt Instrument, Term Realized losses Debt Securities, Available-for-Sale, Realized Loss Reconciliation of revenue from segments to consolidated Segment Reporting, Reconciliation of Revenue by Segment to Consolidated [Table Text Block] Business Combination, Separately Recognized Transaction, Acquisition-Related Cost, Expensed, Statement of Income or Comprehensive Income [Extensible Enumeration] Business Combination, Separately Recognized Transaction, Acquisition-Related Cost, Expensed, Statement of Income or Comprehensive Income [Extensible Enumeration] Reclassification out of Accumulated Other Comprehensive Income [Domain] Reclassification out of Accumulated Other Comprehensive Income [Domain] Milestone Payment, Developmental Milestone Payment, Developmental [Member] Milestone Payment, Developmental Business Combination, Separately Recognized Transaction [Axis] Business Combination, Separately Recognized Transaction [Axis] Fumarate Fumarate [Member] Fumarate [Member] Accumulated other comprehensive income (loss) Accumulated Other Comprehensive Income (Loss), Net of Tax Entity Shell Company Entity Shell Company Changes in fair value Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Gain (Loss) Included in Earnings Trademarks and trade names Trademarks and Trade Names [Member] Earnings per Share Earnings Per Share [Text Block] Securities Litigation Securities Litigation [Member] Securities Litigation Net income per share: Earnings Per Share [Abstract] Earnings Per Share [Abstract] Document Fiscal Period Focus Document Fiscal Period Focus 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 [Member] Peer Group Issuers, Footnote Peer Group Issuers, Footnote [Text Block] BYOOVIZ BYOOVIZ [Member] BYOOVIZ Total liabilities Liabilities Trading Symbol Trading Symbol Discount rate Measurement Input, Discount Rate [Member] Purchases of property, plant and equipment Payments to Acquire Property, Plant, and Equipment Asset acquisition, contingent consideration, range of outcomes, maximum, amount Asset Acquisition, Contingent Consideration, Range of Outcomes, Maximum, Amount Additional milestone payment. 4.050% 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] Business Combination, Recognized Asset Acquired and Liability Assumed Business Combination, Recognized Asset Acquired and Liability Assumed [Table Text Block] Company Selected Measure Name Company Selected Measure Name Purchases of marketable securities Payments to Acquire Debt Securities, Available-for-Sale Income Tax Disclosure [Abstract] Income Tax Disclosure [Abstract] Business Combination, Contingent Consideration, Type [Axis] Business Combination, Contingent Consideration, Type [Axis] Upfront and milestone paymentsmade to collaborative partner Upfront And Milestone Payments Made To Collaborative Partner Total upfront and milestone payments made to collaborative partner. Schedule of Indefinite-Lived Intangible Assets [Table] Intangible Asset, Indefinite-Lived [Table] Swiss franc Switzerland, Francs Amortized Cost Equity Securities, FV-NI, Cost Credit Facility [Domain] Credit Facility [Domain] Cash flow from operating activities: Cash Provided by (Used in) Operating Activity, Including Discontinued Operation [Abstract] Alcyone Therapeutics, Inc. Alcyone Therapeutics, Inc. [Member] Alcyone Therapeutics, Inc. Intangible Asset, Indefinite-Lived [Line Items] Intangible Asset, Indefinite-Lived [Line Items] Equity Valuation Assumption Difference, Footnote Equity Valuation Assumption Difference, Footnote [Text Block] Total identifiable net assets Business Combination, Recognized Asset Acquired to Liability Assumed, Excess (Less) Research and Development Arrangement, Contract to Perform for Others, Type [Domain] Research and Development Arrangement, Contract to Perform for Others, Type [Domain] Alloy Therapeutics, Inc. Alloy Therapeutics, Inc. [Member] Alloy Therapeutics, Inc. 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Expiration Date Trading Arrangement Expiration Date Net increase (decrease) in cash and cash equivalents Cash, Cash Equivalent, Restricted Cash, and Restricted Cash Equivalent, Period Increase (Decrease), Excluding Exchange Rate Effect, Including Discontinued Operation Hedging Relationship [Axis] Hedging Relationship [Axis] Inventory Inventory, Net Current and Noncurrent Inventory, Net Current and Noncurrent Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items] Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items] Other Goodwill, Other Increase (Decrease) Cash equivalents Cash and Cash Equivalents, Fair Value Disclosure Total equity Beginning balance Ending balance Equity, Including Portion Attributable to Noncontrolling Interest Debt Securities, Available-for-Sale, Maturity, Allocated and Single Maturity Date, Amortized Cost [Abstract] Debt Securities, Available-for-Sale, Maturity, Allocated and Single Maturity Date, Amortized Cost [Abstract] Nonrelated Party Nonrelated Party [Member] LUNSUMIO LUNSUMIO [Member] LUNSUMIO Aggregate Pension Adjustments Service Cost Aggregate Pension Adjustments Service Cost [Member] Derivative [Table] Derivative [Table] Schedule of Antidilutive Securities Excluded from Computation of Earnings Per Share [Table] Antidilutive Security, Excluded EPS Calculation [Table] Accounts receivable Business Combination, Recognized Asset Acquired, Receivable, Current Derivative Instruments, Unrealized Gain (Loss) Derivative Instruments, Unrealized Gain (Loss) [Table Text Block] Derivative Instruments, Unrealized Gain (Loss) Cash flow from investing activities: Cash Provided by (Used in) Investing Activity, Including Discontinued Operation [Abstract] Proceeds from maturities and sales Proceeds from Sale and Maturity of Debt Securities, Available-for-Sale Other current assets: Derivative Asset [Abstract] Segments [Domain] Segments [Domain] Indebtedness Debt Disclosure [Text Block] Business Combination [Table] Business Combination [Table] 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 PEO Actually Paid Compensation Amount PEO Actually Paid Compensation Amount Business Combination, Asset Acquisition, Transaction between Entities under Common Control, and Joint Venture Formation [Abstract] Secured Debt Secured Debt [Member] Completed Technology Completed Technology [Member] DGF Company [Member] Total cost and expense Costs and Expenses Euro Euro Member Countries, Euro Beginning Balance Ending Balance Equity, Attributable to Parent Debt instrument, interest rate, stated percentage Debt Instrument, Interest Rate, Stated Percentage Pegcetacoplan Pegcetacoplan [Member] Pegcetacoplan Interferon Interferon [Member] Interferon FUMADERM And ADUHELM FUMADERM And ADUHELM [Member] FUMADERM And ADUHELM ASSETS Assets [Abstract] Amortized Cost Debt Securities, Available-for-Sale, Amortized Cost Insider Trading Policies and Procedures [Line Items] Accounts Receivable, Allowance for Credit Loss, Current Accounts Receivable, Allowance for Credit Loss, Current Business Combination, Separately Recognized Transaction [Domain] Business Combination, Separately Recognized Transaction [Domain] Business Combination, Contingent Consideration, Liability [Roll Forward] Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward] Deferred income taxes Deferred Income Tax Expense (Benefit) Summary of roll forward of the changes in goodwill Goodwill [Table Text Block] Entity Central Index Key Entity Central Index Key Accumulated depreciation Property, Plant, and Equipment, Accumulated Depreciation, Depletion, and Amortization Equity [Abstract] Equity [Abstract] Dividends or Other Earnings Paid on Equity Awards not Otherwise Reflected in Total Compensation for Covered Year Dividends or Other Earnings Paid on Equity Awards not Otherwise Reflected in Total Compensation for Covered Year [Member] Early stage programs Early Stage Programs [Member] Early Stage Programs Litigation Legal Matters and Contingencies [Text Block] Other Stockholders' Equity, Other (Gain) loss on strategic investments Increase (Decrease) in Equity Securities, FV-NI Fair Value Hierarchy and NAV [Axis] Fair Value Hierarchy and NAV [Axis] Investment, Name [Axis] Investment, Name [Axis] Designated as Hedging Instrument Designated as Hedging Instrument [Member] Interest in subsidiary (less than given percentage) Interest In Subsidiary Interest in subsidiary. Other current assets Business Combination, Recognized Asset Acquired, Other Asset, Current Accumulated other comprehensive income (loss) AOCI Attributable to Parent [Member] All Individuals All Individuals [Member] Beginning Balance Ending Balance Total Reserves Revenue-related reserves for discounts and allowances SEC Schedule, 12-09, Valuation Allowances and Reserves, Amount Finished goods Inventory, Finished Goods, Net of Reserves Realized Gain (Loss) on Investments Realized Gain (Loss) on Investments [Table Text Block] Contingent consideration obligations Business Combination, Contingent Consideration, Liability, Noncurrent Previously Reported Previously Reported [Member] Proceeds from sales of strategic investments Proceeds from Sale of Other Investments Statement [Line Items] Statement [Line Items] Total assets Assets Business Combination [Abstract] Business Combination [Abstract] Less: Net (gains) losses realized on equity securities Equity Securities, FV-NI, Realized Gain (Loss) Other, net Convertible Debt [Member] Other revenues Other revenues [Table Text Block] Other revenues [Table Text Block] Milestone Payment, Future Development Milestone Payment, Future Development [Member] Milestone Payment, Future Development Employee stock purchase plan Employee Stock Purchase Plan [Member] Employee stock purchase plan. Share-based compensation expense included in condensed consolidated statements of income Share-Based Payment Arrangement, Expensed and Capitalized, Amount [Table Text Block] Gross unrealized gains Debt Securities, Available-for-Sale, Accumulated Gross Unrealized Gain, before Tax Other non-current assets: Derivative Instruments and Hedges, Noncurrent [Abstract] Share-based Payments Share-Based Payment Arrangement [Text Block] Impairment of intangible assets Intangible Asset, Excluding Goodwill, Indefinite-Lived, Impairment Loss Other comprehensive income (loss) before reclassifications OCI, before Reclassifications, Net of Tax, Attributable to Parent PEO PEO [Member] Convertible note Business Combination, Recognized Liability Assumed, Accrued Expense And Other Business Combination, Recognized Liability Assumed, Accrued Expense And Other 5.050% Senior Notes due January 15, 2031 5.050% Senior Notes due January 15, 2031 [Member] 5.050% Senior Notes due January 15, 2031 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 Other long-term liabilities Other liabilities, noncurrent Other Liabilities, Noncurrent Goodwill Goodwill Goodwill, beginning of period Goodwill, end of period Goodwill Forgone Recovery due to Violation of Home Country Law, Amount Forgone Recovery due to Violation of Home Country Law, Amount Amounts payable Accounts Payable, To Collaborator Accounts Payable, To Collaborator EMPAVELI EMPAVELI [Member] EMPAVELI Senior secured term loan Business Combination, Recognized Liability Assumed, Accounts Payable, Current Net (gains) losses recognized on equity securities Equity Securities, FV-NI, Gain (Loss) Retained earnings Retained Earnings [Member] Long-term operating lease liabilities Operating Lease, Liability, Noncurrent Operating Expenses Operating Expenses Revolving Credit Facility Revolving Credit Facility [Member] Money market funds Money Market Funds [Member] Beginning balance, shares Ending balance, shares Common Stock, Shares, Outstanding Award Timing Disclosures [Line Items] Currency translation adjustments Accumulated Foreign Currency Adjustment Attributable to Parent [Member] Corporate debt securities Corporate Debt Securities [Member] Diluted earnings per share attributable to Biogen Inc. (in USD per share) Earnings Per Share, Diluted Collaborative Arrangement, Transaction with Party to Collaborative Arrangement Collaborative Arrangement, Transaction with Party to Collaborative Arrangement [Member] 6.450% Senior Notes due May 15, 2055 6.450% Senior Notes due May 15, 2055 [Member] 6.450% Senior Notes due May 15, 2055 Litigation Case [Domain] Litigation Case [Domain] Amortization of intangible assets Intangible Asset, Finite-Lived, Amortization Expense Intangible Asset, Finite-Lived [Line Items] Intangible Asset, Finite-Lived [Line Items] Share-based Payment Arrangement [Abstract] Share-Based Payment Arrangement [Abstract] Business Combination, Pro Forma Information, Nonrecurring Adjustment [Domain] Business Combination, Pro Forma Information, Nonrecurring Adjustment [Domain] Other research and development costs Service, Other [Member] IMRALDI Patent Litigation IMRALDI Patent Litigation [Member] IMRALDI Patent Litigation British pound United Kingdom, Pounds Schedule of Restructuring Reserve by Type of Cost Schedule of Restructuring Reserve by Type of Cost [Table Text Block] Equity Awards Adjustments, Footnote Equity Awards Adjustments, Footnote [Text Block] Cost Intangible Asset, Excluding Goodwill, before Accumulated Amortization After First Threshold Date until the Second 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. Floating Rate 364-Day Tranche Floating Rate 364-Day Tranche [Member] Floating Rate 364-Day Tranche Foreign Tax Authority Income Tax Jurisdiction, Foreign [Member] Restructuring and Related Cost, Expected Cost Remaining Restructuring and Related Cost, Expected Cost Remaining Acquisition-related cost, expensed Business Combination, Separately Recognized Transaction, Acquisition-Related Cost, Expensed Proceeds from issuance of debt Proceeds from Issuance of Debt Measurement Input, Probability Rate Measurement Input, Probability Rate [Member] Measurement Input, Probability Rate Acquisitions Business Combination [Text Block] Fair Value Notes Payable, Fair Value Disclosure Fair Value Notes Payable, Current, Fair Value Disclosure Notes Payable, Current, Fair Value Disclosure Government securities, noncurrent Government Securities Non-current [Member] Government Securities Non Current. Rule 10b5-1 Arrangement Adopted Rule 10b5-1 Arrangement Adopted [Flag] Asset impairment charges Asset, Impairment Loss Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items] Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items] Expected receipts to co-fund development costs, noncurrent Collaborative Arrangement, Expected Development Cost Funds, Year Two Collaborative Arrangement, Expected Development Cost Funds, Year Two Collaboration profit sharing/(loss reimbursement) Collaboration profit (loss) sharing Collaboration profit (loss) sharing GAZYVA GAZYVA [Member] GAZYVA [Member] Current portion of marketable securities Debt Securities, Available-for-Sale, Current Accounts receivable Increase (Decrease) in Accounts Receivable Share-based Compensation Expense included in consolidated statements of income Share-Based Payment Arrangement, Recognized Amount [Abstract] Currency [Axis] Currency [Axis] Product and Service [Domain] Product and Service [Domain] Disaggregation of Revenue [Table] Disaggregation of Revenue [Table] Other Other Products [Member] Other Products Deferred tax liability Deferred Income Tax Liabilities, Net Compensation Actually Paid vs. Other Measure Compensation Actually Paid vs. Other Measure [Text Block] Termination Date Trading Arrangement Termination Date Operating lease liabilities Business Combination, Recognized Liability Assumed, Taxes Payable Business Combination, Recognized Liability Assumed, Taxes Payable Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Table] Fair Value, Recurring and Nonrecurring [Table] Business Combination, Recognized Asset Acquired, Identifiable Intangible Asset, Finite-Lived Business Combination, Recognized Asset Acquired, Identifiable Intangible Asset, Finite-Lived Dispositions Disposal Groups, Including Discontinued Operations, Disclosure [Text Block] Compensation Actually Paid vs. Total Shareholder Return Compensation Actually Paid vs. Total Shareholder Return [Text Block] Unrealized gains (losses) on pension benefit obligation, net of tax Other Comprehensive (Income) Loss, Defined Benefit Plan, after Reclassification Adjustment, after Tax Accrued expense and other Total accrued expense and other Other Liabilities, Current Cash flows, revenue Cash flows, revenue [Member] Cash flows, revenue [Member] Derivative contracts Derivative Asset, Current Expense reflected within statements of income Expense reflected within statements of income Expense reflected within statements of income Reclassification Out of Accumulated Other Comprehensive Income (Loss) [Abstract] Reclassification Out of Accumulated Other Comprehensive Income (Loss) [Abstract] Reclassification Out of Accumulated Other Comprehensive Income (Loss) [Abstract] Fair Value as of Grant Date Award Grant Date Fair Value Outstanding unvested equity awards into cash retention awards Share-Based Compensation Arrangement by Share-Based Payment Award, Equity Instruments Other than Options, Aggregate Intrinsic Value, Outstanding Debt instrument, redemption price, percentage Debt Instrument, Redemption Price, Percentage Schedule of Goodwill [Table] Goodwill [Table] Pay vs Performance Disclosure, Table Pay vs Performance [Table Text Block] Arrangement Duration Trading Arrangement Duration Entity Address, Postal Zip Code Entity Address, Postal Zip Code Entity Registrant Name Entity Registrant Name Mortgage and other asset backed securities, noncurrent Mortgage And Other Asset Backed Securities Non Current [Member] Mortgage And Other Asset Backed Securities Non Current. Insider Trading Policies and Procedures Not Adopted Insider Trading Policies and Procedures Not Adopted [Text Block] Intangible Asset, Finite-Lived, before