Comprehensive Income (Details) (USD $)	3 Months Ended		9 Months Ended
	Sep. 30, 2011	Sep. 30, 2010	Sep. 30, 2011	Sep. 30, 2010
Comprehensive income:
Net income	$ 353,662,000	$ 112,179,000	$ 966,476,000	$ 626,821,000
Unrealized gains (losses) on securities available for sale, net of tax of $0.8 and $3.4 for the three months ended and $7.1 and $2.1 for the nine moths ended for September 2011 and 2010 respectively	(1,400,000)	5,800,000	(12,100,000)	(3,600,000)
Unrealized gains (losses) on foreign currency forward contracts, net of tax of $3.8 and $8.0 for the three months ended and $2.6 and $1.5 for the nine moths ended for September 2011 and 2010 respectively	32,900,000	(71,100,000)	21,900,000	(16,800,000)
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 nine moths ended for September 2011 and 2010 respectively		200,000		(100,000)
Currency translation adjustment	(50,500,000)	88,900,000	24,200,000	(40,500,000)
Comprehensive income (loss)	334,700,000	136,000,000	1,000,500,000	565,800,000
Stockholders' Equity attributable to Noncontrolling interest- roll forward
Noncontrolling interests, beginning of period	79,100,000	40,500,000	52,937,000	40,400,000
Fair value of assets and liabilities acquired and assigned to noncontrolling interest		145,000,000		145,000,000
Net income (loss) attributable to noncontrolling interests	1,836,000	(141,936,000)	32,286,000	(138,174,000)
Translation adjustments	(800,000)	4,500,000	4,900,000	(1,300,000)
Distributions to noncontrolling interests	(14,100,000)	(8,900,000)	(24,000,000)	(8,900,000)
Capital contributions from noncontrolling interest		300,000		2,500,000
Acquisition of noncontrolling interests (Note 2)	(61,700,000)		(61,700,000)
Noncontrolling interests, end of period	4,446,000	39,500,000	4,446,000	39,500,000
Biogen Idec Shareholders' Equity [Member]
Comprehensive income:
Net income	351,800,000	254,100,000	934,200,000	765,000,000
Unrealized gains (losses) on securities available for sale, net of tax of $0.8 and $3.4 for the three months ended and $7.1 and $2.1 for the nine moths ended for September 2011 and 2010 respectively	(1,400,000)	5,800,000	(12,100,000)	(3,600,000)
Unrealized gains (losses) on foreign currency forward contracts, net of tax of $3.8 and $8.0 for the three months ended and $2.6 and $1.5 for the nine moths ended for September 2011 and 2010 respectively	32,900,000	(71,100,000)	21,900,000	(16,800,000)
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 nine moths ended for September 2011 and 2010 respectively		200,000		(100,000)
Currency translation adjustment	(49,700,000)	84,400,000	19,300,000	(39,200,000)
Comprehensive income (loss)	333,600,000	273,400,000	963,300,000	705,300,000
Noncontrolling Interests [Member]
Comprehensive income:
Net income	1,900,000	(141,900,000)	32,300,000	(138,200,000)
Currency translation adjustment	(800,000)	4,500,000	4,900,000	(1,300,000)
Comprehensive income (loss)	$ 1,100,000	$ (137,400,000)	$ 37,200,000	$ (139,500,000)