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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