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Business Acquisition (Details Textual) | 1 Months Ended | 3 Months Ended |
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Mar. 31, 2020 USD ($) | Jun. 30, 2019 USD ($) Assets | Mar. 31, 2020 USD ($) | Jun. 30, 2019 USD ($) | Mar. 31, 2019 USD ($) | Jun. 07, 2019 USD ($) $ / shares
Business Acquisition [Line Items] |  |  |  |  |  |
Acquired in-process research and development |  |  | $ 75,000,000.0 |  | $ 0 |
Clinical Assets Acquired | Assets |  | 2 |  |  |  |
Tax Basis of Investments, Cost for Income Tax Purposes | $ 0 |  | $ 0 |  |  |
BIIB118 |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |
Acquired in-process research and development | 75,000,000.0 |  |  |  |  |
Estimated additional payments upon achievement of development and commercial milestones | $ 635,000,000.0 |  |  |  |  |
Nightstar |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |
Price per share | $ / shares |  |  |  |  |  | $ 25.50
Total transaction value |  |  |  | $ 847,600,000 |  |
Payments for pre-combination equity compensation |  |  |  | 4,600,000 |  |
Fair value, in-process research and development, discount rate |  |  |  |  |  | 12.50%
In-process research and development |  |  |  |  |  | $ 700,000,000.0
Deferred tax liabilities, goodwill |  | $ 35,500,000 |  | 35,500,000 |  |
Selling, general and administrative | Nightstar |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |
Fair value of post-combination equity compensation |  |  |  | 18,400,000 |  |
Research and development | Nightstar |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |
Fair value of post-combination equity compensation |  |  |  | 7,800,000 |  |
BIIB111 | Nightstar |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |
In-process research and development |  |  |  |  |  | 480,000,000.0
BIIB112 | Nightstar |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |
In-process research and development |  |  |  |  |  | $ 220,000,000.0
Post-acquisition equity compensation | Nightstar |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |
Fair value of post-combination equity compensation |  |  |  | $ 26,200,000 |  |

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