Intangible Assets and Goodwill (Details) (USD $)	3 Months Ended		9 Months Ended					9 Months Ended				9 Months Ended					9 Months Ended		3 Months Ended	9 Months Ended		9 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012	Dec. 31, 2012	Sep. 30, 2013 Out-licensed patents [Member]	Dec. 31, 2012 Out-licensed patents [Member]	Sep. 30, 2013 Out-licensed patents [Member] Maximum [Member]	Sep. 30, 2013 Out-licensed patents [Member] Minimum [Member]	Sep. 30, 2013 AVONEX core developed technology [Member]	Dec. 31, 2012 AVONEX core developed technology [Member]	Sep. 30, 2013 AVONEX core developed technology [Member] Maximum [Member]	Sep. 30, 2013 AVONEX core developed technology [Member] Minimum [Member]	Sep. 30, 2013 AVONEX core developed technology [Member] AVONEX [Member]	Sep. 30, 2013 Acquired and in-licensed rights and patents [Member]	Dec. 31, 2012 Acquired and in-licensed rights and patents [Member]	Sep. 30, 2013 Acquired and in-licensed rights and patents [Member] Maximum [Member]	Sep. 30, 2013 Acquired and in-licensed rights and patents [Member] Minimum [Member]	Sep. 30, 2013 In-process research and development [Member]	Sep. 30, 2013 In-process research and development [Member]	Dec. 31, 2012 In-process research and development [Member]	Sep. 30, 2013 Trademarks and trade names [Member]	Dec. 31, 2012 Trademarks and trade names [Member]	Sep. 30, 2013 TYSABRI product [Member]	Apr. 02, 2013 TYSABRI product [Member]
Intangible assets
Estimated Life, (In Years)								23 years	13 years			23 years	15 years				17 years	6 years		15 years				17 years
Indefinite Lived Intangible Assets Useful Life																						Indefinite
Cost						$ 578,000,000	$ 578,000,000			$ 3,005,300,000	$ 3,005,300,000				$ 3,236,100,000	$ 53,700,000
Cost and Net																			327,400,000	327,400,000	330,100,000	64,000,000	64,000,000
Total intangible assets, gross	7,210,800,000		7,210,800,000		4,031,100,000
Accumulated Amortization	(2,630,600,000)		(2,630,600,000)		(2,399,600,000)	(443,300,000)	(421,000,000)			(2,107,000,000)	(1,965,700,000)				(80,300,000)	(12,900,000)			0	0	0	0	0
Net						134,700,000	157,000,000			898,300,000	1,039,600,000			887,900,000	3,155,800,000	40,800,000
Intangible assets, net	4,580,199,000		4,580,199,000		1,631,547,000
Write down in carrying value of in-process research and development asset																			2,600,000
Intangible Assets and Goodwill (Additional Textual) [Abstract]
Expected future amortization expense, 2013	113,200,000		113,200,000
Expected future amortization expense, 2014	418,900,000		418,900,000
Expected future amortization expense, 2015	337,400,000		337,400,000
Expected future amortization expense, 2016	323,400,000		323,400,000
Expected future amortization expense, 2017	328,100,000		328,100,000
Expected futured amortization expense, 2018	330,700,000		330,700,000
FiniteLivedIntangibleAssetsFutureAmortization	1,851,700,000		1,851,700,000
Intangible Assets and Goodwill (Textual) [Abstract]
Amortization of acquired intangible assets	99,998,000	53,013,000	233,524,000	151,256,000
Net intangible asset																									3,178,300,000
Accumulated impairment losses related to goodwill	$ 0		$ 0