Accumulated Other Comprehensive Income (Loss) (Details Textual) (USD $) In Millions, unless otherwise specified	12 Months Ended
Dec. 31, 2012	Dec. 31, 2011
Accumulated Other Comprehensive Income Loss (Textual)
Tax on unrealized holding gains (losses) on investments	$ 2.5	$ 0.1
Tax on unrealized holding gains (losses) on foreign currency forward contracts	$ 1.1	$ 3.6

Tax effect on gross appreciation or the gross loss in value of the total of unsold foreign currency forward contracts during the period being reported on.

No definition available.

Name:	biib_TaxOnUnrealizedGainsOnForeignCurrencyForwardContracts
Namespace Prefix:	biib_
Data Type:	xbrli:monetaryItemType
Balance Type:	na
Period Type:	duration