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Gain on Sale of Rights (Details) (USD $) In Thousands, unless otherwise specified | 12 Months Ended
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Dec. 31, 2013 | Dec. 31, 2012 | Dec. 31, 2011
Gain on Sale of Rights [Abstract] |  |  |
Gain on sale of rights | $ 24,898 | $ 46,792 | $ 0

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- Definition Gain on sale of our royalty and other rights related to future sales of BENLYSTA recognized in our consolidated statement of income. + References No definition available. + Details
Name: | biib_GainOnSaleOfRights
Namespace Prefix: | biib_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

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- Details
Name: | biib_GainOnSaleOfRightsAbstract
Namespace Prefix: | biib_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration