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Fair Value Measurements (Details Textual) (USD $)
In Millions, unless otherwise specified
3 Months Ended
Mar. 31, 2015
Mar. 31, 2014
Dec. 31, 2014
Feb. 12, 2015
Business Acquisition, Contingent Consideration [Line Items]        
Asset impairment charges $ 0us-gaap_AssetImpairmentCharges      
Changes in valuation techniques 0      
Contingent consideration obligations 397.5us-gaap_BusinessCombinationContingentConsiderationLiabilityNoncurrent   200.0us-gaap_BusinessCombinationContingentConsiderationLiabilityNoncurrent  
Contingent consideration 238.5us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityPurchases 0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityPurchases    
6.875% Senior Notes due 2018        
Business Acquisition, Contingent Consideration [Line Items]        
Stated rate on debt 6.875%us-gaap_DebtInstrumentInterestRateStatedPercentage
/ us-gaap_DebtInstrumentAxis
= biib_SeniorNotesSixPointEightSevenFivePercentDueTwentyEighteenMember
     
Convergence Pharmaceuticals        
Business Acquisition, Contingent Consideration [Line Items]        
Contingent consideration 238.5us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityPurchases
/ us-gaap_BusinessAcquisitionAxis
= biib_ConvergencePharmaceuticalsMember
     
Potential future milestone payments       450.0biib_PotentialFutureMilestonePaymentsCommitmentToThirdPartyApproximately
/ us-gaap_BusinessAcquisitionAxis
= biib_ConvergencePharmaceuticalsMember
Discount rate 2.00%us-gaap_FairValueInputsDiscountRate
/ us-gaap_BusinessAcquisitionAxis
= biib_ConvergencePharmaceuticalsMember
     
In-process research and development and other intangible assets       $ 424.6us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedIndefiniteLivedIntangibleAssets
/ us-gaap_BusinessAcquisitionAxis
= biib_ConvergencePharmaceuticalsMember