Intangible Assets and Goodwill (Details) (USD $)	3 Months Ended
	Mar. 31, 2012	Mar. 31, 2011	Dec. 31, 2011
Intangible assets
Total intangible assets, gross	$ 4,029,300,000		$ 3,807,500,000
Accumulated Amortization	(2,245,300,000)		(2,199,300,000)
Intangible assets, net	1,783,981,000		1,608,191,000
Intangible Assets and Goodwill Additional (Textual) [Abstract]
Amortization of intangible assets for each of next five years, Minimum	130,000,000
Amortization of intangible assets for each of next five years, Maximum	200,000,000
Intangible asset recognized to reflect the total of upfront and other time-based milestone payments expected to be made	4,029,300,000		3,807,500,000
Intangible Assets and Goodwill (Textual) [Abstract]
Amortization of acquired intangible assets	46,000,000	53,300,000
Business acquisition purchase price allocation to IPR&D	219,200,000
Business acquisition purchase price allocation goodwill amount	47,000,000
Accumulated impairment losses related to goodwill	0
VP1 Protein [Member]
Intangible assets
Total intangible assets, gross	21,800,000		19,200,000
Intangible Assets and Goodwill Additional (Textual) [Abstract]
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	21,800,000		19,200,000
AVONEX [Member]
Intangible assets
Net	1,156,600,000
Intangible Assets and Goodwill Additional (Textual) [Abstract]
Net book value of the core technology intangible asset related to our AVONEX product	1,156,600,000
Amortization of intangible assets for each of next five years, Minimum	100,000,000
Amortization of intangible assets for each of next five years, Maximum	160,000,000
Out-licensed patents [Member]
Intangible assets
Estimated Life, Minimum (In Years)	13
Estimated Life, Maximum (In Years)	23
Cost	578,000,000		578,000,000
Accumulated Amortization	(398,700,000)		(391,300,000)
Net	179,300,000		186,700,000
Intangible Assets and Goodwill Additional (Textual) [Abstract]
Net book value of the core technology intangible asset related to our AVONEX product	179,300,000		186,700,000
Core developed technology [Member]
Intangible assets
Estimated Life, Minimum (In Years)	15
Estimated Life, Maximum (In Years)	23
Cost	3,005,300,000		3,005,300,000
Accumulated Amortization	(1,837,000,000)		(1,801,100,000)
Net	1,168,300,000		1,204,200,000
Intangible Assets and Goodwill Additional (Textual) [Abstract]
Net book value of the core technology intangible asset related to our AVONEX product	1,168,300,000		1,204,200,000
In-licensed rights and patents [Member]
Intangible assets
Estimated Life, Minimum (In Years)	6
Estimated Life, Maximum (In Years)	16
Cost	49,800,000		47,200,000
Accumulated Amortization	(7,500,000)		(4,800,000)
Net	42,300,000		42,400,000
Intangible Assets and Goodwill Additional (Textual) [Abstract]
Net book value of the core technology intangible asset related to our AVONEX product	42,300,000		42,400,000
Assembled workforce [Member]
Intangible assets
Estimated Life (In Years)	4
Cost	2,100,000		2,100,000
Accumulated Amortization	(2,100,000)		(2,100,000)
In-process research and development [Member]
Indefinite-lived Intangible Assets by Major Class [Line Items]
Estimated Life	Up to 15 years upon commercialization
Cost and Net	330,100,000		110,900,000
Trademarks and trade names [Member]
Indefinite-lived Intangible Assets by Major Class [Line Items]
Estimated Life	Indefinite
Cost and Net	$ 64,000,000		$ 64,000,000