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Financial Instruments Financial Instruments (Details Textual 2) - USD ($) $ in Millions | Jun. 30, 2017 | Dec. 31, 2016
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Strategic Investments |  |
Business Acquisition [Line Items] |  |
Strategic investment portfolio | $ 95.6 | $ 99.9

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- Definition Strategic investment portfolio. + 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 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
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Details
Name: | us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis=biib_StrategicInvestmentsMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |