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Estimated additional payments upon achievement of development and commercial milestones.

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Fair Value, Indefinite Lived Intangible Assets, Discount Rate

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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Price of a single share of a number of saleable stocks paid or offered to be paid in a business combination.

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- Definition

Amount of assets, excluding financial assets and goodwill, that lack physical substance, having a projected indefinite period of benefit, acquired at the acquisition date.

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The expenses and losses recorded for each transaction with the acquiree that was recognized separately from the acquisition of assets and assumptions of liabilities in the business combination.

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- Definition

Amount of deferred tax liability attributable to taxable temporary differences from goodwill.

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The cash outflow associated with other payments to acquire businesses including deposit on pending acquisitions and preacquisition costs.

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The cash outflow associated with the acquisition of business during the period. The cash portion only of the acquisition price.

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The amount of purchased research and development assets that are acquired in a business combination have no alternative future use and are therefore written off in the period of acquisition.

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The aggregate cost of investments for Federal income tax purposes.

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Business Acquisition (Details Textual)	1 Months Ended		3 Months Ended
Business Acquisition (Details Textual)	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