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Fair Value Measurements (Details 2) (USD $) In Millions, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Summary of fair and carrying value of debt instruments
Notes payable, carrying value	$ 583.9us-gaap_NotesPayable	$ 585.2us-gaap_NotesPayable
Debt instruments, fair value	646.8us-gaap_DebtInstrumentFairValue	647.2us-gaap_DebtInstrumentFairValue
Notes Payable to Fumedica
Summary of fair and carrying value of debt instruments
Notes payable, fair value	12.9us-gaap_NotesPayableFairValueDisclosure / us-gaap_DebtInstrumentAxis = biib_NotesPayableToFumedicaMember	12.6us-gaap_NotesPayableFairValueDisclosure / us-gaap_DebtInstrumentAxis = biib_NotesPayableToFumedicaMember
Notes payable, carrying value	12.1us-gaap_NotesPayable / us-gaap_DebtInstrumentAxis = biib_NotesPayableToFumedicaMember	11.7us-gaap_NotesPayable / us-gaap_DebtInstrumentAxis = biib_NotesPayableToFumedicaMember
6.875% Senior Notes due 2018
Summary of fair and carrying value of debt instruments
Notes payable, fair value	633.9us-gaap_NotesPayableFairValueDisclosure / us-gaap_DebtInstrumentAxis = biib_SeniorNotesSixPointEightSevenFivePercentDueTwentyEighteenMember	634.6us-gaap_NotesPayableFairValueDisclosure / us-gaap_DebtInstrumentAxis = biib_SeniorNotesSixPointEightSevenFivePercentDueTwentyEighteenMember
Notes payable, carrying value	$ 571.8us-gaap_NotesPayable / us-gaap_DebtInstrumentAxis = biib_SeniorNotesSixPointEightSevenFivePercentDueTwentyEighteenMember	$ 573.5us-gaap_NotesPayable / us-gaap_DebtInstrumentAxis = biib_SeniorNotesSixPointEightSevenFivePercentDueTwentyEighteenMember

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Fair value portion of debt instrument payable, including, but not limited to, notes payable and loans payable.

+ References

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-Publisher FASB

-Name Accounting Standards Codification

-Topic 825

-SubTopic 10

-Section 50

-Paragraph 10

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Including the current and noncurrent portions, aggregate carrying amount of all types of notes payable, as of the balance sheet date, with initial maturities beyond one year or beyond the normal operating cycle, if longer.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 942

-SubTopic 210

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Fair value portion of notes payable.

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-Number 210

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