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Collaborative and Other Relationships Collaborative and Other Relationships (Details 2) - Eisai - USD ($) $ in Millions | 12 Months Ended
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Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Collaborative arrangements and non-collaborative arrangement transactions |  |  |
Total expense incurred by collaboration | $ 146.2 | $ 95.1 | $ 84.1
Biogen's share of expense reflected within our consolidation statements of income | $ 74.3 | $ 50.5 | $ 40.4

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- Definition Total expense incurred by collaboration. + References No definition available. + Details
Name: | biib_ExpenseIncurredByCollaboration
Namespace Prefix: | biib_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- Definition Biogen Idec's share of expense reflected within our consolidated statements of income. + References No definition available. + Details
Name: | biib_ExpenseReflectedWithinStatementsOfIncome
Namespace Prefix: | biib_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
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_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: |