Trading Arrangements, by Individual Trading Arrangements, by Individual [Table] Trading Arrangement: Trading Arrangement [Axis] Maximum Maximum [Member] Title of 12(b) Security Title of 12(b) Security OCREVUS OCREVUS [Member] OCREVUS Adjustment to Compensation, Amount Adjustment to Compensation Amount Financial Instrument [Axis] Financial Instrument [Axis] 5.200% Senior Notes due September 15, 2045 Senior Notes Five Point Two Percent Due Twenty Forty Five [Member] Senior Notes Five Point Two Percent Due Twenty Forty Five [Member] Segment Reporting [Abstract] Denali Therapeutics Denali Therapeutics Inc [Member] Denali Therapeutics Inc 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 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 Government securities US Treasury and Government [Member] 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. 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City Therapeutics, Inc. [Member] City Therapeutics, Inc. 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Preferred stock, par value (in dollars per share) Preferred Stock, Par or Stated Value Per Share Depreciation Depreciation 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. Antidilutive Securities Excluded from Computation of Earnings Per Share, by Antidilutive Securities [Axis] Antidilutive Securities [Axis] Total marketable debt securities Debt Securities, Available-for-Sale, Maturity, Allocated and Single Maturity Date, Amortized Cost Operating lease assets Operating Lease, Right-of-Use Asset Alloy Therapeutics, Inc. Alloy Therapeutics, Inc. [Member] Alloy Therapeutics, Inc. AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward] AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward] Polish zloty Poland, Zlotych Due in one year or less Debt Securities, Available-for-Sale, Amortized Cost, Maturity, Allocated and Single Maturity Date, Year One Total assets acquired and liabilities assumed Business Combination, Recognized Asset Acquired to Liability Assumed, Excess (Less), and Goodwill Fair Value Disclosures [Abstract] Fair Value Disclosures [Abstract] Debt Instrument, Name [Domain] Debt Instrument, Name [Domain] Royalty and other revenue Royalty [Member] Long-Term Line of Credit Long-Term Line of Credit Other revenues Other revenues [Table Text Block] Other revenues [Table Text Block] 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 Samsung Bioepis Samsung Biosimilar Agreement [Member] Samsung bio-similar agreement. 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Reduction in royalty rate Reduction in royalty rate Reduction in royalty rate 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] Contingent Commercialized Rights, Number Of Products Contingent Commercialized Rights, Number Of Products Contingent Commercialized Rights, Number Of Products Restructuring Cost and Reserve [Line Items] Restructuring Cost and Reserve [Line Items] Rule 10b5-1 Arrangement Terminated Rule 10b5-1 Arrangement Terminated [Flag] Aggregate Erroneous Compensation Amount Aggregate Erroneous Compensation Amount Prepaid Taxes Prepaid Taxes Payments to acquire productive assets Payments to Acquire Productive Assets Hedging Designation [Axis] Hedging Designation [Axis] 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] Inventory [Line Items] Inventory [Line Items] Credit Facility [Axis] Credit Facility [Axis] Restructuring, Business Transformation and Other Cost Saving Initiatives Restructuring and Related Activities Disclosure [Text Block] Total Assets, Fair Value Disclosure Canadian dollar Canada, Dollars Foreign currency cash flow hedge gain (loss) to be reclassified during next 15 months Foreign Currency Cash Flow Hedge Gain (Loss) To Be Reclassified During Next 15 Months Foreign Currency Cash Flow Hedge Gain (Loss) To Be Reclassified During Next 15 Months Name Measure Name Amounts receivable Accounts Receivable, From Collaborator Accounts Receivable, From Collaborator Pay vs Performance Disclosure [Line Items] Fair Value Hierarchy and NAV [Domain] Fair Value Hierarchy and NAV [Domain] Debt Instrument [Line Items] Debt Instrument [Line Items] Additional paid-in capital Additional Paid-in Capital [Member] Indefinite-lived Intangible Assets by Major Class [Axis] Intangible Asset, Indefinite-Lived, Class [Axis] Proceeds from sale of intangible assets Proceeds from Sale of Intangible Assets Name Trading Arrangement, Individual Name Raw materials Inventory, Raw Materials, Net of Reserves Foreign currency and other adjustments Restructuring Reserve, Foreign Currency Translation Gain (Loss) Operating lease, expense Operating Lease, Expense Company Selected Measure Amount Company Selected Measure Amount Net unrealized (gains) losses recognized on equity securities Equity Securities, FV-NI, Unrealized Gain (Loss) 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. Depreciation and amortization Depreciation, Depletion and Amortization Issuance of common stock under stock award plan (in shares) Shares Issued, Shares, Share-Based Payment Arrangement, after Forfeiture 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. Non-PEO NEO Non-PEO NEO [Member] Foreign exchange (gains) losses, net Gain (Loss), Foreign Currency Transaction, before Tax 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 Threshold of gross sales of to be achieved in preceding twelve consecutive months under option one. Effect of dilutive securities: Dilutive Securities, Effect on Basic Earnings Per Share [Abstract] 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. 