Intangible Assets and Goodwill (Details) (USD $)	12 Months Ended					12 Months Ended				12 Months Ended				12 Months Ended				12 Months Ended				12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2013 Out-licensed patents	Dec. 31, 2012 Out-licensed patents	Dec. 31, 2013 Out-licensed patents Maximum	Dec. 31, 2013 Out-licensed patents Minimum	Dec. 31, 2013 AVONEX developed technology	Dec. 31, 2012 AVONEX developed technology	Dec. 31, 2013 AVONEX developed technology Maximum	Dec. 31, 2013 AVONEX developed technology Minimum	Dec. 31, 2013 Acquired and in-licensed rights and patents	Dec. 31, 2012 Acquired and in-licensed rights and patents	Dec. 31, 2013 Acquired and in-licensed rights and patents Maximum	Dec. 31, 2013 Acquired and in-licensed rights and patents Minimum	Dec. 31, 2013 In-process research and development	Dec. 31, 2012 In-process research and development	Dec. 31, 2013 Trademarks and trade names	Dec. 31, 2012 Trademarks and trade names	Dec. 31, 2013 AVONEX AVONEX developed technology	Mar. 08, 2012 Stromedix	Dec. 31, 2013 TYSABRI product	Apr. 02, 2013 TYSABRI product
Intangible assets
Estimated life, (In Years)						23 years	13 years			23 years	15 years			17 years	6 years							17 years
Indefinite lived intangible assets useful life																Indefinite		Indefinite
Cost				$ 578,000,000	$ 578,000,000			$ 3,005,300,000	$ 3,005,300,000			$ 3,240,000,000	$ 53,700,000
Cost and net																327,400,000	330,100,000	64,000,000	64,000,000
Total intangible assets, gross	7,214,700,000	4,031,100,000
Accumulated Amortization	(2,740,000,000)	(2,399,600,000)		(450,800,000)	(421,000,000)			(2,165,400,000)	(1,965,700,000)			(123,800,000)	(12,900,000)			0	0	0	0
Net				127,200,000	157,000,000			839,900,000	1,039,600,000			3,116,200,000	40,800,000							829,700,000
Intangible assets, net	4,474,653,000	1,631,547,000
Intangible Assets and Goodwill (Additional Textual)
Expected future amortization expense, 2014	426,300,000
Expected future amortization expense, 2015	336,400,000
Expected future amortization expense, 2016	322,700,000
Expected future amortization expense, 2017	327,500,000
Expected future amortization expense, 2018	330,200,000
FiniteLivedIntangibleAssetsFutureAmortization	1,743,100,000
Amortization of acquired intangible assets	342,948,000	202,204,000	208,566,000
Impairment of intangible asset																2,600,000
Total purchase price allocated to IPR&D																					219,200,000
Net intangible asset																							$ 3,178,300,000