v2.4.0.6

Gain on Sale of Rights Gain on Sale of Rights (Detail) (USD $) In Thousands, unless otherwise specified | 3 Months Ended | 9 Months Ended
--- | --- | ---
Sep. 30, 2012 | Sep. 30, 2011 | Sep. 30, 2012 | Sep. 30, 2011
Gain on Sale of Rights [Abstract] |  |  |  |
Gain on sale of rights | $ 31,719 | $ 0 | $ 31,719 | $ 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