Intangible Assets and Goodwill (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Jun. 07, 2019	Dec. 31, 2018
Intangible assets
Total intangible assets, gross	$ 8,426.9		$ 8,426.9			$ 7,727.3
Accumulated Amortization	(4,745.6)		(4,745.6)			(4,607.3)
Intangible assets, net	3,681.3		3,681.3			3,120.0
Amortization and impairment of acquired intangible assets	70.1	$ 107.4	138.3	$ 211.3
Intangible Assets and Goodwill (Textual) [Abstract]
Expected future amortization expense, 2019 (remaining six months)	130.0		130.0
Expected future amortization expense, 2020	255.0		255.0
Expected future amortization expense, 2021	215.0		215.0
Expected future amortization expense, 2022	215.0		215.0
Expected future amortization expense, 2023	220.0		220.0
Expected future amortization expense, 2024	210.0		210.0
Out-licensed patents
Intangible assets
Cost	543.3		543.3			543.3
Net	0.9		0.9			1.0
Accumulated Amortization	(542.4)		$ (542.4)			(542.3)
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
Net	251.2		251.2			270.5
Accumulated Amortization	(2,754.1)		$ (2,754.1)			(2,734.8)
Developed technology | Minimum
Intangible assets
Estimated life, (in years)			15 years
Developed technology | Maximum
Intangible assets
Estimated life, (in years)			23 years
Developed technology | AVONEX
Intangible assets
Net	246.6		$ 246.6
Acquired and in-licensed rights and patents
Intangible assets
Cost	3,638.7		3,638.7			3,638.7
Net	2,189.6		2,189.6			2,308.5
Accumulated Amortization	(1,449.1)		$ (1,449.1)			(1,330.2)
Acquired and in-licensed rights and patents | Minimum
Intangible assets
Estimated life, (in years)			4 years
Acquired and in-licensed rights and patents | Maximum
Intangible assets
Estimated life, (in years)			18 years
Acquired and in-licensed rights and patents | TYSABRI
Intangible assets
Net	1,933.3		$ 1,933.3
Acquired and in-licensed rights and patents | TECFIDERA
Intangible assets
Net	55.2		$ 55.2
In Process Research and Development
Intangible assets
Indefinite lived intangible assets useful life			Indefinite until commercialization
Cost and Net	1,175.6		$ 1,175.6			476.0
Accumulated Amortization	0.0		$ 0.0			0.0
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
Nightstar
Intangible assets
In-process research and development					$ 700.0