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Collaborative and Other Relationships - Eisai (Details) $ in Millions | Dec. 31, 2023 USD ($)
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Eisai |
Collaborative arrangements and non-collaborative arrangement transactions |
Payments Made To Termination Agreement | $ 16.0
LEQEMBI Collaboration |
Collaborative arrangements and non-collaborative arrangement transactions |
Payable Due To Termination Agreement | $ 31.0

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- Definition Accrued Payment To Termination Agreement + References No definition available. + Details
Name: | biib_AccruedPaymentToTerminationAgreement
Namespace Prefix: | biib_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Definition Payments Made To Termination Agreement + References No definition available. + Details
Name: | biib_PaymentsMadeToTerminationAgreement
Namespace Prefix: | biib_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Topic 808 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 1 -Subparagraph (d) -Publisher FASB -URI https://asc.fasb.org/1943274/2147479402/808-10-50-1 + Details
Name: | us-gaap_CollaborativeArrangementsAndNoncollaborativeArrangementTransactionsLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Details
Name: | us-gaap_TypeOfArrangementAxis=biib_EisaiMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | srt_ProductOrServiceAxis=biib_LEQEMBICollaborationMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |