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Investments in Variable Interest Entities (Details) - USD ($) $ in Millions | 6 Months Ended |
--- | --- | ---
Jun. 30, 2016 | Dec. 31, 2015
Variable Interest Entity [Line Items] |  |
Investment in biotechnology companies that are determined to be unconsolidated variable interest entities | $ 38.0 | $ 29.2
Neurimmune |  |
Variable Interest Entity [Line Items] |  |
Remaining potential development milestone payments and royalties on commercial sales under the terms of collaboration agreement | $ 275.0 |
Research and development cost in support of collaboration agreement | 100.00% |
Other Variable Interest Entities |  |
Variable Interest Entity [Line Items] |  |
Additional investments committed to if certain development milestones are achieved | $ 8.0 |

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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 Expected total payments including upfront and milestone payments as well as the greater of an annual maintenance fee or usage-based royalty payments under the terms of collaboration agreement. + References No definition available. + Details
Name: | biib_RemainingPotentialDevelopmentMilestonePaymentsAndRoyaltiesOnCommercialSalesUnderTermsOfCollaborationAgreement
Namespace Prefix: | biib_
Data Type: | xbrli:monetaryItemType
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- Definition Research and development cost in support of the collaboration agreement. + References No definition available. + Details
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Namespace Prefix: | biib_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

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- Definition Minimum amount of other commitment not otherwise specified in the taxonomy. Excludes commitments explicitly modeled in the taxonomy, including but not limited to, long-term and short-term purchase commitments, recorded and unrecorded purchase obligations, supply commitments, registration payment arrangements, leases, debt, product warranties, guarantees, environmental remediation obligations, and pensions. + References No definition available. + Details
Name: | us-gaap_OtherCommitment
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
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
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- Details
Name: | us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis=biib_NeurimmuneMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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