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Intangible Assets and Goodwill - Narrative (Details) - USD ($) | 3 Months Ended | 12 Months Ended
--- | --- | ---
Dec. 31, 2022 | Sep. 30, 2021 | Mar. 31, 2021 | Dec. 31, 2020 | Dec. 31, 2022 | Dec. 31, 2021 | Dec. 31, 2020
Goodwill [Line Items] |  |  |  |  |  |  |
Amortization and impairment of acquired intangible assets |  |  |  |  | $ 365,900,000 | $ 881,300,000 | $ 464,800,000
Amortization of acquired intangible assets |  |  |  |  | 246,300,000 | $ 252,000,000 | 255,100,000
Impairment, Intangible Asset, Indefinite-Lived (Excluding Goodwill), Statement of Income or Comprehensive Income [Extensible Enumeration] |  |  |  |  |  | Amortization and impairment of acquired intangible assets |
(Gain) loss on fair value remeasurement of contingent consideration |  |  |  |  | (209,100,000) | $ (50,700,000) | (86,300,000)
Accumulated impairment losses related to goodwill | $ 0 |  |  |  | 0 |  |
Goodwill | 5,749,000,000 |  |  | $ 5,762,100,000 | 5,749,000,000 | 5,761,100,000 | 5,762,100,000
Other |  |  |  |  | (12,100,000) | (1,000,000.0) |
In-process research and development |  |  |  |  |  |  |
Goodwill [Line Items] |  |  |  |  |  |  |
Intangible assets (excluding goodwill) | 0 |  |  |  | 0 | 132,700,000 |
BIIB111 |  |  |  |  |  |  |
Goodwill [Line Items] |  |  |  |  |  |  |
Impairment of intangible assets |  |  |  |  |  | 365,000,000 | 115,000,000
BIIB111 | In-process research and development |  |  |  |  |  |  |
Goodwill [Line Items] |  |  |  |  |  |  |
Intangible asset, fair value | 0 | $ 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 |  |  | 0 |  |
BIIB054 |  |  |  |  |  |  |
Goodwill [Line Items] |  |  |  |  |  |  |
Impairment of intangible assets |  |  |  |  |  |  | 75,400,000
In Process Research and Development Other |  |  |  |  |  |  |
Goodwill [Line Items] |  |  |  |  |  |  |
Impairment of intangible assets |  |  |  |  |  |  | 19,300,000
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 | $ 119,600,000 |  | $ 44,300,000 |  |  |  |
BIIB111 and BIIB112 |  |  |  |  |  |  |
Goodwill [Line Items] |  |  |  |  |  |  |
Manufacturing and other costs |  | $ 39,100,000 |  |  |  |  |
Convergence Pharmaceuticals | TGN | In-process research and development |  |  |  |  |  |  |
Goodwill [Line Items] |  |  |  |  |  |  |
Acquired intangible asset |  |  |  |  | 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] |  |  |  |  |  |  |
Impairment of intangible assets |  |  |  | 115,000,000 |  |  |
Acquired intangible asset |  |  |  |  | 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] |  |  |  |  |  |  |
Acquired intangible asset |  |  |  |  | 220,000,000 |  |
Nightstar | BIIB054 |  |  |  |  |  |  |
Goodwill [Line Items] |  |  |  |  |  |  |
Impairment of intangible assets |  |  |  | 75,400,000 |  |  |
Intangible asset, fair value |  |  |  | $ 0 |  |  | $ 0
Biogen International Neuroscience GmbH | BIIB054 | In-process research and development |  |  |  |  |  |  |
Goodwill [Line Items] |  |  |  |  |  |  |
Acquired intangible asset |  |  |  |  | $ 110,900,000 |  |

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