Condensed Consolidated Statements of Comprehensive Income (Unaudited) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Statement of Comprehensive Income [Abstract]
Net income	$ 490,670	$ 387,141	$ 917,419	$ 689,507
Other comprehensive income:
Unrealized gains (losses) on securities available for sale, net of tax of $3,569 and $47 for the three months ended June 30, 2013 and 2012, respectively; and $2,915 and $1,075 for the six months ended June 30, 2013 and 2012, respectively	6,077	(83)	4,960	1,828
Unrealized gains (losses) on foreign currency forward contracts, net of tax of $59 and $2,164 for the three months ended June 30, 2013 and 2012, respectively; and $1,480 and $22 for the six months ended June 30, 2013 and 2012, respectively	(2,305)	18,260	9,298	(103)
Unrealized gains on pension benefit obligation	1,011	202	2,274	391
Currency translation adjustment	8,056	(51,226)	(16,363)	(26,073)
Total other comprehensive income (loss), net of tax	12,839	(32,847)	169	(23,957)
Comprehensive income	503,509	354,294	917,588	665,550
Comprehensive income attributable to noncontrolling interests, net of tax	0	295	0	65
Comprehensive income attributable to Biogen Idec Inc	$ 503,509	$ 353,999	$ 917,588	$ 665,485