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	Sep. 30, 2019	Dec. 31, 2021	Dec. 31, 2020	Jun. 30, 2019
Business Acquisition, Contingent Consideration [Line Items]
Technological and regulatory success, probability						10.90%
In-process research and development
Business Acquisition, Contingent Consideration [Line Items]
Impairment of intangible assets					$ 215,900,000
Intangible asset, fair value				$ 0	0		$ 0
Convergence Pharmaceuticals
Business Acquisition, Contingent Consideration [Line Items]
Additions	$ 274,500,000
Contingent consideration obligations				259,800,000		$ 209,100,000	259,800,000
Maximum contingent consideration in the form of development and approval milestones						$ 400,000,000
Convergence Pharmaceuticals | Discount rate
Business Acquisition, Contingent Consideration [Line Items]
Fair value measurement and measurement inputs, recurring and nonrecurring						1.30%
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
Biogen Idec International Neuroscience GmbH | In-process research and development
Business Acquisition, Contingent Consideration [Line Items]
Acquired intangible asset							110,900,000
Stromedix, Inc.
Business Acquisition, Contingent Consideration [Line Items]
Intangible assets					$ 219,200,000
Other long-term liabilities
Business Acquisition, Contingent Consideration [Line Items]
Contingent consideration obligations				110,300,000		$ 209,100,000	110,300,000
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
Intangible assets								$ 480,000,000
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
Intangible assets								$ 220,000,000
BIIB054
Business Acquisition, Contingent Consideration [Line Items]
Impairment of intangible assets							75,400,000
Reduction in contingent consideration							51,000,000
BIIB054 | Biogen Idec International Neuroscience GmbH
Business Acquisition, Contingent Consideration [Line Items]
Impairment of intangible assets				75,400,000
BIIB054 | Nightstar
Business Acquisition, Contingent Consideration [Line Items]
Impairment of intangible assets				75,400,000
Intangible asset, fair value				$ 0			$ 0