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Investments in Variable Interest Entities (Details) - USD ($) $ in Millions | 3 Months Ended |
--- | --- | ---
Mar. 31, 2017 | Dec. 31, 2016
Variable Interest Entity [Line Items] |  |
Investment in biotechnology companies that are determined to be unconsolidated variable interest entities | $ 53.5 | $ 47.4
Neurimmune |  |
Variable Interest Entity [Line Items] |  |
Research and development cost in support of collaboration agreement | 100.00% |

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- Definition Investment in biotechnology companies that are determined to be unconsolidated variable interest entities. + References No definition available. + Details
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Namespace Prefix: | biib_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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- Definition Research and development cost in support of the collaboration agreement. + References No definition available. + Details
Name: | biib_ResearchAndDevelopmentCostInSupportOfCollaborationAgreement
Namespace Prefix: | biib_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

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

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- Details
Name: | us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis=biib_NeurimmuneMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |