Fair Value Measurements - Narrative (Details) - USD ($)	1 Months Ended		3 Months Ended		12 Months Ended
	Feb. 28, 2015	Dec. 31, 2010	Sep. 30, 2021	Dec. 31, 2020	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Business Acquisition, Contingent Consideration [Line Items]
Impairment of assets					$ 0	$ 0
Technological and regulatory success, probability						10.90%
Convergence Pharmaceuticals
Business Acquisition, Contingent Consideration [Line Items]
Additions	$ 274,500,000
Contingent consideration obligations					209,100,000	$ 209,100,000
Biogen Idec International Neuroscience GmbH
Business Acquisition, Contingent Consideration [Line Items]
Additions		$ 81,200,000					$ 0
Contingent consideration obligations				$ 101,500,000			101,500,000
Other long-term liabilities
Business Acquisition, Contingent Consideration [Line Items]
Contingent consideration obligations						209,100,000
BIIB111
Business Acquisition, Contingent Consideration [Line Items]
Impairment of intangible assets						365,000,000	115,000,000
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]
Impairment of intangible assets							75,400,000
Reduction in contingent consideration					$ 195,400,000
BIIB054 | Nightstar
Business Acquisition, Contingent Consideration [Line Items]
Impairment of intangible assets				75,400,000
Intangible asset, fair value				$ 0			$ 0