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 (loss) income attributable to Biogen Inc. Net (loss) income attributable to Biogen Inc. Net Income (Loss) Attributable to Parent Long-term Debt, Type [Axis] Long-Term Debt, Type [Axis] Erroneous Compensation Analysis Erroneous Compensation Analysis [Text Block] Felzartamab And Izastobart/HIB210 Felzartamab And Izastobart/HIB210 [Member] Felzartamab And Izastobart/HIB210 Balance Sheet Location [Axis] Location, Statement of Financial Position, Balance [Axis] Restatement Determination Date: Restatement Determination Date [Axis] Schedule of Accumulated Other Comprehensive Income (Loss) Schedule of Accumulated Other Comprehensive Income (Loss) [Table Text Block] 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 Acquired and in-licensed rights and patents Acquired And In Licensed Rights And Patents [Member] Acquired And In Licensed Rights And Patents Pension Benefits Adjustments, Footnote Pension Benefits Adjustments, Footnote [Text Block] Weighted-average shares used in calculating: Weighted Average Number of Shares Outstanding, Diluted [Abstract] Statistical Measurement [Domain] Statistical Measurement [Domain] Document Fiscal Year Focus Document Fiscal Year Focus Subtotal Subtotal Share-Based Payment Arrangement, Expense, after Tax Alcyone Therapeutics, Inc. Alcyone Therapeutics, Inc. [Member] Alcyone Therapeutics, Inc. Customer [Axis] Customer [Axis] TECFIDERA TECFIDERA [Member] TECFIDERA [Member] Common stock, par value $0.0005 per share Common Stock, Value, Issued Interest expense Interest Expense, Operating and Nonoperating Payments related to issuance of stock for share-based compensation arrangements, net Payment, Tax Withholding, Share-Based Payment Arrangement Line of Credit Line of Credit [Member] Restatement Determination Date Restatement Determination Date Excess and obsolescence charges related to inventory Inventory Write-down MS Product Revenues MS Product Revenues [Member] MS Product Revenues [Member] Acquisition of Apellis, net of cash acquired Other Payments to Acquire Businesses AVONEX AVONEX [Member] AVONEX Amendment Flag Amendment Flag Fair Value Notes Payable, Current, Fair Value Disclosure Notes Payable, Current, Fair Value Disclosure 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] 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 Gross Unrealized Losses Debt Securities, Available-for-Sale, Accumulated Gross Unrealized Loss, before Tax 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] Genentech Genentech [Member] Genentech 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] Expected profit share percentage Equity Method Investments, Expected Profit Share Equity Method Investments, Expected Profit Share 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] Operating lease liabilities Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Noncurrent Liabilities, Operating Lease Liabilities Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Noncurrent Liabilities, Operating Lease Liabilities Distributor Three Distributor Three [Member] Distributor Three 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 Floating Rate 364-Day Tranche Floating Rate 364-Day Tranche [Member] Floating Rate 364-Day Tranche Accumulated impairment losses related to goodwill Goodwill, Impaired, Accumulated Impairment Loss 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] SEC Schedule, 12-09, Valuation and Qualifying Accounts Disclosure [Table] SEC Schedule, 12-09, Valuation and Qualifying Accounts Disclosure [Table] TYSABRI TYSABRI product [Member] TYSABRI product [Member] Award Timing MNPI Disclosure Award Timing MNPI Disclosure [Text Block] Basis of presentation Basis of Accounting, Policy [Policy Text Block] Investments in Variable Interest Entities [Abstract] Investments in Variable Interest Entities [Abstract] Investments In Variable Interest Entities. 