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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

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