Accumulated Amortization Intangible Asset, Finite-Lived, before Accumulated Amortization Total revenue Total revenue Total revenues from anti-CD20 therapeutic programs Revenues Forgone Recovery due to Expense of Enforcement, Amount Forgone Recovery due to Expense of Enforcement, Amount Total marketable debt securities Debt Securities, Available-for-Sale, Maturity, Allocated and Single Maturity Date, Fair Value Summary of roll forward of the changes in goodwill Goodwill [Roll Forward] RITUXAN RITUXAN [Member] RITUXAN. Intangible Asset, Finite-Lived, Class [Domain] Intangible Asset, Finite-Lived, Class [Domain] Late stage programs Late Stage Programs [Member] Late Stage Programs Payments to acquire in process research and development Payments to Acquire in Process Research and Development Summary of assets and liabilities recorded at fair value Schedule of Fair Value, Assets and Liabilities Measured on Recurring Basis [Table Text Block] Award Type [Domain] Award Type [Domain] Fair Value Measurements Fair Value Disclosures [Text Block] Significant Unobservable Inputs (Level 3) Fair Value, Inputs, Level 3 [Member] 2027 Intangible Asset, Finite-Lived, Estimated Amortization Expense, Year One Revenue recognized Revenue from Collaborative Arrangement, Excluding Revenue from Contract with Customer Geographical [Axis] Geographical [Axis] Accrued expense and other current liabilities Increase (Decrease) in Accrued Liabilities 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. Legal Entity [Axis] Legal Entity [Axis] Marketable securities Debt Securities, Available-for-Sale, Noncurrent Payment Payments for Restructuring Cash and cash equivalents Cash and cash equivalents, beginning of the period Cash and cash equivalents, end of the period Cash, Cash Equivalent, Restricted Cash, and Restricted Cash Equivalent, Continuing Operation Issuance of common stock under stock option and stock purchase plans Stock Issued During Period, Value, Treasury Stock Reissued Schedule of Finite-Lived Intangible Assets [Table] Intangible Asset, Finite-Lived [Table] Collaborative arrangements and non-collaborative arrangement transactions Collaborative Arrangement and Arrangement Other than Collaborative [Line Items] Corporate Debt Securities Current Corporate Debt Securities Current [Member] Corporate debt securities Current. Reclassification out of Accumulated Other Comprehensive Income Reclassification out of Accumulated Other Comprehensive Income [Table Text Block] Non-NEOs Non-NEOs [Member] After LUNSUMIO Approval until the First 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. Business Overview Business Overview [Policy Text Block] Describes an overview of the company and its operations. Cash consideration paid to Apellis shareholders Payments to Acquire Businesses, Gross 2028 Intangible Asset, Finite-Lived, Estimated Amortization Expense, Year Two Investment, Name [Domain] Investment, Name [Domain] Customer [Domain] Customer [Domain] Components of inventory Schedule of Inventory, Current [Table Text Block] Long-term Debt, Type [Domain] Long-Term Debt, Type [Domain] Statistical Measurement [Axis] Statistical Measurement [Axis] 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. Summary of the effect of cash flow derivatives designated as hedging instruments on the condensed consolidated statements of income Derivative Instruments, Gain (Loss) [Table Text Block] Total development expense incurred by the collaboration related to the advancement of LEQEMBI Expense Incurred By Collaboration Total expense incurred by collaboration. Acquired and in-licensed rights and patents Acquired And In Licensed Rights And Patents [Member] Acquired And In Licensed Rights And Patents Award Type [Axis] Award Type [Axis] Named Executive Officers, Footnote Named Executive Officers, Footnote [Text Block] Retained earnings Retained Earnings (Accumulated Deficit) Restructuring and Related Activities [Abstract] Product and Service [Axis] Product and Service [Axis] All Currencies [Domain] All Currencies [Domain] (Gain) loss on fair value remeasurement of contingent consideration Contingent consideration Business Combination, Contingent Consideration, Change in Contingent Consideration, Liability, Increase (Decrease) Awards Close in Time to MNPI Disclosures Awards Close in Time to MNPI Disclosures [Table] Common stock Common Stock [Member] Marketable debt securities: Fair Value Debt Securities, Available-for-Sale Alteogen Inc. Alteogen Inc. [Member] Alteogen Inc. 2025 Senior Notes 2025 Senior Notes [Member] 2025 Senior Notes Reclassification out of Accumulated Other Comprehensive Income [Table] Reclassification out of Accumulated Other Comprehensive Income [Table] Litigation related expense Litigation Settlement, Fee Expense Accounts payable Business Combination, Recognized Asset Acquired, Investments And Other Assets Business Combination, Recognized Asset Acquired, Investments And Other Assets Rest of World Non-US [Member] Derivative Contract [Domain] Derivative Contract [Domain] 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. Total liabilities and equity Liabilities and Equity Milestone Payment, Clinical And Regulatory Milestone Payment, Clinical And Regulatory [Member] Milestone Payment, Clinical And Regulatory (Gains) losses on investments, net Realized Investment Gains (Losses) FLIXABI FLIXABI [Member] FLIXABI [Member] Stock options Share-Based Payment Arrangement, Option [Member] Gain (loss) on foreign currency derivative instruments not designated as hedging instruments Gain (Loss) on Foreign Currency Derivative Instruments Not Designated as Hedging Instruments Segments [Axis] Segments [Axis] Additional 402(v) Disclosure Additional 402(v) Disclosure [Text Block] Asset Class [Axis] Asset Class [Axis] Related Party, Type [Domain] Related and Nonrelated Parties [Domain] Restructuring charges Expense Restructuring Charges Tabular List, Table Tabular List [Table Text Block] Additional