Intangible Assets and Goodwill - Narrative (Details) - USD ($)	3 Months Ended				12 Months Ended
	Dec. 31, 2022	Sep. 30, 2021	Mar. 31, 2021	Dec. 31, 2020	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Goodwill [Line Items]
Amortization and impairment of acquired intangible assets					$ 365,900,000	$ 881,300,000	$ 464,800,000
Amortization of acquired intangible assets					246,300,000	$ 252,000,000	255,100,000
Impairment, Intangible Asset, Indefinite-Lived (Excluding Goodwill), Statement of Income or Comprehensive Income [Extensible Enumeration]						Amortization and impairment of acquired intangible assets
(Gain) loss on fair value remeasurement of contingent consideration					(209,100,000)	$ (50,700,000)	(86,300,000)
Accumulated impairment losses related to goodwill	$ 0				0
Goodwill	5,749,000,000			$ 5,762,100,000	5,749,000,000	5,761,100,000	5,762,100,000
Other					(12,100,000)	(1,000,000.0)
In-process research and development
Goodwill [Line Items]
Intangible assets (excluding goodwill)	0				0	132,700,000
BIIB111
Goodwill [Line Items]
Impairment of intangible assets						365,000,000	115,000,000
BIIB111 | In-process research and development
Goodwill [Line Items]
Intangible asset, fair value	0	$ 0			0
BIIB112
Goodwill [Line Items]
Impairment of intangible assets		220,000,000
BIIB112 | In-process research and development
Goodwill [Line Items]
Intangible asset, fair value	0	0			0
BIIB054
Goodwill [Line Items]
Impairment of intangible assets							75,400,000
In Process Research and Development Other
Goodwill [Line Items]
Impairment of intangible assets							19,300,000
TGN
Goodwill [Line Items]
(Gain) loss on fair value remeasurement of contingent consideration					209,100,000
TGN | In-process research and development
Goodwill [Line Items]
Impairment of intangible assets	$ 119,600,000		$ 44,300,000
BIIB111 and BIIB112
Goodwill [Line Items]
Manufacturing and other costs		$ 39,100,000
Convergence Pharmaceuticals | TGN | In-process research and development
Goodwill [Line Items]
Acquired intangible asset					424,600,000
Nightstar | BIIB111
Goodwill [Line Items]
Impairment of intangible assets						365,000,000
Nightstar | BIIB111 | In-process research and development
Goodwill [Line Items]
Impairment of intangible assets				115,000,000
Acquired intangible asset					480,000,000
Nightstar | BIIB112
Goodwill [Line Items]
Impairment of intangible assets						$ 220,000,000
Nightstar | BIIB112 | In-process research and development
Goodwill [Line Items]
Acquired intangible asset					220,000,000
Nightstar | BIIB054
Goodwill [Line Items]
Impairment of intangible assets				75,400,000
Intangible asset, fair value				$ 0			$ 0
Biogen International Neuroscience GmbH | BIIB054 | In-process research and development
Goodwill [Line Items]
Acquired intangible asset					$ 110,900,000