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.

No definition available.

Name:	us-gaap_ResearchAndDevelopmentInProcess
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

Acquisitions - (Details) - USD ($) $ in Millions	1 Months Ended	12 Months Ended
Acquisitions - (Details) - USD ($) $ in Millions	Mar. 31, 2020	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Business Acquisition [Line Items]
Acquired in-process research and development		$ 0.0	$ 18.0	$ 75.0
BIIB118
Business Acquisition [Line Items]
Acquired in-process research and development	$ 75.0