Condensed Consolidated Statements of Comprehensive Income (Unaudited) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Statement of Comprehensive Income [Abstract]
Net income	$ 487,616	$ 398,400	$ 1,405,035	$ 1,087,906
Other comprehensive income:
Unrealized gains on securities available for sale, net of tax of $3,639 and $883 for the three months ended September 30, 2013 and 2012, respectively; and $6,554 and $1,958 for the nine months ended September 30, 2013 and 2012, respectively	6,211	1,503	11,171	3,331
Unrealized losses on foreign currency forward contracts, net of tax of $298 and $3,140 for the three months ended September 30, 2013 and 2012, respectively; and $1,182 and $3,118 for the nine months ended September 30, 2013 and 2012, respectively	(14,847)	(27,354)	(5,549)	(27,457)
Unrealized gains on pension benefit obligation	892	198	3,167	590
Currency translation adjustment	33,564	25,093	17,201	(980)
Total other comprehensive income (loss), net of tax	25,820	(560)	25,990	(24,516)
Comprehensive income	513,436	397,840	1,431,025	1,063,390
Comprehensive income attributable to noncontrolling interests, net of tax	0	0	0	65
Comprehensive income attributable to Biogen Idec Inc	$ 513,436	$ 397,840	$ 1,431,025	$ 1,063,325