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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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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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 aggregate costs incurred (1) in a planned search or critical investigation aimed at discovery of new knowledge with the hope that such knowledge will be useful in developing a new product or service, a new process or technique, or in bringing about a significant improvement to an existing product or process; or (2) to translate research findings or other knowledge into a plan or design for a new product or process or for a significant improvement to an existing product or process whether intended for sale or the entity's use, during the reporting period charged to research and development projects, including the costs of developing computer software up to the point in time of achieving technological feasibility, and costs allocated in accounting for a business combination to in-process projects deemed to have no alternative future use.

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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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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