Investments in Variable Interest Entities (Details) - USD ($) $ in Millions	Sep. 30, 2019	Dec. 31, 2018
Investment in Variable Interest Entities (Textual)
Investment in biotechnology companies that are determined to be unconsolidated variable interest entities	$ 23.1	$ 28.7

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