3.26.1 html 508 362 1 false 174 0 false 10 false false R1.htm 0000001 - Document - Cover Page Sheet http://www.biogenidec.com/role/CoverPage Cover Page Cover 1 false false R2.htm 9952151 - Statement - Condensed Consolidated Statements of Income (Unaudited) Sheet http://www.biogenidec.com/role/CondensedConsolidatedStatementsofIncomeUnaudited Condensed Consolidated Statements of Income (Unaudited) Statements 2 false false R3.htm 9952152 - Statement - Condensed Consolidated Statements of Comprehensive Income (Unaudited) Sheet http://www.biogenidec.com/role/CondensedConsolidatedStatementsofComprehensiveIncomeUnaudited Condensed Consolidated Statements of Comprehensive Income (Unaudited) Statements 3 false false R4.htm 9952153 - Statement - Condensed Consolidated Balance Sheets (Unaudited) Sheet http://www.biogenidec.com/role/CondensedConsolidatedBalanceSheetsUnaudited Condensed Consolidated Balance Sheets (Unaudited) Statements 4 false false R5.htm 9952154 - Statement - Consolidated Balance Sheets (Parenthetical) Sheet http://www.biogenidec.com/role/ConsolidatedBalanceSheetsParenthetical Consolidated Balance Sheets (Parenthetical) Statements 5 false false R6.htm 9952155 - Statement - Condensed Consolidated Statements of Cash Flows (Unaudited) Sheet http://www.biogenidec.com/role/CondensedConsolidatedStatementsofCashFlowsUnaudited Condensed Consolidated Statements of Cash Flows (Unaudited) Statements 6 false false R7.htm 9952156 - Statement - Condensed Consolidated Statement of Equity Statement Sheet http://www.biogenidec.com/role/CondensedConsolidatedStatementofEquityStatement Condensed Consolidated Statement of Equity Statement Statements 7 false false R8.htm 9952157 - Disclosure - Summary of Significant Accounting Policies Sheet http://www.biogenidec.com/role/SummaryofSignificantAccountingPolicies Summary of Significant Accounting Policies Notes 8 false false R9.htm 9952158 - Disclosure - Acquisitions Sheet http://www.biogenidec.com/role/Acquisitions Acquisitions Notes 9 false false R10.htm 9952159 - Disclosure - Dispositions Sheet http://www.biogenidec.com/role/Dispositions Dispositions Notes 10 false false R11.htm 9952160 - Disclosure - Restructuring Sheet http://www.biogenidec.com/role/Restructuring Restructuring Notes 11 false false R12.htm 9952161 - Disclosure - Revenues Sheet http://www.biogenidec.com/role/Revenues Revenues Notes 12 false false R13.htm 9952162 - Disclosure - Inventory Sheet http://www.biogenidec.com/role/Inventory Inventory Notes 13 false false R14.htm 9952163 - Disclosure - Intangible Assets and Goodwill Sheet http://www.biogenidec.com/role/IntangibleAssetsandGoodwill Intangible Assets and Goodwill Notes 14 false false R15.htm 9952164 - Disclosure - Fair Value Measurements Sheet http://www.biogenidec.com/role/FairValueMeasurements Fair Value Measurements Notes 15 false false R16.htm 9952165 - Disclosure - Financial Instruments Sheet http://www.biogenidec.com/role/FinancialInstruments Financial Instruments Notes 16 false false R17.htm 9952166 - Disclosure - Derivative Instruments Sheet http://www.biogenidec.com/role/DerivativeInstruments Derivative Instruments Notes 17 false false R18.htm 9952167 - Disclosure - Property, Plant and Equipment Sheet http://www.biogenidec.com/role/PropertyPlantandEquipment Property, Plant and Equipment Notes 18 false false R19.htm 9952168 - Disclosure - Indebtedness Sheet http://www.biogenidec.com/role/Indebtedness Indebtedness Notes 19 false false R20.htm 9952169 - Disclosure - Equity Sheet http://www.biogenidec.com/role/Equity Equity Notes 20 false false R21.htm 9952170 - Disclosure - Earnings per Share Sheet http://www.biogenidec.com/role/EarningsperShare Earnings per Share Notes 21 false false R22.htm 9952171 - Disclosure - Share-based Payments Sheet http://www.biogenidec.com/role/SharebasedPayments Share-based Payments Notes 22 false false R23.htm 9952172 - Disclosure - Income Taxes Sheet http://www.biogenidec.com/role/IncomeTaxes Income Taxes Notes 23 false false R24.htm 9952173 - Disclosure - Other Consolidated Financial Statement Detail Sheet http://www.biogenidec.com/role/OtherConsolidatedFinancialStatementDetail Other Consolidated Financial Statement Detail Notes 24 false false R25.htm 9952174 - Disclosure - Collaborative and Other Relationships Sheet http://www.biogenidec.com/role/CollaborativeandOtherRelationships Collaborative and Other Relationships Notes 25 false false R26.htm 9952175 - Disclosure - Investments in Variable Interest Entities Sheet http://www.biogenidec.com/role/InvestmentsinVariableInterestEntities Investments in Variable Interest Entities Notes 26 false false R27.htm 9952176 - Disclosure - Litigation Sheet http://www.biogenidec.com/role/Litigation Litigation Notes 27 false false R28.htm 9952177 - Disclosure - Segment Information Sheet http://www.biogenidec.com/role/SegmentInformation Segment Information Notes 28 false false R29.htm 9952178 - Disclosure - Subsequent Events Sheet http://www.biogenidec.com/role/SubsequentEvents Subsequent Events Notes 29 false false R30.htm 995445 - Disclosure - Insider Trading Arrangements Sheet http://xbrl.sec.gov/ecd/role/InsiderTradingArrangements Insider Trading Arrangements Notes 30 false false R31.htm 9955511 - Disclosure - Summary of Significant Accounting Policies Summary of Significant Accounting Policies (Policies) Sheet http://www.biogenidec.com/role/SummaryofSignificantAccountingPoliciesSummaryofSignificantAccountingPoliciesPolicies Summary of Significant Accounting Policies Summary of Significant Accounting Policies (Policies) Policies http://www.biogenidec.com/role/SummaryofSignificantAccountingPolicies 31 false false R32.htm 9955512 - Disclosure - Restructuring (Tables) Sheet http://www.biogenidec.com/role/RestructuringTables Restructuring (Tables) Tables http://www.biogenidec.com/role/Restructuring 32 false false R33.htm 9955513 - Disclosure - Revenues (Tables) Sheet http://www.biogenidec.com/role/RevenuesTables Revenues (Tables) Tables http://www.biogenidec.com/role/Revenues 33 false false R34.htm 9955514 - Disclosure - Inventory (Tables) Sheet http://www.biogenidec.com/role/InventoryTables Inventory (Tables) Tables http://www.biogenidec.com/role/Inventory 34 false false R35.htm 9955515 - Disclosure - Intangible Assets and Goodwill (Tables) Sheet http://www.biogenidec.com/role/IntangibleAssetsandGoodwillTables Intangible Assets and Goodwill (Tables) Tables http://www.biogenidec.com/role/IntangibleAssetsandGoodwill 35 false false R36.htm 9955516 - Disclosure - Fair Value Measurements (Tables) Sheet http://www.biogenidec.com/role/FairValueMeasurementsTables Fair Value Measurements (Tables) Tables http://www.biogenidec.com/role/FairValueMeasurements 36 false false R37.htm 9955517 - Disclosure - Financial Instruments (Tables) Sheet http://www.biogenidec.com/role/FinancialInstrumentsTables Financial Instruments (Tables) Tables http://www.biogenidec.com/role/FinancialInstruments 37 false false R38.htm 9955518 - Disclosure - Derivative Instruments (Tables) Sheet http://www.biogenidec.com/role/DerivativeInstrumentsTables Derivative Instruments (Tables) Tables http://www.biogenidec.com/role/DerivativeInstruments 38 false false R39.htm 9955519 - Disclosure - Equity (Tables) Sheet http://www.biogenidec.com/role/EquityTables Equity (Tables) Tables http://www.biogenidec.com/role/Equity 39 false false R40.htm 9955520 - Disclosure - Earnings per Share (Tables) Sheet http://www.biogenidec.com/role/EarningsperShareTables Earnings per Share (Tables) Tables http://www.biogenidec.com/role/EarningsperShare 40 false false R41.htm 9955521 - Disclosure - Share-Based Payments (Tables) Sheet http://www.biogenidec.com/role/ShareBasedPaymentsTables Share-Based Payments (Tables) Tables 41 false false R42.htm 9955522 - Disclosure - Other Consolidated Financial Statement Detail (Tables) Sheet http://www.biogenidec.com/role/OtherConsolidatedFinancialStatementDetailTables Other Consolidated Financial Statement Detail (Tables) Tables http://www.biogenidec.com/role/OtherConsolidatedFinancialStatementDetail 42 false false R43.htm 9955523 - Disclosure - Collaborative and Other Relationships (Tables) Sheet http://www.biogenidec.com/role/CollaborativeandOtherRelationshipsTables Collaborative and Other Relationships (Tables) Tables http://www.biogenidec.com/role/CollaborativeandOtherRelationships 43 false false R44.htm 9955524 - Disclosure - Segment Information (Tables) Sheet http://www.biogenidec.com/role/SegmentInformationTables Segment Information (Tables) Tables http://www.biogenidec.com/role/SegmentInformation 44 false false R45.htm 9955525 - Disclosure - Summary of Significant Accounting Policies (Details) Sheet http://www.biogenidec.com/role/SummaryofSignificantAccountingPoliciesDetails Summary of Significant Accounting Policies (Details) Details http://www.biogenidec.com/role/SummaryofSignificantAccountingPoliciesSummaryofSignificantAccountingPoliciesPolicies 45 false false R46.htm 9955526 - Disclosure - Acquisitions - Narrative (Details) Sheet http://www.biogenidec.com/role/AcquisitionsNarrativeDetails Acquisitions - Narrative (Details) Details 46 false false R47.htm 9955527 - Disclosure - Dispositions - Narrative (Details) Sheet http://www.biogenidec.com/role/DispositionsNarrativeDetails Dispositions - Narrative (Details) Details 47 false false R48.htm 9955528 - Disclosure - Restructuring- Fit for Growth Initiative (Details) Sheet http://www.biogenidec.com/role/RestructuringFitforGrowthInitiativeDetails Restructuring- Fit for Growth Initiative (Details) Details 48 false false R49.htm 9955529 - Disclosure - Restructuring - Restructuring Reserve Roll Forward (Details) Sheet http://www.biogenidec.com/role/RestructuringRestructuringReserveRollForwardDetails Restructuring - Restructuring Reserve Roll Forward (Details) Details 49 false false R50.htm 9955530 - Disclosure - Revenues - Revenues by product (Details) Sheet http://www.biogenidec.com/role/RevenuesRevenuesbyproductDetails Revenues - Revenues by product (Details) Details 50 false false R51.htm 9955531 - Disclosure - Revenues - Narrative (Details) Sheet http://www.biogenidec.com/role/RevenuesNarrativeDetails Revenues - Narrative (Details) Details 51 false false R52.htm 9955532 - Disclosure - Revenues Reserves for Discounts and Allowances (Details 1) Sheet http://www.biogenidec.com/role/RevenuesReservesforDiscountsandAllowancesDetails1 Revenues Reserves for Discounts and Allowances (Details 1) Details 52 false false R53.htm 9955533 - Disclosure - Revenues- Reserves for Discounts and Allowances (Details 2) Sheet http://www.biogenidec.com/role/RevenuesReservesforDiscountsandAllowancesDetails2 Revenues- Reserves for Discounts and Allowances (Details 2) Details 53 false false R54.htm 9955534 - Disclosure - Revenues - Revenues from Anti-CD20 Therapeutic Programs (Details) Sheet http://www.biogenidec.com/role/RevenuesRevenuesfromAntiCD20TherapeuticProgramsDetails Revenues - Revenues from Anti-CD20 Therapeutic Programs (Details) Details 54 false false R55.htm 9955535 - Disclosure - Revenues - Contract Manufacturing, Royalty and Other Revenue (Details) Sheet http://www.biogenidec.com/role/RevenuesContractManufacturingRoyaltyandOtherRevenueDetails Revenues - Contract Manufacturing, Royalty and Other Revenue (Details) Details 55 false false R56.htm 9955536 - Disclosure - Inventory - Components of Inventory (Details) Sheet http://www.biogenidec.com/role/InventoryComponentsofInventoryDetails Inventory - Components of Inventory (Details) Details 56 false false R57.htm 9955537 - Disclosure - Inventory - Narrative (Details) Sheet http://www.biogenidec.com/role/InventoryNarrativeDetails Inventory - Narrative (Details) Details 57 false false R58.htm 9955538 - Disclosure - Intangible Assets and Goodwill - Summary of Intangible Assets (Details) Sheet http://www.biogenidec.com/role/IntangibleAssetsandGoodwillSummaryofIntangibleAssetsDetails Intangible Assets and Goodwill - Summary of Intangible Assets (Details) Details 58 false false R59.htm 9955539 - Disclosure - Intangible Assets and Goodwill - Narrative (Details) Sheet http://www.biogenidec.com/role/IntangibleAssetsandGoodwillNarrativeDetails Intangible Assets and Goodwill - Narrative (Details) Details 59 false false R60.htm 9955540 - Disclosure - Intangible Assets and Goodwill - Estimated Future Amortization of Intangible Assets (Details) Sheet http://www.biogenidec.com/role/IntangibleAssetsandGoodwillEstimatedFutureAmortizationofIntangibleAssetsDetails Intangible Assets and Goodwill - Estimated Future Amortization of Intangible Assets (Details) Details 60 false false R61.htm 9955541 - Disclosure - Intangible Assets and Goodwill - Changes in Goodwill (Details) Sheet http://www.biogenidec.com/role/IntangibleAssetsandGoodwillChangesinGoodwillDetails Intangible Assets and Goodwill - Changes in Goodwill (Details) Details 61 false false R62.htm 9955542 - Disclosure - Fair Value Measurements - Summary of Assets and Liabilities Recorded at Fair Value (Details) Sheet http://www.biogenidec.com/role/FairValueMeasurementsSummaryofAssetsandLiabilitiesRecordedatFairValueDetails Fair Value Measurements - Summary of Assets and Liabilities Recorded at Fair Value (Details) Details 62 false false R63.htm 9955543 - Disclosure - Fair Value Measurements - Narrative (Details) Sheet http://www.biogenidec.com/role/FairValueMeasurementsNarrativeDetails Fair Value Measurements - Narrative (Details) Details 63 false false R64.htm 9955544 - Disclosure - Fair Value Measures - Level 3 Assets and Liabilities Held at Fair Value (Details) Sheet http://www.biogenidec.com/role/FairValueMeasuresLevel3AssetsandLiabilitiesHeldatFairValueDetails Fair Value Measures - Level 3 Assets and Liabilities Held at Fair Value (Details) Details 64 false false R65.htm 9955545 - Disclosure - Fair Value Measurements - Contingent Consideration Obligations (Details) Sheet http://www.biogenidec.com/role/FairValueMeasurementsContingentConsiderationObligationsDetails Fair Value Measurements - Contingent Consideration Obligations (Details) Details 65 false false R66.htm 9955546 - Disclosure - Fair Value Measurements - Debt Instruments (Details) Sheet http://www.biogenidec.com/role/FairValueMeasurementsDebtInstrumentsDetails Fair Value Measurements - Debt Instruments (Details) Details 66 false false R67.htm 9955547 - Disclosure - Financial Instruments - Summary of financial assets with maturities of less than 90 days included within cash and cash equivalents (Details) Sheet http://www.biogenidec.com/role/FinancialInstrumentsSummaryoffinancialassetswithmaturitiesoflessthan90daysincludedwithincashandcashequivalentsDetails Financial Instruments - Summary of financial assets with maturities of less than 90 days included within cash and cash equivalents (Details) Details 67 false false R68.htm 9955548 - Disclosure - Financial Instruments - Summary of Marketable Securities (Details) Sheet http://www.biogenidec.com/role/FinancialInstrumentsSummaryofMarketableSecuritiesDetails Financial Instruments - Summary of Marketable Securities (Details) Details 68 false false R69.htm 9955549 - Disclosure - Financial Instruments - Summary of Contractual Maturities (Details) Sheet http://www.biogenidec.com/role/FinancialInstrumentsSummaryofContractualMaturitiesDetails Financial Instruments - Summary of Contractual Maturities (Details) Details 69 false false R70.htm 9955550 - Disclosure - Financial Instruments - Narrative (Details) Sheet http://www.biogenidec.com/role/FinancialInstrumentsNarrativeDetails Financial Instruments - Narrative (Details) Details 70 false false R71.htm 9955551 - Disclosure - Financial Instruments - Proceeds from Marketable Debt Securities (Details) Sheet http://www.biogenidec.com/role/FinancialInstrumentsProceedsfromMarketableDebtSecuritiesDetails Financial Instruments - Proceeds from Marketable Debt Securities (Details) Details 71 false false R72.htm 9955552 - Disclosure - Derivative Instruments - Narrative (Details) Sheet http://www.biogenidec.com/role/DerivativeInstrumentsNarrativeDetails Derivative Instruments - Narrative (Details) Details 72 false false R73.htm 9955553 - Disclosure - Derivative Instruments - Notional Amount of Foreign Currency Forward Contracts and Foreign Currency Options (Details) Sheet http://www.biogenidec.com/role/DerivativeInstrumentsNotionalAmountofForeignCurrencyForwardContractsandForeignCurrencyOptionsDetails Derivative Instruments - Notional Amount of Foreign Currency Forward Contracts and Foreign Currency Options (Details) Details 73 false false R74.htm 9955554 - Disclosure - Derivative Instruments - Pre-tax Portion of the Fair Value of Foreign Currency Forward Contracts and Foreign Currency Options (Details) Sheet http://www.biogenidec.com/role/DerivativeInstrumentsPretaxPortionoftheFairValueofForeignCurrencyForwardContractsandForeignCurrencyOptionsDetails Derivative Instruments - Pre-tax Portion of the Fair Value of Foreign Currency Forward Contracts and Foreign Currency Options (Details) Details 74 false false R75.htm 9955555 - Disclosure - Derivative Instruments - Effect of Foreign Currency Forward Contracts and Forward Currency Options Designated as Hedging Instruments (Details) Sheet http://www.biogenidec.com/role/DerivativeInstrumentsEffectofForeignCurrencyForwardContractsandForwardCurrencyOptionsDesignatedasHedgingInstrumentsDetails Derivative Instruments - Effect of Foreign Currency Forward Contracts and Forward Currency Options Designated as Hedging Instruments (Details) Details 75 false false R76.htm 9955556 - Disclosure - Derivative Instruments - Summary of Fair Value and Presentation of Derivatives (Details) Sheet http://www.biogenidec.com/role/DerivativeInstrumentsSummaryofFairValueandPresentationofDerivativesDetails Derivative Instruments - Summary of Fair Value and Presentation of Derivatives (Details) Details 76 false false R77.htm 9955557 - Disclosure - Property, Plant and Equipment (Details) Sheet http://www.biogenidec.com/role/PropertyPlantandEquipmentDetails Property, Plant and Equipment (Details) Details http://www.biogenidec.com/role/PropertyPlantandEquipment 77 false false R78.htm 9955558 - Disclosure - Indebtedness - Narrative (Details) Sheet http://www.biogenidec.com/role/IndebtednessNarrativeDetails Indebtedness - Narrative (Details) Details 78 false false R79.htm 9955559 - Disclosure - Equity - Accumulated Other Comprehensive Income (Loss) (Details) Sheet http://www.biogenidec.com/role/EquityAccumulatedOtherComprehensiveIncomeLossDetails Equity - Accumulated Other Comprehensive Income (Loss) (Details) Details 79 false false R80.htm 9955560 - Disclosure - Equity - Reclassified From AOCI (Details) Sheet http://www.biogenidec.com/role/EquityReclassifiedFromAOCIDetails Equity - Reclassified From AOCI (Details) Details 80 false false R81.htm 9955561 - Disclosure - Earnings per Share (Details) Sheet http://www.biogenidec.com/role/EarningsperShareDetails Earnings per Share (Details) Details http://www.biogenidec.com/role/EarningsperShareTables 81 false false R82.htm 9955562 - Disclosure - Share-Based Payments - Share-Based Compensation Expense Included In Statements Of Income (Details) Sheet http://www.biogenidec.com/role/ShareBasedPaymentsShareBasedCompensationExpenseIncludedInStatementsOfIncomeDetails Share-Based Payments - Share-Based Compensation Expense Included In Statements Of Income (Details) Details 82 false false R83.htm 9955563 - Disclosure - Share-Based Payments - Share-Based Compensation Expense by Share-Based Compensation Program (Details) Sheet http://www.biogenidec.com/role/ShareBasedPaymentsShareBasedCompensationExpensebyShareBasedCompensationProgramDetails Share-Based Payments - Share-Based Compensation Expense by Share-Based Compensation Program (Details) Details 83 false false R84.htm 9955564 - Disclosure - Income Taxes - Narrative (Details) Sheet http://www.biogenidec.com/role/IncomeTaxesNarrativeDetails Income Taxes - Narrative (Details) Details 84 false false R85.htm 9955565 - Disclosure - Other Consolidated Financial Statement Detail - Other (Income) Expense, Net (Details) Sheet http://www.biogenidec.com/role/OtherConsolidatedFinancialStatementDetailOtherIncomeExpenseNetDetails Other Consolidated Financial Statement Detail - Other (Income) Expense, Net (Details) Details http://www.biogenidec.com/role/OtherConsolidatedFinancialStatementDetailTables 85 false false R86.htm 9955566 - Disclosure - Other Consolidated Financial Statement Detail - (Gains) Losses on Investments, Net (Details) Sheet http://www.biogenidec.com/role/OtherConsolidatedFinancialStatementDetailGainsLossesonInvestmentsNetDetails Other Consolidated Financial Statement Detail - (Gains) Losses on Investments, Net (Details) Details http://www.biogenidec.com/role/OtherConsolidatedFinancialStatementDetailTables 86 false false R87.htm 9955567 - Disclosure - Other Consolidated Financial Statement - Narrative (Details) Sheet http://www.biogenidec.com/role/OtherConsolidatedFinancialStatementNarrativeDetails Other Consolidated Financial Statement - Narrative (Details) Details 87 false false R88.htm 9955568 - Disclosure - Other Consolidated Financial Statement Detail - Accrued Expense and Other (Details) Sheet http://www.biogenidec.com/role/OtherConsolidatedFinancialStatementDetailAccruedExpenseandOtherDetails Other Consolidated Financial Statement Detail - Accrued Expense and Other (Details) Details 88 false false R89.htm 9955569 - Disclosure - Collaborative and Other Relationships - OCREVUS (Details) Sheet http://www.biogenidec.com/role/CollaborativeandOtherRelationshipsOCREVUSDetails Collaborative and Other Relationships - OCREVUS (Details) Details 89 false false R90.htm 9955570 - Disclosure - Collaborative and Other Relationships - Co-promotion Profit Sharing Formula (Details) Sheet http://www.biogenidec.com/role/CollaborativeandOtherRelationshipsCopromotionProfitSharingFormulaDetails Collaborative and Other Relationships - Co-promotion Profit Sharing Formula (Details) Details 90 false false R91.htm 9955571 - Disclosure - Collaborative and Other Relationships - Profit Sharing (Details) Sheet http://www.biogenidec.com/role/CollaborativeandOtherRelationshipsProfitSharingDetails Collaborative and Other Relationships - Profit Sharing (Details) Details 91 false false R92.htm 9955572 - Disclosure - Collaborative and Other Relationships - Pretax Profit Sharing Formula (Details) Sheet http://www.biogenidec.com/role/CollaborativeandOtherRelationshipsPretaxProfitSharingFormulaDetails Collaborative and Other Relationships - Pretax Profit Sharing Formula (Details) Details 92 false false R93.htm 9955573 - Disclosure - Collaborative and Other Relationships - Ionis Pharmaceuticals, Inc (Details) Sheet http://www.biogenidec.com/role/CollaborativeandOtherRelationshipsIonisPharmaceuticalsIncDetails Collaborative and Other Relationships - Ionis Pharmaceuticals, Inc (Details) Details 93 false false R94.htm 9955574 - Disclosure - Collaborative and Other Relationships - Eisai (Details) Sheet http://www.biogenidec.com/role/CollaborativeandOtherRelationshipsEisaiDetails Collaborative and Other Relationships - Eisai (Details) Details http://www.biogenidec.com/role/CollaborativeandOtherRelationshipsTables 94 false false R95.htm 9955575 - Disclosure - Collaborative and Other Relationships - Summary of Activity Related to LEQEMBI Collaboration (Details) Sheet http://www.biogenidec.com/role/CollaborativeandOtherRelationshipsSummaryofActivityRelatedtoLEQEMBICollaborationDetails Collaborative and Other Relationships - Summary of Activity Related to LEQEMBI Collaboration (Details) Details 95 false false R96.htm 9955576 - Disclosure - Collaborative and Other Relationships - Summary of Activity Related to the UCB Collaboration (Details) Sheet http://www.biogenidec.com/role/CollaborativeandOtherRelationshipsSummaryofActivityRelatedtotheUCBCollaborationDetails Collaborative and Other Relationships - Summary of Activity Related to the UCB Collaboration (Details) Details 96 false false R97.htm 9955577 - Disclosure - Collaborative and Other Relationships - Sage Therapeutics, Inc. (Details) Sheet http://www.biogenidec.com/role/CollaborativeandOtherRelationshipsSageTherapeuticsIncDetails Collaborative and Other Relationships - Sage Therapeutics, Inc. (Details) Details 97 false false R98.htm 9955578 - Disclosure - Collaborative and Other Relationships - Summary of Activity Related to Sage Therapeutics (Details) Sheet http://www.biogenidec.com/role/CollaborativeandOtherRelationshipsSummaryofActivityRelatedtoSageTherapeuticsDetails Collaborative and Other Relationships - Summary of Activity Related to Sage Therapeutics (Details) Details 98 false false R99.htm 9955579 - Disclosure - Collaborative and Other Relationships - Summary of Activity Related To Denali Therapeutics (Details) Sheet http://www.biogenidec.com/role/CollaborativeandOtherRelationshipsSummaryofActivityRelatedToDenaliTherapeuticsDetails Collaborative and Other Relationships - Summary of Activity Related To Denali Therapeutics (Details) Details 99 false false R100.htm 9955580 - Disclosure - Collaborative and Other Relationships - Stoke Therapeutics, Inc. (Details) Sheet http://www.biogenidec.com/role/CollaborativeandOtherRelationshipsStokeTherapeuticsIncDetails Collaborative and Other Relationships - Stoke Therapeutics, Inc. (Details) Details 100 false false R101.htm 9955581 - Disclosure - Collaborative and Other Relationships Summary of Activity Related to Stoke Therapeutics (Details) Sheet http://www.biogenidec.com/role/CollaborativeandOtherRelationshipsSummaryofActivityRelatedtoStokeTherapeuticsDetails Collaborative and Other Relationships Summary of Activity Related to Stoke Therapeutics (Details) Details 101 false false R102.htm 9955582 - Disclosure - Collaborative and Other Relationships - Samsung Bioepis (Details) Sheet http://www.biogenidec.com/role/CollaborativeandOtherRelationshipsSamsungBioepisDetails Collaborative and Other Relationships - Samsung Bioepis (Details) Details 102 false false R103.htm 9955583 - Disclosure - Collaborative and Other Relationships - Other Research and Discovery Arrangements and Funding Arrangements (Details) Sheet http://www.biogenidec.com/role/CollaborativeandOtherRelationshipsOtherResearchandDiscoveryArrangementsandFundingArrangementsDetails Collaborative and Other Relationships - Other Research and Discovery Arrangements and Funding Arrangements (Details) Details 103 false false R104.htm 9955584 - Disclosure - Investments in Variable Interest Entities (Details) Sheet http://www.biogenidec.com/role/InvestmentsinVariableInterestEntitiesDetails Investments in Variable Interest Entities (Details) Details http://www.biogenidec.com/role/InvestmentsinVariableInterestEntities 104 false false R105.htm 9955585 - Disclosure - Litigation (Details) Sheet http://www.biogenidec.com/role/LitigationDetails Litigation (Details) Details http://www.biogenidec.com/role/Litigation 105 false false R106.htm 9955586 - Disclosure - Segment Information - Narrative (Details) Sheet http://www.biogenidec.com/role/SegmentInformationNarrativeDetails Segment Information - Narrative (Details) Details 106 false false R107.htm 9955587 - Disclosure - Segment Information - Additional Segment Information (Details) Sheet http://www.biogenidec.com/role/SegmentInformationAdditionalSegmentInformationDetails Segment Information - Additional Segment Information (Details) Details 107 false false R108.htm 9955588 - Disclosure - Subsequent Events (Details) Sheet http://www.biogenidec.com/role/SubsequentEventsDetails Subsequent Events (Details) Details http://www.biogenidec.com/role/SubsequentEvents 108 false false All Reports Book All Reports biib-20260331.htm biib-20260331.xsd biib-20260331_cal.xml biib-20260331_def.xml biib-20260331_lab.xml biib-20260331_pre.xml biib-20260331_g1.jpg biib-20260331_g2.jpg biib-20260331_g3.jpg biib-20260331_g4.jpg biib-20260331_g5.jpg biib-20260331_g6.jpg biib-20260331_g7.jpg biib-20260331_g8.jpg biib-20260331_g9.jpg http://fasb.org/us-gaap/2025 http://xbrl.sec.gov/dei/2025 http://xbrl.sec.gov/ecd/2025 true true