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Acquisitions - Narrative (Details) $ / shares in Units, $ in Millions	1 Months Ended					12 Months Ended
	Mar. 31, 2020 USD ($)	Jun. 30, 2019 USD ($) product	Jul. 30, 2018 USD ($)	Apr. 30, 2018 USD ($)	Jan. 31, 2018 USD ($)	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Jun. 07, 2019 USD ($) $ / shares
Business Acquisition [Line Items]
Acquired in-process research and development						$ 75.0	$ 0.0	$ 112.5
Clinical assets acquired | product		2
BIIB118
Business Acquisition [Line Items]
Acquired in-process research and development	$ 75.0
Estimated additional payments upon achievement of development and commercial milestones	$ 635.0
Nightstar
Business Acquisition [Line Items]
Price per share | $ / shares									$ 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%
Intangible assets									$ 585.0
Deferred tax liabilities, goodwill									60.9
Nightstar | BIIB111
Business Acquisition [Line Items]
Intangible assets									480.0
Nightstar | BIIB112
Business Acquisition [Line Items]
Intangible assets									$ 220.0
Nightstar | Post-acquisition equity compensation
Business Acquisition [Line Items]
Fair-value post-combination equity compensation		26.2
Nightstar | Post-acquisition equity compensation | Selling, general and administrative
Business Acquisition [Line Items]
Fair-value post-combination equity compensation		18.4
Nightstar | Post-acquisition equity compensation | Research and development
Business Acquisition [Line Items]
Fair-value post-combination equity compensation		$ 7.8
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
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

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