Intangible Assets and Goodwill (Details) (USD $)	3 Months Ended		6 Months Ended		12 Months Ended	3 Months Ended			6 Months Ended				6 Months Ended					6 Months Ended				6 Months Ended				6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013	Dec. 31, 2013	Mar. 31, 2014 Out-licensed patents	Jun. 30, 2014 Out-licensed patents	Dec. 31, 2013 Out-licensed patents	Jun. 30, 2014 Out-licensed patents Minimum	Jun. 30, 2014 Out-licensed patents Maximum	Jun. 30, 2014 Developed technology	Dec. 31, 2013 Developed technology	Jun. 30, 2014 Developed technology Minimum	Jun. 30, 2014 Developed technology Maximum	Jun. 30, 2014 Developed technology AVONEX	Jun. 30, 2014 Acquired and in-licensed rights and patents	Dec. 31, 2013 Acquired and in-licensed rights and patents	Jun. 30, 2014 Acquired and in-licensed rights and patents Minimum	Jun. 30, 2014 Acquired and in-licensed rights and patents Maximum	Jun. 30, 2014 In-process research and development	Dec. 31, 2013 In-process research and development	Jun. 30, 2014 Trademarks and trade names	Dec. 31, 2013 Trademarks and trade names	Apr. 02, 2013 TYSABRI product	Jun. 30, 2014 TYSABRI product Acquired and in-licensed rights and patents	Jun. 30, 2014 ALPROLIX product	Jun. 30, 2014 ALPROLIX product Acquired and in-licensed rights and patents	Jun. 30, 2014 Fumapharm AG [Member] TECFIDERA [Member]	Jun. 30, 2014 Two billion [Member] Fumapharm AG [Member]
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				$ 3,271,500,000	$ 3,240,000,000
Cost and Net																				330,900,000	327,400,000	64,000,000	64,000,000
Total intangible assets, gross	7,215,000,000		7,215,000,000		7,214,700,000
Accumulated Amortization	(2,965,600,000)		(2,965,600,000)		(2,740,000,000)		(465,900,000)	(450,800,000)			(2,283,700,000)	(2,165,400,000)				(216,000,000)	(123,800,000)			0	0	0	0
Net							77,400,000	127,200,000			721,600,000	839,900,000			711,900,000	3,055,500,000	3,116,200,000								2,995,100,000
Intangible assets, net	4,249,378,000		4,249,378,000		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
Income tax benefit (expense)	268,521,000	159,140,000	446,935,000	224,648,000																								18,100,000
Intangible Assets and Goodwill (Textual) [Abstract]
Amortization of acquired intangible assets	116,826,000	82,225,000	260,084,000	133,526,000
Increase to goodwill			131,900,000		35,700,000																							150,000,000
Cumulative sales level																													2,000,000,000
Accumulated impairment losses related to goodwill	0		0
Intangible Assets And Goodwill (Additional Textual) [Abstract]
Expected future amortization expense, 2014 (remaining six months)	217,200,000		217,200,000
Expected future amortization expense, 2015	333,400,000		333,400,000
Expected future amortization expense, 2016	318,800,000		318,800,000
Expected future amortization expense, 2017	322,000,000		322,000,000
Expected future amortization expense, 2018	323,100,000		323,100,000
Expected future amortization expense, 2019	302,900,000		302,900,000
Total	$ 1,817,400,000		$ 1,817,400,000