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] Debt instrument, make whole provision redemption price, percentage Debt Instrument, Make Whole Provision Redemption Price, Percentage Debt Instrument, Make Whole Provision Redemption Price, Percentage Capitalized share-based compensation costs Share-Based Payment Arrangement, Amount Capitalized RayThera Inc. RayThera Inc. [Member] RayThera Inc. Treasury stock, at cost Treasury Stock, Value New accounting pronouncements New Accounting Pronouncements, Policy [Policy Text Block] Litigation Case [Axis] Litigation Case [Axis] Accrued expenses and other Schedule of Accrued Liabilities [Table Text Block] Government securities, noncurrent Government Securities Non-current [Member] Government Securities Non Current. Notes payable and term loan Long-Term Debt Property, Plant and Equipment Property, Plant, and Equipment [Text Block] Right-of-use asset, derecognized Operating Lease, Right-Of-Use Asset, Derecognized Operating Lease, Right-Of-Use Asset, Derecognized Alzheimer's Collaboration Alzheimer's Collaboration [Member] Alzheimer's Collaboration Income tax assets and liabilities Increase (Decrease) in Income Taxes Payable 2023 Cost Saving Initiatives 2023 Cost Saving Initiatives [Member] 2023 Cost Saving Initiatives Changes in operating assets and liabilities, net of effects of business acquired: Adjustment to Reconcile Net Income to Cash Provided by (Used in) Operating Activity, Increase (Decrease) in Operating Capital [Abstract] Net cash flow provided by (used in) investing activities Cash Provided by (Used in) Investing Activity, Including Discontinued Operation Equity Awards Adjustments, Excluding Value Reported in Compensation Table Equity Awards Adjustments, Excluding Value Reported in the Compensation Table [Member] Equity Equity [Text Block] Cost and expenses: Costs and Expenses [Abstract] Security Exchange Name Security Exchange Name Ionis Ionis Pharmaceuticals Inc. [Member] Ionis Pharmaceuticals Inc. 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] 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. 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. Total Liabilities, Fair Value Disclosure 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] 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. Geographical [Domain] Geographical [Domain] Cost and net Intangible Asset, Excluding Goodwill, Indefinite-Lived 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] Amounts reclassified from AOCI Reclassification from AOCI, Current Period, Net of Tax, Attributable to Parent Apellis Integration Apellis Integration [Member] Apellis Integration 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] 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] Entity Emerging Growth Company Entity Emerging Growth Company Underlying Security Market Price Change Underlying Security Market Price Change, Percent 2026 Term Loan two-year tranche 2026 Term Loan two-year tranche [Member] 2026 Term Loan two-year tranche Commercial Paper Commercial Paper [Member] Derivative Instruments Derivative Instruments and Hedging Activities Disclosure [Text Block] Contractual adjustments Contractual Adjustments [Member] Contractual adjustments. 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] Treasury stock Treasury Stock, Common [Member] Intangible Asset, Goodwill and Other [Abstract] Intangible Asset, Goodwill and Other [Abstract] Summary of Activity Related to Stoke Therapeutics Collaboration Summary Of Activity Related To Collaboration [Table Text Block] Summary Of Activity Related To Collaboration UCB Pharma S.A. UCB Pharma S.A. [Member] UCB Pharma S.A. 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] Performance stock units settled in stock Performance Stock Units Settled in Stock [Member] Performance Stock Units Settled in Stock [Member] All Executive Categories All Executive Categories [Member] Milestone Payment, Developmental Milestone Payment, Developmental [Member] Milestone Payment, Developmental 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] 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 Asset Acquisition [Domain] Asset Acquisition [Domain] Payments Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Settlements 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 Segment Information Segment Reporting [Text Block] Minimum Minimum [Member] QALSODY QALSODY [Member] QALSODY 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] 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. Non-Rule 10b5-1 Arrangement Adopted Non-Rule 10b5-1 Arrangement Adopted [Flag] Sales trigger gross sales threshold Sales Trigger Gross Sales Threshold Sales trigger gross sales threshold. 