Intangible Assets and Goodwill (Details) (USD $)	12 Months Ended			3 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Jun. 30, 2013	Apr. 02, 2013
Intangible assets
Total intangible assets, gross	$ 7,207,100,000	$ 7,214,700,000
Accumulated Amortization	(3,178,600,000)	(2,740,000,000)
Intangible assets, net	4,028,507,000	4,474,653,000
Intangible Assets and Goodwill (Additional Textual)
Expected future amortization expense, 2015	344,300,000
Expected future amortization expense, 2016	313,200,000
Expected future amortization expense, 2017	286,300,000
Expected future amortization expense, 2018	283,700,000
Expected future amortization expense, 2019	273,600,000
FiniteLivedIntangibleAssetsFutureAmortization	1,501,100,000
Amortization of acquired intangible assets	489,761,000	342,948,000	202,204,000
Total impairment charges	50,900,000
Changes in fair value	38,893,000	547,000	(27,202,000)
Out-licensed patents
Intangible assets
Cost	543,300,000	578,000,000
Accumulated Amortization	(481,700,000)	(450,800,000)
Net	61,600,000	127,200,000
Intangible Assets and Goodwill (Additional Textual)
Impairment of out-licensed patent	34,700,000
Out-licensed patents | Minimum
Intangible assets
Estimated life, (In Years)	13 years
Out-licensed patents | Maximum
Intangible assets
Estimated life, (In Years)	23 years
Developed technology
Intangible assets
Cost	3,005,300,000	3,005,300,000
Accumulated Amortization	(2,396,800,000)	(2,165,400,000)
Net	608,500,000	839,900,000
Developed technology | Minimum
Intangible assets
Estimated life, (In Years)	15 years
Developed technology | Maximum
Intangible assets
Estimated life, (In Years)	23 years
Acquired and in-licensed rights and patents
Intangible assets
Cost	3,280,400,000	3,240,000,000
Accumulated Amortization	(300,100,000)	(123,800,000)
Net	2,980,300,000	3,116,200,000
Intangible Assets and Goodwill (Additional Textual)
Increase in amortization of acquired intangible assets	60,200,000
Acquired and in-licensed rights and patents | Minimum
Intangible assets
Estimated life, (In Years)	6 years
Acquired and in-licensed rights and patents | Maximum
Intangible assets
Estimated life, (In Years)	17 years
In-process research and development
Intangible assets
Indefinite lived intangible assets useful life	Indefinite
Cost and net	314,100,000	327,400,000
Accumulated Amortization	0	0
Intangible Assets and Goodwill (Additional Textual)
Impairment of in-process research and development asset	16,200,000
Changes in fair value	49,400,000
Trademarks and trade names
Intangible assets
Indefinite lived intangible assets useful life	Indefinite
Cost and net	64,000,000	64,000,000
Accumulated Amortization	0	0
AVONEX | Developed technology
Intangible assets
Net	599,300,000
TYSABRI product | Acquired and in-licensed rights and patents
Intangible assets
Net	2,921,000,000
ALPROLIX
Intangible Assets and Goodwill (Additional Textual)
Contingent payment	20,000,000
ALPROLIX | Acquired and in-licensed rights and patents
Intangible Assets and Goodwill (Additional Textual)
Increase in in-licensed rights and patents	7,800,000
TYSABRI product
Intangible assets
Estimated life, (In Years)				17 years
Intangible Assets and Goodwill (Additional Textual)
Net intangible asset					$ 3,178,300,000