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Fair Value Measurements (Details 2) (USD $)
In Millions, unless otherwise specified
Dec. 31, 2014
Dec. 31, 2013
Summary of fair and carrying value of debt instruments    
Total fair value $ 647.2us-gaap_DebtInstrumentFairValue $ 665.4us-gaap_DebtInstrumentFairValue
Notes Payable to Fumedica    
Summary of fair and carrying value of debt instruments    
Notes payable, fair value 12.6us-gaap_NotesPayableFairValueDisclosure
/ us-gaap_DebtInstrumentAxis
= biib_NotesPayableToFumedicaMember		 17.5us-gaap_NotesPayableFairValueDisclosure
/ us-gaap_DebtInstrumentAxis
= biib_NotesPayableToFumedicaMember
6.875% Senior Notes due 2018    
Summary of fair and carrying value of debt instruments    
Notes payable, fair value $ 634.6us-gaap_NotesPayableFairValueDisclosure
/ us-gaap_DebtInstrumentAxis
= biib_SeniorNotesSixPointEightSevenFivePercentDueTwentyEighteenMember		 $ 647.9us-gaap_NotesPayableFairValueDisclosure
/ us-gaap_DebtInstrumentAxis
= biib_SeniorNotesSixPointEightSevenFivePercentDueTwentyEighteenMember