Condensed Consolidated Statements of Comprehensive Income (Unaudited) (USD $) In Thousands, unless otherwise specified	3 Months Ended
Mar. 31, 2013	Mar. 31, 2012
Statement of Other Comprehensive Income [Abstract]
Net income	$ 426,749	$ 302,366
Other comprehensive income:
Unrealized gains (losses) on securities available for sale, net of tax of $654 and $1,122	(1,117)	1,911
Unrealized gains (losses) on foreign currency forward contracts, net of tax of $1,421 and $2,143	11,603	(18,363)
Unrealized gains (losses) on pension benefit obligation	1,263	189
Currency translation adjustment	(24,419)	25,154
Total other comprehensive income (loss), net of tax	(12,670)	8,891
Comprehensive income	414,079	311,257
Comprehensive loss attributable to noncontrolling interests, net of tax	0	(230)
Comprehensive income attributable to Biogen Idec Inc	$ 414,079	$ 311,487

The change in equity [net assets] of a business enterprise during a period from transactions and other events and circumstances from non-owner sources which are attributable to the reporting entity. It includes all changes in equity during a period except those resulting from investments by owners and distributions to owners, but excludes any and all transactions which are directly or indirectly attributable to that ownership interest in subsidiary equity which is not attributable to the parent.

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