Consolidated Statements of Comprehensive Income (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Statement of Comprehensive Income [Abstract]
Net income attributable to Biogen Idec Inc.	$ 2,934,784	$ 1,862,341	$ 1,380,033
Other comprehensive income:
Unrealized gains (losses) recognized during the period, net of tax of $80, $6,394 and $2,940	411	11,770	5,080
Less: reclassification adjustment for (gains) losses included in net income, net of tax of $3,462, $5,576 and $486	(6,429)	(10,355)	(903)
Unrealized gains (losses) on securities available for sale, net of tax of $3,542, $818 and $2,454	(6,018)	1,415	4,177
Unrealized gains (losses) recognized during the period, net of tax of $760, $1,721 and $1,396	101,792	(26,679)	(11,808)
Less: reclassification adjustment for (gains) losses included in net income, net of tax of $500, $533 and $3,360	(6,349)	13,716	(31,713)
Unrealized gains (losses) on foreign currency forward contracts, net of tax of $260, $1,187 and $4,756	95,443	(12,963)	(43,521)
Unrealized gains (losses) on pension benefit obligation	(11,950)	2,096	(12,656)
Currency translation adjustment	(109,218)	37,012	23,230
Total other comprehensive income (loss), net of tax	(31,743)	27,560	(28,770)
Comprehensive income attributable to Biogen Idec Inc.	2,903,041	1,889,901	1,351,263
Comprehensive income attributable to noncontrolling interests, net of tax	6,781	0	65
Comprehensive income	$ 2,909,822	$ 1,889,901	$ 1,351,328