Intangible Assets and Goodwill (Details) (USD $)	12 Months Ended					3 Months Ended	12 Months Ended					12 Months Ended				12 Months Ended		12 Months Ended		12 Months Ended				12 Months Ended		12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009	Dec. 31, 2011 Biogen Idec International Neuroscience GmbH [Member]	Dec. 31, 2010 Biogen Idec International Neuroscience GmbH [Member]	Sep. 30, 2011 Acorda [Member]	Dec. 31, 2011 Acorda [Member]	Dec. 31, 2011 VP1 Protein [Member]	Dec. 31, 2011 AVONEX [Member]	Dec. 31, 2011 Out-licensed patents [Member] Year	Dec. 31, 2010 Out-licensed patents [Member]	Dec. 31, 2011 Core developed technology [Member] Year	Dec. 31, 2010 Core developed technology [Member]	Dec. 31, 2011 In-Process Research and Development [Member]	Dec. 31, 2010 In-Process Research and Development [Member]	Dec. 31, 2011 In-licensed rights and patents [Member] Year	Dec. 31, 2010 In-licensed rights and patents [Member]	Dec. 31, 2011 Assembled workforce [Member] Year	Dec. 31, 2010 Assembled workforce [Member]	Dec. 31, 2011 Distribution rights [Member] Year	Dec. 31, 2010 Distribution rights [Member]	Dec. 31, 2011 Trademarks and trade names [Member]	Dec. 31, 2010 Trademarks and trade names [Member]	Dec. 31, 2011 In-Process Research and Development [Member]	Dec. 31, 2010 In-Process Research and Development [Member]	Dec. 31, 2011 Trademarks and trade names [Member]	Dec. 31, 2010 Trademarks and trade names [Member]
Indefinite-lived Intangible Assets by Major Class [Line Items]
Estimated Life																								Up to 15 years upon commercialization		Indefinite
Cost and Net																								$ 110,900,000	$ 110,900,000	$ 64,000,000	$ 64,000,000
Intangible assets
Estimated Life (In Years)																		4		2
Estimated Life, Minimum (In Years)										12		15				6
Estimated Life, Maximum (In Years)										23		23				16
Cost										578,000,000	578,000,000	3,005,300,000	3,005,300,000			47,200,000	3,000,000	2,100,000	2,100,000	12,700,000	12,700,000
Total intangible assets, gross	3,820,200,000	3,776,000,000						19,200,000
Accumulated Amortization	(2,212,000,000)	(2,003,200,000)								(391,300,000)	(350,200,000)	(1,801,100,000)	(1,636,900,000)	0	0	(4,800,000)	(1,300,000)	(2,100,000)	(2,100,000)	(12,700,000)	(12,700,000)	0	0
Net	0								1,192,100,000	186,700,000	227,800,000	1,204,200,000	1,368,400,000			42,400,000	1,700,000		0		0
Intangible assets, net	1,608,191,000	1,772,826,000
Intangible Assets and Goodwill Additional (Textual) [Abstract]
Net book value of the core technology intangible asset related to our AVONEX product	0								1,192,100,000	186,700,000	227,800,000	1,204,200,000	1,368,400,000			42,400,000	1,700,000		0		0
Amortization of intangible assets for each of next five years, Minimum	160,000,000								100,000,000
Amortization of intangible assets for each of next five years, Maximum	200,000,000								150,000,000
Expected usage-based royalties to be earned through 2016 from license agreement								58,900,000
Intangible asset recognized to reflect the total of upfront and other time-based milestone payments expected to be made	3,820,200,000	3,776,000,000						19,200,000
Collaborative arrangements and noncollaborative arrangement transactions milestone payment based on sales
Milestone payment payable to Acorda Therapeutics, Inc. (Acorda)				5,000,000		25,000,000
Amount due on next expected milestone payment							15,000,000
Net sales over four consecutive quarters							100,000,000
Business Acquisition [Line Items]
Business acquisition, purchase price allocation, in process research and development					110,900,000
Business acquisition purchase price allocation goodwill amount					25,600,000
Intangible Assets and Goodwill (Textual) [Abstract]
Amortization of acquired intangible assets	208,600,000	208,900,000	289,800,000
Increase (decrease) in goodwill		17,900,000
Business acquisition purchase price allocation goodwill amount					$ 25,600,000