Financial Instruments (Details Textual)	6 Months Ended
Jun. 30, 2020	Jun. 30, 2019
Investments, Debt and Equity Securities [Abstract]
Average maturity of marketable securities, months	14 months	14 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