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v2.4.1.9

Litigation (Details) (Italy) | 3 Months Ended
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Dec. 31, 2011
Italy |
Loss Contingencies [Line Items] |
Reimbursement limit stated in the resolution | 24 months

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- Definition Reimbursement limit stated in the resolution. + References No definition available. + Details
Name: | biib_ReimbursementLimitStatedInResolution
Namespace Prefix: | biib_
Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | duration

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- Details
Name: | us-gaap_LossContingenciesLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration