Intangible Assets and Goodwill (Details) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2022	Jun. 30, 2021	Jun. 30, 2022	Jun. 30, 2021	Dec. 31, 2021	Mar. 31, 2021
Intangible assets
Total intangible assets, gross	$ 7,598,200,000		$ 7,598,200,000		$ 7,609,800,000
Accumulated Amortization	(5,522,900,000)		(5,522,900,000)		(5,388,500,000)
Intangible assets, net	2,075,300,000		2,075,300,000		2,221,300,000
Amortization and impairment of acquired intangible assets	67,500,000	$ 604,100,000	134,400,000	$ 702,200,000
Expected future amortization expense, 2019 (remaining three months)	130,000,000.0		130,000,000.0
Expected future amortization expense, 2020	210,000,000.0		210,000,000.0
Expected future amortization expense, 2021	195,000,000.0		195,000,000.0
Expected future amortization expense, 2022	195,000,000.0		195,000,000.0
Expected future amortization expense, 2023	180,000,000.0		180,000,000.0
Expected future amortization expense, 2024	165,000,000.0		165,000,000.0
Completed technology
Intangible assets
Cost	7,415,000,000		7,415,000,000		7,413,100,000
Net	1,892,100,000		1,892,100,000		2,024,600,000
Accumulated Amortization	(5,522,900,000)		$ (5,522,900,000)		(5,388,500,000)
Completed technology | Minimum
Intangible assets
Useful life			4 years
Completed technology | Maximum
Intangible assets
Useful life			28 years
In-process research and development
Intangible assets
Cost and Net	119,200,000		$ 119,200,000		132,700,000
Accumulated Amortization	0		0		0
Indefinite-lived Intangible Assets (Excluding Goodwill)	119,200,000		119,200,000		132,700,000
Trademarks and Trade Names
Intangible assets
Cost and Net	64,000,000.0		64,000,000.0		64,000,000.0
Accumulated Amortization	0		0		0
Indefinite-lived Intangible Assets (Excluding Goodwill)	64,000,000.0		64,000,000.0		64,000,000.0
BIIB111
Intangible assets
Amortization and impairment of acquired intangible assets		350,000,000		350,000,000
Finite-lived Intangible Assets, Fair Value Disclosure		15,000,000		15,000,000		$ 365,000,000
BIIB112
Intangible assets
Amortization and impairment of acquired intangible assets		191,600,000		191,600,000
Finite-lived Intangible Assets, Fair Value Disclosure		$ 28,400,000		28,400,000		$ 220,000,000
Vixotrigine
Intangible assets
Amortization and impairment of acquired intangible assets				$ 44,300,000
TGN | In-process research and development
Intangible assets
Cost and Net	119,200,000		119,200,000
Indefinite-lived Intangible Assets (Excluding Goodwill)	$ 119,200,000		$ 119,200,000
BIIB111 and BIIB112
Intangible assets
Finite-lived Intangible Assets, Fair Value Disclosure					$ 0