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Reserves for Discounts and Allowances (Details 1) (USD $) In Millions, unless otherwise specified | Mar. 31, 2014 | Dec. 31, 2013
--- | --- | ---
Summary of total product revenue reserves included in consolidated balance sheets |  |
Reduction of accounts receivable | $ 134.5 | $ 151.4
Component of accrued expenses and other | 330.5 | 264.9
Total reserves | $ 465.0 | $ 416.3

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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
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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_SummaryOfProductRevenueReservesIncludedInConsolidatedBalanceSheetsAbstract
Namespace Prefix: | biib_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration