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Inventory Inventory (Details Textual) - USD ($) $ in Millions | Sep. 30, 2015 | Dec. 31, 2014
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Inventory Disclosure [Abstract] |  |
Inventory, Various Programs, Gross | $ 54.3 | $ 6.3

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- Definition Gross inventory of various programs which have been capitalized in advance of regulatory approval. + References No definition available. + Details
Name: | biib_InventoryVariousProgramsGross
Namespace Prefix: | biib_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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- References No definition available. + Details
Name: | us-gaap_InventoryDisclosureAbstract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration