Intangible Assets and Goodwill - Narrative (Details) - USD ($)	3 Months Ended		12 Months Ended
	Sep. 30, 2022	Mar. 31, 2022	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021	Sep. 27, 2023	Sep. 26, 2023
Goodwill [Line Items]
Amortization and impairment of acquired intangible assets			$ 240,600,000	$ 365,900,000	$ 881,300,000
Amortization of acquired intangible assets			240,600,000	246,300,000	252,000,000
(Gain) loss on fair value remeasurement of contingent consideration			0	$ (209,100,000)	(50,700,000)
Impairment, Intangible Asset, Indefinite-Lived (Excluding Goodwill), Statement of Income or Comprehensive Income [Extensible Enumeration]				Amortization and impairment of acquired intangible assets
Accumulated impairment losses related to goodwill			$ 0
Acquired and in-licensed rights and patents
Goodwill [Line Items]
Weighted-Average remaining useful life			12 years
Acquired and in-licensed rights and patents | Minimum
Goodwill [Line Items]
Estimated life (in years)			2 years
Acquired and in-licensed rights and patents | Maximum
Goodwill [Line Items]
Estimated life (in years)			22 years
Developed technology and other
Goodwill [Line Items]
Weighted-Average remaining useful life			10 years
Developed technology and other | Minimum
Goodwill [Line Items]
Estimated life (in years)			13 years
Developed technology and other | Maximum
Goodwill [Line Items]
Estimated life (in years)			31 years
Completed Technology | Minimum
Goodwill [Line Items]
Estimated life (in years)			2 years
Completed Technology | Maximum
Goodwill [Line Items]
Estimated life (in years)			31 years
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		$ 44,300,000		$ 119,600,000
BIIB111
Goodwill [Line Items]
Impairment of intangible assets				365,000,000
BIIB111 | In-process research and development
Goodwill [Line Items]
Intangible asset, fair value	$ 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
BIIB111 and BIIB112
Goodwill [Line Items]
Manufacturing and other costs	$ 39,100,000
Convergence Pharmaceuticals | TGN | In-process research and development
Goodwill [Line Items]
Finite-Lived Intangible Assets Acquired			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]
Finite-Lived Intangible Assets Acquired			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]
Finite-Lived Intangible Assets Acquired			220,000,000
Reata Pharmaceuticals, Inc | In-process research and development
Goodwill [Line Items]
Intangible assets:			2,300,000,000			$ 2,300,000,000	$ 1,900,000,000
Reata Pharmaceuticals, Inc | Completed technology for SKYCLARYS (U.S.)
Goodwill [Line Items]
Intangible assets:			4,200,000,000			$ 4,200,000,000	3,600,000,000
Reata Pharmaceuticals, Inc | Priority review voucher
Goodwill [Line Items]
Intangible assets:			$ 100,000,000.0				$ 100,000,000.0