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Subsequent Events Subsequent Events (Details Textual) - Subsequent Event $ in Millions | 1 Months Ended
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Jul. 31, 2018 USD ($)
Subsequent Event [Line Items] |
Upfront payment for asset acquisition | $ 27.5
AliveGen |
Subsequent Event [Line Items] |
Estimated additional payments upon achievement of milestones | $ 535.0

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- Definition Estimated additional payments upon achievement of development and commercial milestones. + References No definition available. + Details
Name: | biib_EstimatedAdditionalPaymentsUponAchievementOfDevelopmentAndCommercialMilestones
Namespace Prefix: | biib_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- Definition Upfront payment for asset acquisition + References No definition available. + Details
Name: | biib_Upfrontpaymentforassetacquisition
Namespace Prefix: | biib_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- Definition Detail information of subsequent event by type. User is expected to use existing line items from elsewhere in the taxonomy as the primary line items for this disclosure, which is further associated with dimension and member elements pertaining to a subsequent event. + References No definition available. + Details
Name: | us-gaap_SubsequentEventLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Details
Name: | us-gaap_SubsequentEventTypeAxis=us-gaap_SubsequentEventMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | us-gaap_BusinessAcquisitionAxis=biib_AliveGenMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |