Intangible Assets and Goodwill (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Feb. 12, 2015
Finite-Lived Intangible Assets [Line Items]
Increase to goodwill		$ 1,026.9	$ 908.1
Goodwill	$ 3,669.3	3,669.3	2,663.8	$ 1,760.2
Intangible assets
Total intangible assets, gross	7,742.3	7,742.3	7,646.3
Accumulated Amortization	(3,934.0)	(3,934.0)	(3,561.2)
Intangible assets, net	3,808.3	3,808.3	4,085.1
Intangible Assets and Goodwill (Additional Textual)
Expected future amortization expense, 2017	334.8	334.8
Expected future amortization expense, 2018	312.7	312.7
Expected future amortization expense, 2019	295.2	295.2
Expected future amortization expense, 2020	259.7	259.7
Expected future amortization expense, 2021	242.8	242.8
Amortization of acquired intangible assets		(385.6)	(382.6)	(489.8)
Deferred tax liabilities	376.6	376.6	440.1
Out-licensed patents
Intangible assets
Cost	543.3	543.3	543.3
Accumulated Amortization	(523.6)	(523.6)	(506.0)
Net	19.7	$ 19.7	37.3
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	3,005.3
Accumulated Amortization	(2,634.3)	(2,634.3)	(2,552.9)
Net	371.0	$ 371.0	452.4
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	3,481.7	$ 3,481.7	3,303.2
Accumulated Amortization	(776.1)	(776.1)	(502.3)
Net	2,705.6	$ 2,705.6	2,800.9
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	648.0	$ 648.0	730.5
Accumulated Amortization	0.0	0.0	0.0
Intangible Assets and Goodwill (Additional Textual)
Impairment of out-licensed patent		$ 12.2		$ 16.2
Trademarks and trade names
Intangible assets
Indefinite lived intangible assets useful life		Indefinite
Cost and net	64.0	$ 64.0	64.0
Accumulated Amortization	0.0	0.0	0.0
AVONEX | Developed technology
Intangible assets
Net	363.3	363.3
TYSABRI | Acquired and in-licensed rights and patents
Intangible assets
Net	2,493.2	2,493.2
SPINRAZA | Acquired and in-licensed rights and patents
Intangible Assets and Goodwill (Additional Textual)
Net change in acquired and in-licensed rights and patents		60.0
Biosimilars | Acquired and in-licensed rights and patents
Intangible Assets and Goodwill (Additional Textual)
Net change in acquired and in-licensed rights and patents		50.0
ZINBRYTA | Acquired and in-licensed rights and patents
Intangible Assets and Goodwill (Additional Textual)
Net change in acquired and in-licensed rights and patents		32.0
ALPROLIX | Acquired and in-licensed rights and patents
Intangible Assets and Goodwill (Additional Textual)
Net change in acquired and in-licensed rights and patents		26.5
Milestone payment made during period		20.0
Deferred tax liabilities	6.5	6.5
Convergence Pharmaceuticals
Finite-Lived Intangible Assets [Line Items]
Goodwill					$ 128.3
Intangible Assets and Goodwill (Additional Textual)
In-process research and development					$ 424.6
Milestone payment made during period	$ 50.0
Fumapharm AG | TECFIDERA
Finite-Lived Intangible Assets [Line Items]
Increase to goodwill		1,200.0	$ 900.0
Rodin Therapeutics | In-process research and development
Intangible Assets and Goodwill (Additional Textual)
Impairment of out-licensed patent		8.7
Ataxion | In-process research and development
Intangible Assets and Goodwill (Additional Textual)
Impairment of out-licensed patent		$ 3.5