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Collaborative and Other Relationships - Eisai (Details) - USD ($) $ in Millions | Mar. 31, 2025 | Dec. 31, 2024
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Eisai |  |
Collaborative arrangements and non-collaborative arrangement transactions |  |
Amounts receivable | $ 115.5 | $ 16.7
Amounts payable | $ 122.6 | $ 138.0
Alzheimer's collaboration revenue |  |
Collaborative arrangements and non-collaborative arrangement transactions |  |
Share of non-controling interest recognized | 50.00% |

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- Definition Accounts Payable, To Collaborator + References No definition available. + Details
Name: | biib_AccountsPayableToCollaborator
Namespace Prefix: | biib_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Definition Accounts Receivable, From Collaborator + References No definition available. + Details
Name: | biib_AccountsReceivableFromCollaborator
Namespace Prefix: | biib_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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- Definition Share Of Non-Controling Interest Recognized + References No definition available. + Details
Name: | biib_ShareOfNonControlingInterestRecognized
Namespace Prefix: | biib_
Data Type: | dtr-types:percentItemType
Balance Type: | na
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: |