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] 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 Acquired in-process research and development Acquired In-Process Research And Development Acquired In-Process Research And Development Accumulated Other Comprehensive Income (Loss) [Line Items] Accumulated Other Comprehensive Income (Loss) [Line Items] Realized gains Debt Securities, Available-for-Sale, Realized Gain Returns Sales Returns and Allowances [Member] 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 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] FAMPYRA FAMPYRA [Member] FAMPYRA [Member] 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] Marketable equity securities, current Equity Securities, Current [Member] Equity Securities, Current Pre-acquisition services Business Combination, Consideration Transferred, Pre-Acquisition Services Business Combination, Consideration Transferred, Pre-Acquisition Services Accounts receivable Accounts Receivable, after Allowance for Credit Loss Repayments of borrowings Repayments of Long-Term Debt RITUXAN RITUXAN [Member] RITUXAN. 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] Total reserves included in consolidated balance sheets Summary Of Product Revenue Reserves Included In Consolidated Balance Sheets [Table Text Block] Summary of total product revenue reserves included in consolidated balance sheets. Divestitures [Abstract] Divestitures [Abstract] Divestitures [Abstract] Measurement Input Type [Domain] Measurement Input Type [Domain] Foreign Exchange Contract Foreign Exchange Contract [Member] 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] Restructuring Type [Axis] Restructuring Type [Axis] Fair Value Equity Securities, FV-NI, Current 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 Title Trading Arrangement, Individual Title Forgone Recovery due to Disqualification of Tax Benefits, Amount Forgone Recovery due to Disqualification of Tax Benefits, Amount 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] Compensation Actually Paid vs. Net Income Compensation Actually Paid vs. Net Income [Text Block] 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. Other revenues [Abstract] Other revenues [Abstract] Other revenues [Abstract] 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 Restructuring Plan [Domain] Restructuring Plan [Domain] Deferred tax asset Deferred Income Tax Assets, Net Stock Appreciation Rights (SARs) Stock Appreciation Rights (SARs) [Member] Total development expense incurred by the collaboration related to the advancement of LEQEMBI Expense Incurred By Collaboration Total expense incurred by collaboration. Reata Pharmaceuticals, Inc Reata Pharmaceuticals, Inc [Member] Reata Pharmaceuticals, Inc SKYCLARYS SKYCLARYS [Member] SKYCLARYS Interferon Interferon [Member] Interferon Unrealized gains Unrealized Gain on Derivatives Unrealized Gain on Derivatives Acquired in-process research and development, upfront and milestone expense Research And Development Expense (Including Acquired in Process Cost) Research And Development Expense (Including Acquired in Process Cost) City Area Code City Area Code Lease, liability, derecognized Operating Lease, Liability, Derecognized Operating Lease, Liability, Derecognized Name Forgone Recovery, Individual Name Non-GAAP Measure Description Non-GAAP Measure Description [Text Block] Entity [Domain] Entity [Domain] Accrued expense and other Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustment, Accrued Expense And Other Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustment, Accrued Expense And Other Sage Therapeutics Sage Therapeutics Inc. [Member] Sage Therapeutics Inc. BENEPALI BENEPALI [Member] BENEPALI [Member] 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. Cash flows, revenue Cash flows, revenue [Member] Cash flows, revenue [Member] 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 Share-based payment arrangement, expense Share-based compensation expense Share-Based Payment Arrangement, Expense Collaboration agreement term Period Of Collaboration Agreement Period Of Collaboration Agreement Inventory* Business Combination, Recognized Asset Acquired, Inventory, Current Equity Securities, Non-Current Equity Securities, Non-Current [Member] Equity Securities, Non-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] Collaboration expense Collaboration Expenses Accrual Collaboration Expenses Accrual Cash equivalents, at carrying value Cash Equivalents, at Carrying Value Due in one year or less Debt Securities, Available-for-Sale, Fair Value, Maturity, Allocated and Single Maturity Date, Year One Corporate Debt Securities Current Corporate Debt Securities Current [Member] Corporate debt securities Current. 