paid-in capital Additional Paid in Capital SEC Schedule, 12-09, Valuation and Qualifying Accounts [Abstract] SEC Schedule, 12-09, Valuation and Qualifying Accounts [Abstract] Acquisitions of intangible assets Payments to Acquire Intangible Assets Human Immunology Biosciences Pre-Existing In License Commitments Human Immunology Biosciences Pre-Existing In License Commitments [Member] Human Immunology Biosciences Pre-Existing In License Commitments 2029 Intangible Asset, Finite-Lived, Estimated Amortization Expense, Year Three 2026 (remaining six months) Intangible Asset, Finite-Lived, Estimated Amortization Expense, Remainder of Fiscal Year Other (income) expense, net Other (income) expense, net Other (income) expense, net Other Operating Income (Expense), Net Name Awards Close in Time to MNPI Disclosures, Individual Name Year-end Fair Value of Equity Awards Granted in Covered Year that are Outstanding and Unvested Year-end Fair Value of Equity Awards Granted in Covered Year that are Outstanding and Unvested [Member] Business Combination, Contingent Consideration, Type [Domain] Business Combination, Contingent Consideration, Type [Domain] Preferred stock, par value $0.001 per share Preferred Stock, Value, Issued Adjustment to Compensation: Adjustment to Compensation [Axis] 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. 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Apellis Pharmaceuticals, Inc. Apellis Pharmaceuticals, Inc. [Member] Apellis Pharmaceuticals, Inc. Non-PEO NEO Non-PEO NEO [Member] Distributor One Distributor One [Member] Distributor one. Foreign exchange (gains) losses, net Gain (Loss), Foreign Currency Transaction, before Tax Effect of dilutive securities: Dilutive Securities, Effect on Basic Earnings Per Share [Abstract] Cost of sales, excluding amortization and impairment of acquired intangible assets Cost of Product and Service Sold Award Timing Method Award Timing Method [Text Block] Derivative, remaining maturity Derivative, Remaining Maturity Contract manufacturing, royalty and other revenue Product and Service, Other [Member] Forgone Recovery, Explanation of Impracticability Forgone Recovery, Explanation of Impracticability [Text Block] Net income attributable to Biogen Inc. Net income attributable to Biogen Inc. 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Line of Credit Line of Credit [Member] Restatement Determination Date Restatement Determination Date Excess and obsolescence charges related to inventory Inventory Write-down Summary of Activity Related to Stoke Therapeutics Collaboration Summary Of Activity Related To Collaboration [Table Text Block] Summary Of Activity Related To Collaboration Acquisition of Apellis, net of cash acquired Other Payments to Acquire Businesses Amendment Flag Amendment Flag Goodwill [Line Items] Goodwill [Line Items] Current liabilities: Liabilities, Current [Abstract] Foreign currency forward contracts that were entered into to hedge forecasted revenue Schedule of Notional Amounts of Outstanding Derivative Positions [Table Text Block] TYSABRI TYSABRI product [Member] TYSABRI product [Member] Gross Unrealized Losses Debt Securities, Available-for-Sale, Accumulated Gross Unrealized Loss, before Tax 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. Marketable debt and equity securities Debt Securities, Available-for-Sale [Table Text Block] Loss Contingencies [Table] Loss Contingencies [Table] Investment Type Categorization [Domain] Investments [Domain] Debt Securities, Trading, and Equity Securities, FV-NI, Cost [Abstract] Debt Securities, Trading, and Equity Securities, FV-NI, Cost [Abstract] Business Combination, Pro Forma Information, Nonrecurring Adjustment [Axis] Business Combination, Pro Forma Information, Nonrecurring Adjustment [Axis] Adjustment to Non-PEO NEO Compensation Footnote Adjustment to Non-PEO NEO Compensation Footnote [Text Block] Property, Plant, and Equipment [Abstract] Property, Plant, and Equipment [Abstract] Entity Address, State or Province Entity Address, State or Province PEO Total Compensation Amount PEO Total Compensation Amount Currency translation adjustments, net of tax Other Comprehensive Income (Loss), Foreign Currency Transaction and Translation Adjustment, Net of Tax Assets: Assets, Fair Value Disclosure [Abstract] Derivative, notional amount Total foreign currency forward contracts and options Derivative, Notional Amount Basic earnings per share attributable to Biogen Inc. (in USD per share) Weighted average number of common shares outstanding Weighted Average Number of Shares Outstanding, Basic Accumulated impairment losses related to goodwill Goodwill, Impaired, Accumulated Impairment Loss Weighted-Average remaining useful life Finite-Lived Intangible Asset, Weighted-Average Remaining Useful Life Finite-Lived Intangible Asset, Weighted-Average Remaining Useful Life Loss Contingencies [Line Items] Loss Contingencies [Line Items] SEC Schedule, 12-09, Valuation Allowances and Reserves Type [Axis] SEC Schedule, 12-09, Valuation Allowances and Reserves Type [Axis] Apellis acquisition SEC Schedule, 12-09, Valuation Allowances and Reserves, Business Acquired Business Combination, Pro Forma Information, Nonrecurring Adjustment, Inventory, Fair Value Adjustment Business Combination, Pro Forma Information, Nonrecurring Adjustment, Inventory, Fair Value Adjustment [Member] Contractual adjustments Contractual Adjustments [Member] Contractual adjustments. SEC Schedule, 12-09, Valuation and Qualifying Accounts Disclosure [Table] SEC Schedule, 12-09, Valuation and Qualifying Accounts Disclosure [Table] Award Timing MNPI Disclosure Award Timing MNPI Disclosure [Text Block] Basis of presentation Basis of Accounting, Policy [Policy Text Block] Pre-acquisition services Business Combination, Consideration Transferred, Pre-Acquisition Services Business Combination, Consideration Transferred, Pre-Acquisition Services SKYCLARYS SKYCLARYS [Member] SKYCLARYS Accumulated Amortization Accumulated Amortization Intangible Asset, Finite-Lived, Accumulated Amortization Maximum borrowing capacity Line of Credit Facility, Maximum Borrowing Capacity Collaborative Arrangement and Arrangement Other than Collaborative [Table] Collaborative Arrangement and Arrangement Other than Collaborative [Table] 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 Costs And Expenses Share based compensation expense included in costs and expenses. 