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v3.19.1

Inventory Inventory (Details Textual) $ in Millions | 3 Months Ended
--- | ---
Mar. 31, 2019 USD ($)
Bioverativ |
Inventory [Line Items] |
Inventory sold to Bioverativ, cost | $ 173.5

X
- Definition Inventory sold to Bioverativ, cost + References No definition available. + Details
Name: | biib_InventorysoldtoBioverativcost
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_InventoryLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Details
Name: | srt_MajorCustomersAxis=biib_BioverativMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |