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Subsequent Events Subsequent Events (Details) (USD $)
In Millions, unless otherwise specified
 1 Months Ended
Jan. 31, 2015
Dec. 31, 2014
Subsequent Event [Line Items]    
Potential future milestone payments   $ 2,800.0biib_PotentialFutureMilestonePaymentsCommitmentToThirdPartyApproximately
Subsequent event | Convergence Pharmaceuticals    
Subsequent Event [Line Items]    
Amount paid in cash 200.0us-gaap_PaymentsToAcquireBusinessesGross
/ us-gaap_SubsequentEventTypeAxis
= us-gaap_SubsequentEventMember
/ us-gaap_TypeOfArrangementAxis
= biib_ConvergencePharmaceuticalsMember		  
Potential future milestone payments $ 475.0biib_PotentialFutureMilestonePaymentsCommitmentToThirdPartyApproximately
/ us-gaap_SubsequentEventTypeAxis
= us-gaap_SubsequentEventMember
/ us-gaap_TypeOfArrangementAxis
= biib_ConvergencePharmaceuticalsMember