Investments in Variable Interest Entities (Details) - USD ($) $ in Millions	Mar. 31, 2025	Dec. 31, 2024
Investments in Variable Interest Entities [Abstract]
Investment in biotechnology companies that are determined to be unconsolidated variable interest entities	$ 19.3	$ 23.6

Investment in biotechnology companies that are determined to be unconsolidated variable interest entities.

No definition available.

Name:	biib_InvestmentInBiotechnologyCompaniesThatAreDeterminedToBeUnconsolidatedVariableInterestEntities
Namespace Prefix:	biib_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant