Consolidated Statements of Comprehensive Income (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Statement of Comprehensive Income [Abstract]
Net income attributable to Biogen Idec Inc.	$ 1,862,341	$ 1,380,033	$ 1,234,428
Other comprehensive income:
Unrealized gains (losses) recognized during the period, net of tax of $6,394, $2,940 and $133	11,770	5,080	(224)
Less: reclassification adjustment for (gains) losses included in net income, net of tax of $5,576, $486 and $7,155	(10,355)	(903)	(12,184)
Unrealized gains (losses) on securities available for sale, net of tax of $818, $2,454 and $7,288	1,415	4,177	(12,408)
Unrealized gains (losses) recognized during the period, net of tax of $1,721, $1,396 and $3,647	(26,679)	(11,808)	32,830
Less: reclassification adjustment for (gains) losses included in net income, net of tax of $533, $3,360 and $1,268	13,716	(31,713)	9,767
Unrealized gains (losses) on foreign currency forward contracts, net of tax of $1,187, $4,756 and $4,915	(12,963)	(43,521)	42,597
Unrealized gains (losses) on pension benefit obligation	2,096	(12,656)	(9,280)
Currency translation adjustment	37,012	23,230	(25,834)
Total other comprehensive income (loss), net of tax	27,560	(28,770)	(4,925)
Comprehensive income attributable to Biogen Idec Inc.	1,889,901	1,351,263	1,229,503
Comprehensive income attributable to noncontrolling interests, net of tax	0	65	37,161
Comprehensive income	$ 1,889,901	$ 1,351,328	$ 1,266,664