XML 93 R61.htm IDEA: XBRL DOCUMENT v3.21.2

Financial Instruments (Details Textual)	6 Months Ended	9 Months Ended
	Jun. 30, 2021	Sep. 30, 2020
Investments, Debt and Equity Securities [Abstract]
Average maturity of marketable securities, months	13 months	11 months

X

- Definition

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

+ References

No definition available.

+ Details

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

X

- References

No definition available.

+ Details

Name:	us-gaap_InvestmentsDebtAndEquitySecuritiesAbstract
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration