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Intangible Assets and Goodwill (Details) - USD ($)	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Dec. 31, 2019	Jun. 07, 2019
Finite-Lived Intangible Assets [Line Items]
Impairment of Intangible Assets (Excluding Goodwill)	$ 0	$ 0
Intangible assets
Contingent consideration impairment	4,600,000	(11,500,000)
Total intangible assets, gross	8,399,800,000		$ 8,408,800,000
Accumulated Amortization	(4,952,900,000)		(4,881,400,000)
Intangible assets, net	3,446,900,000		3,527,400,000
Amortization and impairment of acquired intangible assets	71,500,000	$ 68,200,000
Expected future amortization expense, 2019 (remaining three months)	193,000,000.0
Expected future amortization expense, 2020	220,000,000.0
Expected future amortization expense, 2021	220,000,000.0
Expected future amortization expense, 2022	230,000,000.0
Expected future amortization expense, 2023	230,000,000.0
Expected future amortization expense, 2024	220,000,000.0
Out-licensed patents
Intangible assets
Cost	7,379,300,000		7,379,300,000
Net	2,426,400,000		2,497,900,000
Accumulated Amortization	$ (4,952,900,000)		(4,881,400,000)
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 | Minimum
Intangible assets
Estimated life, (in years)	15 years
Developed technology | Maximum
Intangible assets
Estimated life, (in years)	28 years
Developed technology | AVONEX
Intangible assets
Net	$ 154,600,000
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	$ 1,800,000,000
In Process Research and Development
Intangible assets
Indefinite lived intangible assets useful life	Indefinite until commercialization
Cost and Net	$ 956,500,000		965,500,000
Accumulated Amortization	$ 0		0
Trademarks and Trade Names
Intangible assets
Indefinite lived intangible assets useful life	Indefinite
Cost and Net	$ 64,000,000.0		64,000,000.0
Accumulated Amortization	0		$ 0
Nightstar
Intangible assets
In-process research and development				$ 700,000,000.0
TGN [Member] | In Process Research and Development
Intangible assets
Cost and Net	$ 161,700,000

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Indefinite lived intangible assets useful life.

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Amount of assets, excluding financial assets and goodwill, that lack physical substance, having a projected indefinite period of benefit, acquired at the acquisition date.

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Amount of gain (loss) recognized in income from liability measured at fair value on recurring basis using unobservable input (level 3).

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Useful life of finite-lived intangible assets, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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Amount of amortization expense for assets, excluding financial assets and goodwill, lacking physical substance with a finite life expected to be recognized during the next fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

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Amount of amortization expense for assets, excluding financial assets and goodwill, lacking physical substance with a finite life expected to be recognized during the fifth fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

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Amount before amortization of assets, excluding financial assets and goodwill, lacking physical substance with a finite life.

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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