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Intangible Assets and Goodwill (Details) - USD ($) $ in Millions	3 Months Ended			9 Months Ended
	Sep. 30, 2021	Mar. 31, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020
Intangible assets
Contingent consideration impairment	$ 15.6		$ 29.0	$ 49.1	$ 23.5
Total intangible assets, gross	7,607.2			7,607.2		$ 8,220.8
Accumulated Amortization	(5,320.4)			(5,320.4)		(5,136.5)
Intangible assets, net	2,286.8			2,286.8		3,084.3
Amortization and impairment of acquired intangible assets	111.0		82.6	813.2	215.6
Expected future amortization expense, 2019 (remaining three months)	70.0			70.0
Expected future amortization expense, 2020	255.0			255.0
Expected future amortization expense, 2021	210.0			210.0
Expected future amortization expense, 2022	195.0			195.0
Expected future amortization expense, 2023	195.0			195.0
Expected future amortization expense, 2024	180.0			180.0
Impairment of intangible assets				629.3	19.3
Completed technology
Intangible assets
Cost	7,411.2			7,411.2		7,394.3
Net	2,090.8			2,090.8		2,257.8
Accumulated Amortization	(5,320.4)			$ (5,320.4)		(5,136.5)
Completed technology | Minimum
Intangible assets
Estimated life, (in years)				4 years
Completed technology | Maximum
Intangible assets
Estimated life, (in years)				28 years
In-process research and development
Intangible assets
Amortization and impairment of acquired intangible assets	19.3			$ 19.3
In-process research and development
Intangible assets
Indefinite lived intangible assets useful life				Indefinite until commercialization
Cost and Net	132.0			$ 132.0		762.5
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
Vixotrigine
Intangible assets
Amortization and impairment of acquired intangible assets				44.3
TGN | In-process research and development
Intangible assets
Cost and Net	132.0			132.0
BIIB111
Intangible assets
Impairment of intangible assets	15.0			365.0
BIIB111 | Nightstar
Intangible assets
Amortization and impairment of acquired intangible assets	15.0			365.0
Impairment of intangible assets		$ 115.0
BIIB112
Intangible assets
Impairment of intangible assets			$ 28.4	220.0	$ 220.0
BIIB112 | Nightstar
Intangible assets
Amortization and impairment of acquired intangible assets	28.4			$ 220.0
BIIB111 and BIIB112
Intangible assets
Manufacturing and other costs	$ 39.1

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