Comprehensive Income (Details) (USD $)	3 Months Ended		6 Months Ended
	Jun. 30, 2011	Jun. 30, 2010	Jun. 30, 2011	Jun. 30, 2010	Mar. 31, 2011	Dec. 31, 2010	Mar. 31, 2010	Dec. 31, 2009
Comprehensive income:
Net income	$ 304,049,000	$ 294,649,000	$ 612,814,000	$ 514,642,000
Unrealized gains (losses) on securities available for sale, net of tax of $0.6 and $3.8 for the three months ended and $6.3 and $5.5 for the six moths ended for June 2011 and 2010 respectively	1,100,000	(6,500,000)	(10,700,000)	(9,400,000)
Unrealized gains (losses) on foreign currency forward contracts, net of tax of $0.8 and $3.6 for the three months ended and $1.2 and $6.5 for the six moths ended for June 2011 and 2010 respectively	6,300,000	26,500,000	(11,100,000)	54,300,000
Unrealized gains (losses) on pension benefit obligation, net of tax of $0 and $0 for the three months ended and $0 and $0 for the six moths ended for June 2011 and 2010 respectively		(200,000)		(300,000)
Currency translation adjustment	22,200,000	(75,200,000)	74,800,000	(130,200,000)
Comprehensive income (loss)	333,600,000	239,200,000	665,800,000	429,100,000
Stockholders' Equity attributable to Noncontrolling interest- roll forward
Net income attributable to noncontrolling interests	16,015,000	1,211,000	30,450,000	3,762,000
Translation adjustments	2,900,000	(3,400,000)	5,700,000	(6,000,000)
Distributions to noncontrolling interests	(9,900,000)		(9,900,000)
Capital contributions from noncontrolling interest		1,400,000		2,200,000
Noncontrolling interests, end of period	79,144,000	40,400,000	79,144,000	40,400,000	70,100,000	52,937,000	41,200,000	40,400,000
Biogen Idec Shareholders' Equity [Member]
Comprehensive income:
Net income	288,000,000	293,400,000	582,400,000	510,900,000
Unrealized gains (losses) on securities available for sale, net of tax of $0.6 and $3.8 for the three months ended and $6.3 and $5.5 for the six moths ended for June 2011 and 2010 respectively	1,100,000	(6,500,000)	(10,700,000)	(9,400,000)
Unrealized gains (losses) on foreign currency forward contracts, net of tax of $0.8 and $3.6 for the three months ended and $1.2 and $6.5 for the six moths ended for June 2011 and 2010 respectively	6,300,000	26,500,000	(11,100,000)	54,300,000
Unrealized gains (losses) on pension benefit obligation, net of tax of $0 and $0 for the three months ended and $0 and $0 for the six moths ended for June 2011 and 2010 respectively		(200,000)		(300,000)
Currency translation adjustment	19,300,000	(71,800,000)	69,100,000	(124,200,000)
Comprehensive income (loss)	314,700,000	241,400,000	629,700,000	431,300,000
Noncontrolling Interests [Member]
Comprehensive income:
Net income	16,000,000	1,200,000	30,400,000	3,800,000
Currency translation adjustment	2,900,000	(3,400,000)	5,700,000	(6,000,000)
Comprehensive income (loss)	$ 18,900,000	$ (2,200,000)	$ 36,100,000	$ (2,200,000)