Intangible Assets and Goodwill (Details) (USD $)	3 Months Ended			3 Months Ended		3 Months Ended				3 Months Ended					3 Months Ended				3 Months Ended				3 Months Ended
	Mar. 31, 2014	Mar. 31, 2013	Dec. 31, 2013	Mar. 31, 2014 Out-licensed patents	Dec. 31, 2013 Out-licensed patents	Mar. 31, 2014 Out-licensed patents Minimum	Mar. 31, 2014 Out-licensed patents Maximum	Mar. 31, 2014 Developed technology	Dec. 31, 2013 Developed technology	Mar. 31, 2014 Developed technology Minimum	Mar. 31, 2014 Developed technology Maximum	Mar. 31, 2014 Developed technology AVONEX	Mar. 31, 2014 Acquired and in-licensed rights and patents	Dec. 31, 2013 Acquired and in-licensed rights and patents	Mar. 31, 2014 Acquired and in-licensed rights and patents Minimum	Mar. 31, 2014 Acquired and in-licensed rights and patents Maximum	Mar. 31, 2014 In-process research and development	Dec. 31, 2013 In-process research and development	Mar. 31, 2014 Trademarks and trade names	Dec. 31, 2013 Trademarks and trade names	Mar. 31, 2014 TYSABRI product	Apr. 02, 2013 TYSABRI product	Mar. 31, 2014 ALPROLIX product	Mar. 31, 2014 ALPROLIX product Acquired and in-licensed rights and patents
Intangible assets
Estimated Life, (In Years)						13 years	23 years			15 years	23 years				6 years	17 years
Indefinite Lived Intangible Assets Useful Life																	Indefinite until commercialization		Indefinite
Cost				$ 543,300,000	$ 578,000,000			$ 3,005,300,000	$ 3,005,300,000				$ 3,269,600,000	$ 3,240,000,000
Cost and Net																	330,900,000	327,400,000	64,000,000	64,000,000
Total intangible assets, gross	7,213,100,000		7,214,700,000
Accumulated Amortization	(2,848,800,000)		(2,740,000,000)	(458,300,000)	(450,800,000)			(2,224,300,000)	(2,165,400,000)				(166,200,000)	(123,800,000)			0	0	0	0
Net				85,000,000	127,200,000			781,000,000	839,900,000			771,100,000	3,103,400,000	3,116,200,000							3,041,000,000
Intangible assets, net	4,364,384,000		4,474,653,000
Intangible asset impairment charges				34,700,000
Net intangible asset																						3,178,300,000
Contingent payment																							20,000,000
Increase in in-licensed rights and patents																								7,800,000
Intangible Assets and Goodwill (Textual) [Abstract]
Amortization of acquired intangible assets	143,258,000	51,301,000
Accumulated impairment losses related to goodwill	0
Intangible Assets And Goodwill (Additional Textual) [Abstract]
Expected future amortization expense, 2014 (remaining nine months)	316,300,000
Expected future amortization expense, 2015	334,700,000
Expected future amortization expense, 2016	319,900,000
Expected future amortization expense, 2017	323,200,000
Expected future amortization expense, 2018	324,300,000
Expected future amortization expense, 2019	304,100,000
Total	$ 1,922,500,000