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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. Total Liabilities, Fair Value Disclosure Debt instrument, make whole provision redemption price, percentage Debt Instrument, Make Whole Provision Redemption Price, Percentage Debt Instrument, Make Whole Provision Redemption Price, Percentage Statement of Comprehensive Income [Abstract] Statement of Comprehensive Income [Abstract] Overnight reverse repurchase agreements Repurchase Agreements [Member] Name Outstanding Recovery, Individual Name Proceeds from sales and maturities of marketable securities Proceeds from Sale and Maturity of Marketable Securities Award Timing, How MNPI Considered Award Timing, How MNPI Considered [Text Block] Apellis Integration Apellis Integration [Member] Apellis Integration Milestone Payment, Regulatory Milestone Payment, Regulatory [Member] Milestone Payment, Regulatory Discounts Reserve for Cash Discounts [Member] Reserve for Cash Discounts [Member] Number of wholesalers Number of Wholesalers Number of Wholesalers Geographical [Domain] Geographical [Domain] Cost and net Intangible Asset, Excluding Goodwill, Indefinite-Lived 5.750% Senior Notes due May 15, 2035 5.750% Senior Notes due May 15, 2035 [Member] 5.750% Senior Notes due May 15, 2035 Prior Year End Fair Value of Equity Awards Granted in Any Prior Year that Fail to Meet Applicable Vesting Conditions During Covered Year Prior Year End Fair Value of Equity Awards Granted in Any Prior Year that Fail to Meet Applicable Vesting Conditions During Covered Year [Member] Pension Adjustments Prior Service Cost Pension Adjustments Prior Service Cost [Member] Discount rate Fair Value, Indefinite Lived Intangible Assets, Discount Rate Fair Value, Indefinite Lived Intangible Assets, Discount Rate Expected profit share percentage Equity Method Investments, Expected Profit Share Equity Method Investments, Expected Profit Share Amounts reclassified from AOCI Reclassification from AOCI, Current Period, Net of Tax, Attributable to Parent Unrealized gains (losses) on securities available for sale, net of tax OCI, Debt Securities, Available-for-Sale, Gain (Loss), after Adjustment and Tax Net unrealized gains (losses) Unrealized Gain (Loss) on Derivatives Current portion of term loan Secured Debt, Current Unfunded status of postretirement benefit plans Accumulated Defined Benefit Plans Adjustment Attributable to Parent [Member] Short-term debt securities Debt Securities [Member] Asset Class [Domain] Asset Class [Domain] 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. Divestitures [Abstract] Divestitures [Abstract] Divestitures [Abstract] Compensation Amount Outstanding Recovery Compensation Amount Individual: Individual [Axis] Diluted earnings per share attributable to Biogen Inc. (in USD per share) Shares used in calculating diluted earnings per share Weighted Average Number of Shares Outstanding, Diluted Disaggregation of Revenue [Line Items] Disaggregation of Revenue [Line Items] Felzartamab Felzartamab [Member] Felzartamab Entity Emerging Growth Company Entity Emerging Growth Company Underlying Security Market Price Change Underlying Security Market Price Change, Percent Commercial Paper Commercial Paper [Member] Derivative Instruments Derivative Instruments and Hedging Activities Disclosure [Text Block] 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. Derivative liability, fair value Derivative Liability, Subject to Master Netting Arrangement, before Offset Revenue from Contract with Customer [Abstract] Revenue from Contract with Customer [Abstract] Distributor Three Distributor Three [Member] Distributor Three Treasury stock Treasury Stock, Common [Member] Intangible Asset, Goodwill and Other [Abstract] Intangible Asset, Goodwill and Other [Abstract] Co Promotion Profit Sharing Formula Co Promotion Profit Sharing Formula [Table Text Block] Co-promotion profit sharing formula. Organization, Consolidation and Presentation of Financial Statements [Abstract] Organization, Consolidation and Presentation of Financial Statements [Abstract] Indefinite-lived Intangible Assets, Major Class Name [Domain] Intangible Asset, Indefinite-Lived, Class [Domain] All Executive Categories All Executive Categories [Member] Other revenues from anti-CD20 therapeutic programs Other revenues from anti-CD20 therapeutic programs Other revenues from anti-CD20 therapeutic programs consists of revenue on sales of RITUXAN outside the U.S., which consist of our share of pre-tax co-promotion profits in Canada and royalty revenue on sales of Ocrevus. Pension Adjustments Service Cost Pension Adjustments Service Cost [Member] Other comprehensive income (loss): Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Parent [Abstract] Asset Acquisition [Domain] Asset Acquisition [Domain] Payments Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Settlements Human Immunology Biosciences Human Immunology Biosciences [Member] Human Immunology Biosciences Issuance of common stock under stock option and stock purchase plans (in shares) Stock Issued During Period, Shares, Treasury Stock Reissued Basic and diluted earnings per share Schedule of Earnings Per Share, Basic and Diluted [Table Text Block] Debt Disclosure [Abstract] Debt Disclosure [Abstract] Estimate of possible loss Income Tax Examination, Estimate of Possible Loss Derivative Actions Derivative Actions [Member] Derivative Actions Segment Information Segment Reporting [Text Block] Minimum Minimum [Member] Strategic Investments Strategic Investments [Member] Strategic investments. Investment Type [Axis] Investment Type [Axis] Marketable equity securities Investments, Fair Value Disclosure Revenues by product Disaggregation of Revenue [Table Text Block] Investments in Variable Interest Entities Variable Interest Entity Disclosure [Text Block] Effect of exchange rate changes on cash and cash equivalents Effect of Exchange Rate on Cash, Cash Equivalent, Restricted Cash, and Restricted Cash Equivalent, Continuing Operation Income Statement Location [Axis] Location, Statement of Income, Balance [Axis] Non-Rule 10b5-1 Arrangement Adopted Non-Rule 10b5-1 Arrangement Adopted [Flag] Aggregate Grant Date Fair Value of Equity Award Amounts Reported in Summary Compensation Table Aggregate Grant Date Fair Value of Equity Award Amounts Reported in Summary Compensation Table [Member] Contingent consideration Business Combination, Consideration Transferred, Liabilities Incurred Business Combination [Line Items] Business Combination [Line Items] Estimated life (in years) Intangible Asset, Finite-Lived, Useful Life Accumulated Other Comprehensive Income (Loss) [Line Items] Accumulated Other Comprehensive Income (Loss) [Line Items] TECFIDERA TECFIDERA [Member] TECFIDERA [Member] Realized gains Debt Securities, Available-for-Sale, Realized Gain Returns Sales Returns and Allowances [Member] Summary of fair and carrying value of debt instruments Schedule of Carrying Values and Estimated Fair Values of Debt Instruments [Table Text Block] Exercise Price Award Exercise Price Schedule of Restructuring and Related Costs [Table] Restructuring Cost [Table] Issuance of common stock under stock award plan Stock Issued During Period, Value, Restricted Stock Award, Net of Forfeitures Segment Reporting [Line Items] Segment Reporting [Line Items] Document Quarterly Report Document Quarterly Report Year-over-Year Change in Fair Value of Equity Awards Granted in Prior Years That are Outstanding and Unvested Year-over-Year Change in Fair Value of Equity Awards Granted in Prior Years That are Outstanding and Unvested [Member] Share-based Payment Arrangement, Expensed and Capitalized, Amount [Line Items] Share-Based Payment Arrangement, Expensed and Capitalized, Amount [Line Items] Percentage of future development costs Percentage Of Future Development Costs Percentage Of Future Development Costs OCREVUS OCREVUS [Member] OCREVUS Accounts receivable Accounts Receivable, after Allowance for Credit Loss Repayments of borrowings Repayments of Long-Term Debt Lloyd Minor [Member] Lloyd Minor Other current liabilities: Derivative Liability [Abstract] Awards Close in Time to MNPI Disclosures, Table Awards Close in Time to MNPI Disclosures [Table Text Block] Restructuring reserve, beginning Restructuring reserve, ending Restructuring Reserve Equity Component [Domain] Equity Component [Domain] Measurement Input Type [Domain] Measurement Input Type [Domain] Foreign Exchange Contract Foreign Exchange Contract [Member] Gross Unrealized Losses Equity Securities, Available-for-Sale, Accumulated Gross Unrealized Loss, Before Tax Equity Securities, Available-for-Sale, Accumulated Gross Unrealized Loss, Before Tax Milestone Payment, One Milestone Payment, One [Member] Milestone Payment, One Adoption Date Trading Arrangement Adoption Date Other income (expense), net Schedule of Other Nonoperating Income (Expense) [Table Text Block] Counterparty Name [Domain] Counterparty Name [Domain] Performance stock units settled in stock Performance Stock Units Settled in Stock [Member] Performance Stock Units Settled in Stock [Member] Restructuring Type [Axis] Restructuring Type [Axis] Fair Value Equity Securities, FV-NI, Current Corporate debt securities, noncurrent Corporate Debt Securities Non Current [Member] Corporate Debt Securities Non Current. Title Trading Arrangement, Individual Title Forgone Recovery due to Disqualification of Tax Benefits, Amount Forgone Recovery due to Disqualification of Tax Benefits, Amount Samsung Bioepis Samsung Biosimilar Agreement [Member] Samsung bio-similar agreement. Accounting Policies [Abstract] Accounting Policies [Abstract] Restatement does not require Recovery Restatement Does Not Require Recovery [Text Block] Other Proceeds from (Payment for) Other Financing Activity Insider Trading Policies and Procedures Adopted Insider Trading Policies and Procedures Adopted [Flag] Accrued income taxes Accrued Income Taxes, Noncurrent Selling, general and administrative Selling, general and administrative Selling, General and Administrative Expense Type of Restructuring [Domain] Type of Restructuring [Domain] Business Combination [Domain] Business Combination [Domain] Unrealized gains Unrealized Gain on Derivatives Unrealized Gain on Derivatives Compensation Actually Paid vs. Net Income Compensation Actually Paid vs. Net Income [Text Block] Genentech Genentech [Member] Genentech Weighted Average Weighted Average [Member] Current portion of term loan Notes Payable, Current All Adjustments to Compensation All Adjustments to Compensation [Member] Business Combination [Axis] Business Combination [Axis] Net cash flow provided by (used in) operating activities Cash Provided by (Used in) Operating Activity, Including Discontinued Operation Biogen Inc. shareholders’ equity: Equity, Attributable to Parent [Abstract] Income Statement [Abstract] Income Statement [Abstract] Document Transition Report Document Transition Report Clinical Inventory Clinical Inventory [Member] Clinical Inventory Restructuring Plan [Domain] Restructuring Plan [Domain] Deferred tax asset Deferred Income Tax Assets, Net Stock Appreciation Rights (SARs) Stock Appreciation Rights (SARs) [Member] Sobi Sobi [Member] Sobi Notes payable Business Combination, Recognized Liability Assumed, Notes Payable Business Combination, Recognized Liability Assumed, Notes Payable City Area Code City Area Code