Financial Instruments (Details Textual)	9 Months Ended
Sep. 30, 2023
Investments, Debt and Equity Securities [Abstract]
Average maturity of marketable securities, months	8 months

Average remaining time to maturity of marketable debt securities, available-for-sale securities.

No definition available.

Name:	biib_AverageMaturityOfMarketableSecurities
Namespace Prefix:	biib_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration