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Intangible Assets and Goodwill - Narrative (Details) - USD ($) | 3 Months Ended | 12 Months Ended |  |
--- | --- | --- | --- | ---
Dec. 31, 2020 | Sep. 30, 2019 | Sep. 30, 2018 | Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018 | Sep. 30, 2020 | Jun. 07, 2019
Goodwill [Line Items] |  |  |  |  |  |  |  |
Amortization and impairment of acquired intangible assets |  |  |  | $ 464,800,000 | $ 489,900,000 | $ 747,300,000 |  |
Amortization of acquired intangible assets |  |  |  | 255,100,000 | 274,000,000.0 | 381,200,000 |  |
Accumulated impairment losses related to goodwill | $ 0 |  |  | 0 |  |  |  |
Intangible Assets, Net (Excluding Goodwill) | 3,084,300,000 |  |  | 3,084,300,000 | 3,527,400,000 |  |  |
TECFIDERA |  |  |  |  |  |  |  |
Goodwill [Line Items] |  |  |  |  |  |  |  |
Impairment |  |  |  |  |  | 176,800,000 |  |
In-process research and development |  |  |  |  |  |  |  |
Goodwill [Line Items] |  |  |  |  |  |  |  |
Amortization and impairment of acquired intangible assets |  |  |  | 19,300,000 |  |  |  |
In-process research and development |  |  |  |  |  |  |  |
Goodwill [Line Items] |  |  |  |  |  |  |  |
Impairment |  | $ 215,900,000 | $ 189,300,000 |  | 215,900,000 | $ 189,300,000 |  |
Intangible assets (excluding goodwill) | 762,500,000 |  |  | 762,500,000 | $ 965,500,000 |  |  |
BIIB111 | In-process research and development |  |  |  |  |  |  |  |
Goodwill [Line Items] |  |  |  |  |  |  |  |
Intangible Assets, Net (Excluding Goodwill) | 365,000,000.0 |  |  | 365,000,000.0 |  |  | $ 480,000,000.0 |
TGN | In-process research and development |  |  |  |  |  |  |  |
Goodwill [Line Items] |  |  |  |  |  |  |  |
Intangible assets (excluding goodwill) | 177,500,000 |  |  | 177,500,000 |  |  |  |
Nightstar |  |  |  |  |  |  |  |
Goodwill [Line Items] |  |  |  |  |  |  |  |
Intangible assets |  |  |  |  |  |  |  | $ 585,000,000.0
Nightstar | BIIB111 |  |  |  |  |  |  |  |
Goodwill [Line Items] |  |  |  |  |  |  |  |
Impairment |  |  |  | 115,000,000.0 |  |  |  |
Intangible assets |  |  |  |  |  |  |  | $ 480,000,000.0
Nightstar | BIIB111 | In-process research and development |  |  |  |  |  |  |  |
Goodwill [Line Items] |  |  |  |  |  |  |  |
Impairment | 115,000,000.0 |  |  |  |  |  |  |
Nightstar | BIIB054 |  |  |  |  |  |  |  |
Goodwill [Line Items] |  |  |  |  |  |  |  |
Impairment | $ 75,400,000 |  |  | $ 75,400,000 |  |  |  |

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