Intangible Assets and Goodwill (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Feb. 12, 2015
Finite-Lived Intangible Assets [Line Items]
Goodwill	$ 2,663.8	$ 1,760.2	$ 1,232.9
Intangible assets
Total intangible assets, gross	7,646.3	7,207.1
Accumulated Amortization	(3,561.2)	(3,178.6)
Intangible assets, net	4,085.1	4,028.5
Intangible Assets and Goodwill (Additional Textual)
Expected future amortization expense, 2016	346.4
Expected future amortization expense, 2017	318.6
Expected future amortization expense, 2018	291.0
Expected future amortization expense, 2019	275.1
Expected future amortization expense, 2020	269.1
FiniteLivedIntangibleAssetsFutureAmortization	1,500.2
Amortization of acquired intangible assets	382.6	489.8	$ 342.9
Out-licensed patents
Intangible assets
Cost	543.3	543.3
Accumulated Amortization	(506.0)	(481.7)
Net	$ 37.3	61.6
Intangible Assets and Goodwill (Additional Textual)
Impairment of out-licensed patent		34.7
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	$ 3,005.3	3,005.3
Accumulated Amortization	(2,552.9)	(2,396.8)
Net	$ 452.4	608.5
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	$ 3,303.2	3,280.4
Accumulated Amortization	(502.3)	(300.1)
Net	$ 2,800.9	2,980.3
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)	18 years
In-process research and development
Intangible assets
Indefinite lived intangible assets useful life	Indefinite
Cost and net	$ 730.5	314.1
Accumulated Amortization	$ 0.0	0.0
Intangible Assets and Goodwill (Additional Textual)
Impairment of in-process research and development asset		16.2
Trademarks and trade names
Intangible assets
Indefinite lived intangible assets useful life	Indefinite
Cost and net	$ 64.0	64.0
Accumulated Amortization	0.0	$ 0.0
AVONEX | Developed technology
Intangible assets
Net	443.9
TYSABRI product | Acquired and in-licensed rights and patents
Intangible assets
Net	$ 2,742.9
Convergence Pharmaceuticals
Finite-Lived Intangible Assets [Line Items]
Goodwill				$ 128.3
Intangible Assets and Goodwill (Additional Textual)
In-process research and development				$ 424.6