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v3.20.2

Summary of Significant Accounting Policies (Details) | 6 Months Ended
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Jun. 30, 2020 segment
Accounting Policies [Abstract] |
Number of reportable segments | 1
Interest in subsidiary (less than given percentage) | 100.00%

X
- Definition Interest in subsidiary. + References No definition available. + Details
Name: | biib_InterestInSubsidiary
Namespace Prefix: | biib_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

X
- References No definition available. + Details
Name: | us-gaap_AccountingPoliciesAbstract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Definition Number of segments reported by the entity. A reportable segment is a component of an entity for which there is an accounting requirement to report separate financial information on that component in the entity's financial statements. + References No definition available. + Details
Name: | us-gaap_NumberOfReportableSegments
Namespace Prefix: | us-gaap_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | duration