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Acquisitions - Narrative (Details) $ / shares in Units, $ in Millions | 1 Months Ended | 12 Months Ended |
--- | --- | --- | ---
Mar. 31, 2020 USD ($) | Jun. 30, 2019 USD ($) product | Jul. 30, 2018 USD ($) | Apr. 30, 2018 USD ($) | Jan. 31, 2018 USD ($) | Dec. 31, 2020 USD ($) | Dec. 31, 2019 USD ($) | Dec. 31, 2018 USD ($) | Jun. 07, 2019 USD ($) $ / shares
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |
Acquired in-process research and development |  |  |  |  |  | $ 75.0 | $ 0.0 | $ 112.5 |
Clinical assets acquired | product |  | 2 |  |  |  |  |  |  |
BIIB118 |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |
Acquired in-process research and development | $ 75.0 |  |  |  |  |  |  |  |
Estimated additional payments upon achievement of development and commercial milestones | $ 635.0 |  |  |  |  |  |  |  |
Nightstar |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |
Price per share | $ / shares |  |  |  |  |  |  |  |  | $ 25.50
Total transaction value |  | $ 847.6 |  |  |  |  |  |  |
Payments for pre-combination equity compensation |  | 4.6 |  |  |  |  |  |  |
Fair value, in-process research and development, discount rate |  |  |  |  |  |  |  |  | 12.50%
Intangible assets |  |  |  |  |  |  |  |  | $ 585.0
Deferred tax liabilities, goodwill |  |  |  |  |  |  |  |  | 60.9
Nightstar | BIIB111 |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |
Intangible assets |  |  |  |  |  |  |  |  | 480.0
Nightstar | BIIB112 |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |
Intangible assets |  |  |  |  |  |  |  |  | $ 220.0
Nightstar | Post-acquisition equity compensation |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |
Fair-value post-combination equity compensation |  | 26.2 |  |  |  |  |  |  |
Nightstar | Post-acquisition equity compensation | Selling, general and administrative |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |
Fair-value post-combination equity compensation |  | 18.4 |  |  |  |  |  |  |
Nightstar | Post-acquisition equity compensation | Research and development |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |
Fair-value post-combination equity compensation |  | $ 7.8 |  |  |  |  |  |  |
Karyopharm |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |
Acquired in-process research and development |  |  |  |  | $ 10.0 |  |  |  |
Estimated additional payments upon achievement of development and commercial milestones |  |  |  |  |  | 207.0 |  |  |
Pfizer |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |
Acquired in-process research and development |  |  |  | $ 75.0 |  |  |  |  |
Estimated additional payments upon achievement of development and commercial milestones |  |  |  |  |  | 515.0 |  |  |
AliveGen |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |
Acquired in-process research and development |  |  | $ 27.5 |  |  |  |  |  |
Estimated additional payments upon achievement of development and commercial milestones |  |  |  |  |  | $ 535.0 |  |  |

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