Intangible Assets and Goodwill (Details) (USD $)	3 Months Ended		6 Months Ended			6 Months Ended		6 Months Ended		6 Months Ended		6 Months Ended		6 Months Ended		6 Months Ended		6 Months Ended			3 Months Ended
	Jun. 30, 2011	Jun. 30, 2010	Jun. 30, 2011	Jun. 30, 2010	Dec. 31, 2010	Jun. 30, 2011 In-Process Research and Development [Member]	Dec. 31, 2010 In-Process Research and Development [Member]	Jun. 30, 2011 Trademarks and tradenames [Member]	Dec. 31, 2010 Trademarks and tradenames [Member]	Jun. 30, 2011 Out-licensed patents [Member]	Dec. 31, 2010 Out-licensed patents [Member]	Jun. 30, 2011 Core developed technology [Member]	Dec. 31, 2010 Core developed technology [Member]	Jun. 30, 2011 In-licensed patents [Member]	Dec. 31, 2010 In-licensed patents [Member]	Jun. 30, 2011 Assembled workforce [Member]	Dec. 31, 2010 Assembled workforce [Member]	Jun. 30, 2011 Distribution rights [Member]	Dec. 31, 2010 Distribution rights [Member]	Jun. 30, 2011 Avonex [Member]	Mar. 31, 2011 VP1 Protein [Member]	Jun. 30, 2011 VP1 Protein [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)										12				14		4		2
Estimated Life, Minimum (In Years)												15
Estimated Life, Maximum (In Years)												23
Cost										578,000,000	578,000,000	3,005,300,000	3,005,300,000	17,500,000	3,000,000	2,100,000	2,100,000	12,700,000	12,700,000
Total intangible assets, gross	3,790,500,000		3,790,500,000		3,776,000,000																	14,500,000
Accumulated Amortization	(2,111,600,000)		(2,111,600,000)		(2,003,200,000)					(370,900,000)	(350,200,000)	(1,723,400,000)	(1,636,900,000)	(2,500,000)	(1,300,000)	(2,100,000)	(2,100,000)	(12,700,000)	(12,700,000)
Net										207,100,000	227,800,000	1,281,900,000	1,368,400,000	15,000,000	1,700,000	0	0	0	0	1,268,800,000
Intangible assets, net	1,678,867,000		1,678,867,000		1,772,826,000
Intangible Assets and Goodwill Additional (Textuals) [Abstract]
Net book value of the core technology intangible asset related to our AVONEX product										207,100,000	227,800,000	1,281,900,000	1,368,400,000	15,000,000	1,700,000	0	0	0	0	1,268,800,000
Expected usage-based royalties to be earned through 2016 from license agreement																					47,100,000
Intangible asset recognized to reflect the total of upfront and other time-based milestone payments expected to be made	3,790,500,000		3,790,500,000		3,776,000,000																	14,500,000
Intangible Assets and Goodwill (Textuals) [Abstract]
Amortization of acquired intangible assets	55,100,000	53,100,000	108,400,000	102,000,000
Amortization of intangible assets for each of next five years, Minimum			180,000,000
Amortization of intangible assets for each of next five years, Maximum			220,000,000
Accumulated impairment loss on goodwill	$ 0		$ 0