Summary of share based compensation expense associated with different programs [Abstract] Summary of share based compensation expense associated with different programs [Abstract] Summary of share-based compensation expense associated with different programs. Name Forgone Recovery, Individual Name Non-GAAP Measure Description Non-GAAP Measure Description [Text Block] Entity [Domain] Entity [Domain] Priya Singhal [Member] Priya Singhal Collaboration agreement term Collaborative Arrangement, Term Extension Collaborative Arrangement, Term Extension Share-based payment arrangement, expense Share-based compensation expense Share-Based Payment Arrangement, Expense Inventory Business Combination, Recognized Asset Acquired, Inventory, Current Goodwill resulting from acquisition Goodwill, Acquired During Period Fair Value Measurement Inputs and Valuation Techniques [Line Items] Fair Value Measurement Inputs and Valuation Techniques [Line Items] Cash equivalents, at carrying value Cash Equivalents, at Carrying Value 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 [Member] Due in one year or less Debt Securities, Available-for-Sale, Fair Value, Maturity, Allocated and Single Maturity Date, Year One Intangible Asset, Excluding Goodwill, after Accumulated Amortization [Abstract] Intangible Asset, Excluding Goodwill, after Accumulated Amortization [Abstract] Taxes payable Taxes Payable, Current Revision of Prior Period [Domain] Revision of Prior Period [Domain] Gains (losses) on securities available for sale AOCI, Accumulated Gain (Loss), Debt Securities, Available-for-Sale, Parent [Member] Cash and cash equivalents Business Combination, Recognized Asset Acquired, Cash and Cash Equivalent Erroneously Awarded Compensation Recovery Erroneously Awarded Compensation Recovery [Table] Fair Value Measurement Inputs and Valuation Techniques Fair Value Measurement Inputs and Valuation Techniques [Table Text Block] Disaggregation of Revenue [Abstract] Disaggregation of Revenue [Abstract] Accumulated Other Comprehensive Income (Loss) [Table] Accumulated Other Comprehensive Income (Loss) [Table] Entity Current Reporting Status Entity Current Reporting Status Net cash flow provided by (used in) financing activities Cash Provided by (Used in) Financing Activity, Including Discontinued Operation Total Shareholder Return Amount Total Shareholder Return Amount Cash Flow Hedging [Member] Cash Flow Hedging [Member] Liabilities: Liabilities, Fair Value Disclosure [Abstract] Business Combination Business Combination [Table Text Block] Royalty Attributed To OCREVUS Royalty Attributed To OCREVUS [Member] Royalty Attributed To OCREVUS Biosimilars Biosimilars [Member] Biosimilars Adjustments to reconcile net income to net cash flow from operating activities: Adjustment to Reconcile Net Income to Cash Provided by (Used in) Operating Activity [Abstract] Foreign currency cash flow hedge gain (loss) to be reclassified during next 12 months Foreign Currency Cash Flow Hedge Gain (Loss) to be Reclassified During Next 12 Months Cash flow from financing activities: Cash Provided by (Used in) Financing Activity, Including Discontinued Operation [Abstract] Property, plant and equipment, net Property, Plant, and Equipment, after Accumulated Depreciation, Depletion, and Amortization Net Investment Income [Line Items] Net Investment Income [Line Items] Unrealized (losses) Unrealized Loss on Derivatives Unrealized Loss on Derivatives Financial Instruments [Domain] Financial Instruments [Domain] Derivative contracts Derivative Asset Inventory Inventory Disclosure [Text Block] Share of net profit from sage therapeutics, Percent Share Of Net Profit From Sage Therapeutics, Percent Share Of Net Profit From Sage Therapeutics, Percent Intangible Asset, Finite-Lived, Class [Axis] Intangible Asset, Finite-Lived, Class [Axis] Intangible Assets and Goodwill Intangible Asset and Goodwill [Text Block] Underlying Securities Award Underlying Securities Amount Revenues Revenue from Contract with Customer [Text Block] Fair Values Derivatives, Balance Sheet Location, by Derivative Contract Type [Table] Fair Values Derivatives, Balance Sheet Location, by Derivative Contract Type [Table] After Second Threshold Date Percentage Of Co Promotion Operating Profits Greater Than First Fifty Million Option Two Sub Option Four Percentage Of Co Promotion Operating Profits Greater Than First Fifty Million Option Two Sub Option Four Government Securities Current Government Securities Current [Member] Government securities Current. 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[Member] Ionis Pharmaceuticals Inc. 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Inventory Disclosure [Abstract] Inventory Disclosure [Abstract] Intangible Asset, Finite-Lived, after Accumulated Amortization [Abstract] Intangible Asset, Finite-Lived, after Accumulated Amortization [Abstract] Other, net Other Nonoperating Income (Expense) 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] 2031 Intangible Asset, Finite-Lived, Estimated Amortization Expense, Year Five Parent Parent [Member] Equity: Equity, Including Portion Attributable to Noncontrolling Interest [Abstract] Derivatives [Line Items] Derivatives, Fair Value [Line Items] Loss Contingency, Information about Litigation Matters [Abstract] Loss Contingency, Information about Litigation Matters [Abstract] Loss contingency, pending claims, number Loss Contingency, Pending Claims, Number Hedging Designation [Domain] Hedging Designation [Domain] Sage Therapeutics Sage Therapeutics Inc. [Member] Sage Therapeutics Inc. Statement of Cash Flows [Abstract] Statement of Cash Flows [Abstract] Collaborative arrangements, investments in counterparty Collaborative Arrangements, Investments In Counterparty Collaborative Arrangements, Investments In Counterparty City Therapeutics, Inc. City Therapeutics, Inc. [Member] City Therapeutics, Inc. 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