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Business Acquisition (Details Textual) - Nightstar - USD ($) $ / shares in Units, $ in Millions | 6 Months Ended |
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Jun. 30, 2019 | Jun. 07, 2019
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 | 35.0 |
Selling, general and administrative |  |
Business Acquisition [Line Items] |  |
Fair value of post-combination equity compensation | 18.4 |
Research and development |  |
Business Acquisition [Line Items] |  |
Fair value of post-combination equity compensation | 7.8 |
BIIB111 |  |
Business Acquisition [Line Items] |  |
In-process research and development |  | 480.0
BIIB112 |  |
Business Acquisition [Line Items] |  |
In-process research and development |  | $ 220.0
Post-acquisition equity compensation |  |
Business Acquisition [Line Items] |  |
Fair value of post-combination equity compensation | $ 26.2 |

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