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Acquisitions (Details Textual) - USD ($) $ / shares in Units, $ in Millions | 3 Months Ended | 12 Months Ended |
--- | --- | --- | ---
Jun. 30, 2019 | Sep. 30, 2018 | Jun. 30, 2018 | Mar. 31, 2018 | Jun. 30, 2017 | Dec. 31, 2019 | Dec. 31, 2018 | Dec. 31, 2017 | Jun. 07, 2019
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |
Acquired in-process research and development |  |  |  |  |  | $ 0.0 | $ 112.5 | $ 120.0 |
Nightstar |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |
Price per share |  |  |  |  |  |  |  |  | $ 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%
In-process research and development |  |  |  |  |  |  |  |  | $ 700.0
Deferred tax liabilities, goodwill | 35.5 |  |  |  |  |  |  |  |
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 |  |  |
TMS |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |
Acquired in-process research and development |  |  |  |  |  | 18.0 |  |  |
Estimated additional payments upon achievement of development and commercial milestones |  |  |  |  |  | 335.0 |  |  |
Research and development expense asset acquired |  |  | $ 4.0 |  |  |  |  |  |
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 |  |  |
Remedy Pharmaceutical |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |
Acquired in-process research and development |  |  |  |  | $ 120.0 |  |  |  |
Post-acquisition equity compensation | Nightstar |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |
Fair-value post-combination equity compensation | 26.2 |  |  |  |  |  |  |  |
Selling, General and Administrative Expenses | Nightstar |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |
Fair-value post-combination equity compensation | 18.4 |  |  |  |  |  |  |  |
Research and development | Nightstar |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |
Fair-value post-combination equity compensation | $ 7.8 |  |  |  |  |  |  |  |
BIIB111 | Nightstar |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |
In-process research and development |  |  |  |  |  |  |  |  | 480.0
BIIB112 | Nightstar |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |
In-process research and development |  |  |  |  |  |  |  |  | $ 220.0

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