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Statement of Stockholders' Equity [Abstract] Statement of Stockholders' Equity [Abstract] Other non-current liabilities: Derivative Instruments and Hedges, Liabilities, Noncurrent [Abstract] Proceeds from borrowings, net Proceeds from Issuance of Long-Term Debt Local Phone Number Local Phone Number Other Consolidated Financial Statement Detail Additional Financial Information Disclosure [Text Block] Marketed products Marketed Products [Member] Marketed Products (Loss) Income before income tax (benefit) expense (Loss) Income before income tax (benefit) expense Income (Loss) from Continuing Operations before Income Taxes, Noncontrolling Interest Payments/Returns Relating To Sales in Prior Year Payments Returns Applied Against Product Revenue Reserves Relating To Prior Year Payments/ returns applied against product revenue reserves relating to prior year. Commitments, contingencies and guarantees Commitments and Contingencies Antidilutive Securities, Name [Domain] Antidilutive Securities, Name [Domain] Changed Peer Group, Footnote Changed Peer Group, Footnote [Text Block] Reclassification out of Accumulated Other Comprehensive Income [Axis] Reclassification out of Accumulated Other Comprehensive Income [Axis] 2030 Intangible Asset, Finite-Lived, Estimated Amortization Expense, Year Four Executive Category: Executive Category [Axis] Counterparty Name [Axis] Counterparty Name [Axis] Derivative Instruments, Unrealized Gain (Loss) Derivative Instruments, Unrealized Gain (Loss) [Table Text Block] Derivative Instruments, Unrealized Gain (Loss) LIABILITIES AND EQUITY Liabilities and Equity [Abstract] Debt Securities, Available-for-Sale, Amortized Cost [Abstract] Debt Securities, Available-for-Sale, Amortized Cost [Abstract] Inventory Inventory, current Inventory, Net Non-PEO NEO Average Compensation Actually Paid Amount Non-PEO NEO Average Compensation Actually Paid Amount Felzartamab Felzartamab [Member] Felzartamab Milestone Payment, Sales-Based Milestone Payment, Sales-Based [Member] Milestone Payment, Sales-Based PEO Name PEO Name 3.250% Senior Notes due February 15, 2051 Senior Notes 3.250%, Due February 15, 2051 [Member] Senior Notes 3.250%, Due February 15, 2051 Gains (losses) on cash flow hedges Accumulated Gain (Loss), Net, Cash Flow Hedge, Parent [Member] Other segment expense Segment Reporting, Other Segment Item, Amount Stock Price or TSR Estimation Method Stock Price or TSR Estimation Method [Text Block] Entity Tax Identification Number Entity Tax Identification Number Discount rate Business Combination, Contingent Consideration, Liability, Measurement Input Carrying Value Notes Payable Income tax effect Share-Based Payment Arrangement, Expense, Tax Benefit GAZYVA GAZYVA [Member] GAZYVA [Member] Interest in subsidiary (less than given percentage) Interest In Subsidiary Interest in subsidiary. 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 Revenues from anti-CD20 therapeutic programs Revenues from anti-CD20 therapeutic programs [Table Text Block] Revenues from anti-CD20 therapeutic programs Other Other Accrued Liabilities, Current Entity Incorporation, State or Country Code Entity Incorporation, State or Country Code SYFOVRE SYFOVRE [Member] SYFOVRE Aggregate Erroneous Compensation Not Yet Determined Aggregate Erroneous Compensation Not Yet Determined [Text Block] Contingent consideration related to the acquisition of Apellis Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Purchases Material Terms of Trading Arrangement Material Terms of Trading Arrangement [Text Block] Vesting Date Fair Value of Equity Awards Granted and Vested in Covered Year Vesting Date Fair Value of Equity Awards Granted and Vested in Covered Year [Member] Debt Instrument [Axis] Debt Instrument [Axis] Share-based compensation Share-Based Payment Arrangement, Noncash Expense Expected number of positions eliminated Restructuring and Related Cost, Expected Number of Positions Eliminated Summary of fair value and presentation of derivatives Schedule of Derivative Instruments in Statement of Financial Position, Fair Value [Table Text Block] Basic earnings per share attributable to Biogen Inc. 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Contingent consideration obligations Business Combination, Contingent Consideration, Liability Share of non-controling interest recognized Share Of Non-Controling Interest Recognized Share Of Non-Controling Interest Recognized Compensation Actually Paid vs. Company Selected Measure Compensation Actually Paid vs. Company Selected Measure [Text Block] Cover [Abstract] Cover [Abstract] Fair Value Measurement Inputs and Valuation Techniques [Table] Fair Value Measurement Inputs and Valuation Techniques [Table] Equity Awards Adjustments Equity Awards Adjustments [Member] Collaborative and Other Relationships Collaborative Arrangement Disclosure [Text Block] Hedging Relationship [Domain] Hedging Relationship [Domain] Derivative contracts Derivative Liability, Noncurrent Inventory Disclosure [Abstract] Inventory Disclosure [Abstract] Expected receipts to co-fund development costs, current Collaborative Arrangement, Expected Development Cost Funds, Year One Collaborative Arrangement, Expected Development Cost Funds, Year One Intangible Asset, Finite-Lived, after Accumulated Amortization [Abstract] Intangible Asset, Finite-Lived, after Accumulated Amortization [Abstract] Collaboration profit sharing/(loss reimbursement) Collaboration profit (loss) sharing Collaboration profit (loss) sharing Other, net Other Nonoperating Income (Expense) Operating lease assets 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 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] Floating Rate Two-Year Tranche Floating Rate Two-Year Tranche [Member] Floating Rate Two-Year Tranche Derivatives [Line Items] Derivatives, Fair Value [Line Items] The Credit Facility Floating Rate Three-Year Tranche The Credit Facility Floating Rate Three-Year Tranche [Member] The Credit Facility Floating Rate Three-Year Tranche Sage and Denali Sage and Denali [Member] Sage and Denali 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] Statement of Cash Flows [Abstract] Statement of Cash Flows [Abstract] In-process research and development In-process research and development In Process Research and Development [Member] Phase 3 Clinical Trial, Felzartamab AMR Phase 3 Clinical Trial, Felzartamab AMR [Member] Phase 3 Clinical Trial, Felzartamab AMR Apellis Pharmaceuticals, Inc. Apellis Pharmaceuticals, Inc. [Member] Apellis Pharmaceuticals, Inc. Aggregate Available Trading Arrangement, Securities Aggregate Available Amount Accounts payable Accounts Payable Discontinued Operations and Disposal Groups [Abstract] Income Taxes Income Tax Disclosure [Text Block] 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] Accounts payable Accounts Payable, Current Other Other Noncash Income (Expense) Biosimilars Biosimilars [Member] Biosimilars Pay vs Performance Disclosure Pay vs Performance Disclosure [Table] Total consideration Business Combination, Consideration Transferred Entity Filer Category Entity Filer Category Other research and discovery Other research and discovery [Member] Other research and discovery [Member] Revision of Prior Period [Axis] Revision of Prior Period [Axis] Net Gains/(Losses) Excluded from Effectiveness Testing and Recognized in Operating Income (in millions) Gain (Loss) on Derivative Instruments, Net, Pretax SEC Schedule, 12-09, Valuation and Qualifying Accounts Disclosure [Line Items] SEC Schedule, 12-09, Valuation and Qualifying Accounts Disclosure [Line Items] Not designated as hedging instrument Not Designated as Hedging Instrument [Member] Investments, Debt and Equity Securities [Abstract] Investments, Debt and Equity Securities [Abstract] Gain (loss) on investments in equity securities Gain (Loss) on Securities [Table Text Block] Derivative Actions Derivative Actions [Member] Derivative Actions Net income Net income Net Income (Loss), Including Portion Attributable to Noncontrolling Interest Price per share (in dollars per share) Business Combination, Price Per Share Current assets: Assets, Current [Abstract] Summary of Activity Related to Sage Therapeutics Summary of Activity Related to Sage Therapeutics [Table Text Block] Summary of Activity Related to Sage Therapeutics Senior secured term loan Senior Notes [Member] Entity Address, Address Line One Entity Address, Address Line One Income tax (benefit) expense Income Tax Expense (Benefit) Research and Development Arrangement, Contract to Perform for Others, Type [Axis] Research and Development Arrangement, Contract to Perform for Others, Type [Axis] Consolidation Consolidation, Policy [Policy Text Block] Financial Instruments Cash, Cash Equivalent, and Marketable Security [Text Block] Numerator: Net Income (Loss) Attributable to Parent [Abstract] Research and development Research and development: Research and Development Expense (Excluding Acquired in Process Cost) Deferred tax liability Business Combination, Recognized Liability Assumed, Deferred Tax Liability Clinical Inventory Clinical Inventory [Member] Clinical Inventory BYOOVIZ BYOOVIZ [Member] BYOOVIZ Statement [Table] Statement [Table] Share-based Payment Arrangement, Expensed and Capitalized, Amount [Table] Share-Based Payment Arrangement, Expensed and Capitalized, Amount [Table] Milestone Payment, One Milestone Payment, One [Member] Milestone Payment, One Interest income Investment Income, Interest Operating lease area Operating Lease Area Operating Lease Area 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 Share of net profit from sage therapeutics, Percent Share Of Net Profit From Sage Therapeutics, Percent Share Of Net Profit From Sage Therapeutics, Percent Derivative asset, fair value Derivative Asset, Subject to Master Netting Arrangement, before Offset Claims asserted Loss Contingency, Damages Sought, Value 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. TOFIDENCE TOFIDENCE [Member] TOFIDENCE Derivative Instruments and Hedging Activities Disclosure [Abstract] Derivative Instruments and Hedging Activities Disclosure [Abstract]