Fair Value Measurements - Narrative (Details) - USD ($)	1 Months Ended	3 Months Ended	12 Months Ended
	Feb. 28, 2015	Mar. 31, 2022	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021	Sep. 30, 2022
Business Acquisition, Contingent Consideration [Line Items]
Impairment of assets			$ 0	$ 0
Convergence Pharmaceuticals
Business Acquisition, Contingent Consideration [Line Items]
Additions	$ 274,500,000		0
Contingent consideration obligations					$ 209,100,000
Business Combination, Contingent Consideration Arrangements, Change in Amount of Contingent Consideration, Liability				209,100,000
BIIB111
Business Acquisition, Contingent Consideration [Line Items]
Impairment of intangible assets				365,000,000
BIIB111 | In-process research and development
Business Acquisition, Contingent Consideration [Line Items]
Intangible asset, fair value			0			$ 0
BIIB111 | Nightstar
Business Acquisition, Contingent Consideration [Line Items]
Impairment of intangible assets					365,000,000
BIIB112
Business Acquisition, Contingent Consideration [Line Items]
Impairment of intangible assets					$ 220,000,000
BIIB112 | In-process research and development
Business Acquisition, Contingent Consideration [Line Items]
Intangible asset, fair value			$ 0			$ 0
BIIB112 | Nightstar
Business Acquisition, Contingent Consideration [Line Items]
Impairment of intangible assets				220,000,000
BIIB054
Business Acquisition, Contingent Consideration [Line Items]
Reduction in contingent consideration				195,400,000
TGN | In-process research and development
Business Acquisition, Contingent Consideration [Line Items]
Impairment of intangible assets		$ 44,300,000		$ 119,600,000