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Revenue Reserves (Details 1) (USD $) In Millions, unless otherwise specified | 12 Months Ended |  |
--- | --- | --- | ---
Dec. 31, 2012 | Dec. 31, 2011 | Dec. 31, 2013 | Dec. 31, 2010
Reserves for Discounts and Allowances [Abstract] |  |  |  |
Revenue reserves reclassification | $ 17.3 | $ 11.7 |  |
Summary of total product revenue reserves included in consolidated balance sheets |  |  |  |
Reduction of accounts receivable | 46.1 |  | 151.4 |
Components of accrued expenses | 191.0 |  | 264.9 |
Total reserves | $ 237.1 | $ 155.6 | $ 416.3 | $ 142.0

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- Definition Period balance related to product revenue reserves. + References No definition available. + Details
Name: | biib_ProductRevenueReserves
Namespace Prefix: | biib_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Definition Product revenue reserves classified as a reduction of accounts receivable. + References No definition available. + Details
Name: | biib_ProductRevenueReservesClassifiedAsReductionOfAccountsReceivable
Namespace Prefix: | biib_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Definition Product revenue reserves classified as a reduction of current liabilities. + References No definition available. + Details
Name: | biib_ProductRevenueReservesClassifiedAsReductionOfCurrentLiabilities
Namespace Prefix: | biib_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Details
Name: | biib_ReservesForDiscountsAndAllowancesAbstract
Namespace Prefix: | biib_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Definition Revenue reserves reclassification from discounts to contractual adjustments + References No definition available. + Details
Name: | biib_Revenuereservesreclassification
Namespace Prefix: | biib_
Data Type: | xbrli:monetaryItemType
Balance Type: | na
Period Type: | duration

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- Details
Name: | biib_SummaryOfProductRevenueReservesIncludedInConsolidatedBalanceSheetsAbstract
Namespace Prefix: | biib_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration