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v2.4.0.8

Litigation (Details) (Italy) | 3 Months Ended
--- | ---
Dec. 31, 2011
Italy |
Loss Contingencies [Line Items] |
Limited period for government reimbursement | 24 months

X
- Definition Limited period for government reimbursement. + References No definition available. + Details
Name: | biib_LimitedPeriodForGovernmentReimbursement
Namespace Prefix: | biib_
Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | duration

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