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Litigation (Details) (ITALY) | 3 Months Ended
--- | ---
Jun. 30, 2014 | Dec. 31, 2011
ITALY |  |
Loss Contingencies [Line Items] |  |
Limited period for government reimbursement |  | 24 months
Automatic renewal period for the resolution |  | 24 months
Term of agreement to eliminate reimbursement limit | 24 months |

X
- Definition Automatic renewal period for the resolution. + References No definition available. + Details
Name: | biib_AutomaticRenewalPeriodForResolution
Namespace Prefix: | biib_
Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | duration

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
- Definition Term Of Agreement To Eliminate Reimbursement Limit + References No definition available. + Details
Name: | biib_TermOfAgreementToEliminateReimbursementLimit
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