Condensed Consolidated Statements of Comprehensive Income (Unaudited) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
Statement of Other Comprehensive Income [Abstract]
Net income	$ 398,400	$ 353,662	$ 1,087,906	$ 966,476
Other comprehensive income:
Unrealized gains (losses) on securities available for sale, net of tax of $883 and $794 for the three months ended September 30, 2012 and 2011, respectively; and $1,958 and $7,101 for the nine months ended September 30, 2012 and 2011, respectively	1,503	(1,353)	3,331	(12,092)
Unrealized gains (losses) on foreign currency forward contracts, net of tax of $3,140 and $3,848 for the three months ended September 30, 2012 and 2011, respectively; and $3,118 and $2,634 for the nine months ended September 30, 2012 and 2011, respectively	(27,354)	32,921	(27,457)	21,870
Unrealized gains (losses) on pension benefit obligation	198	(11)	590	5
Currency translation adjustment	25,093	(50,505)	(980)	24,279
Total other comprehensive income, net of tax	(560)	(18,948)	(24,516)	34,062
Comprehensive income	397,840	334,714	1,063,390	1,000,538
Comprehensive income attributable to noncontrolling interests, net of tax	0	1,030	65	37,167
Comprehensive income attributable to Biogen Idec Inc	$ 397,840	$ 333,684	$ 1,063,325	$ 963,371