Condensed Consolidated Statements of Comprehensive Income (Unaudited) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
Statement of Comprehensive Income [Abstract]
Net income attributable to Biogen Idec Inc.	$ 714,506	$ 490,670	$ 1,194,462	$ 917,419
Other comprehensive income:
Unrealized gains (losses) on securities available for sale, net of tax of $4,029 and $3,569 for the three months ended June 30, 2014 and 2013, respectively; and $3,015 and $2,915 for the six months ended June 30, 2014 and 2013, respectively	(6,865)	6,077	(5,140)	4,960
Unrealized gains (losses) on foreign currency forward contracts, net of tax of $260 and $59 for the three months ended June 30, 2014 and 2013, respectively; and $5 and $1,480 for the six months ended June 30, 2014 and 2013, respectively	10,760	(2,305)	16,551	9,298
Unrealized gains (losses) on pension benefit obligation	(170)	1,011	646	2,274
Currency translation adjustment	(8,047)	8,056	(10,991)	(16,363)
Total other comprehensive income (loss), net of tax	(4,322)	12,839	1,066	169
Comprehensive income attributable to Biogen Idec Inc.	710,184	503,509	1,195,528	917,588
Comprehensive income attributable to noncontrolling interests, net of tax	8,626	0	8,398	0
Comprehensive income	$ 718,810	$ 503,509	$ 1,203,926	$ 917,588