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Property, Plant and Equipment (Details Textual) (USD $)
In Millions, unless otherwise specified
 12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Property, Plant and Equipment      
Depreciation expense $ 198.4us-gaap_Depreciation $ 187.8us-gaap_Depreciation $ 164.3us-gaap_Depreciation
Interest cost capitalization related to construction in progress 6.4us-gaap_InterestCostsCapitalized 7.8us-gaap_InterestCostsCapitalized 25.4us-gaap_InterestCostsCapitalized
Cambridge Leases      
Property, Plant and Equipment      
Charges recognized to vacate building   $ 27.2biib_Chargesrecognizedtovacatebuilding
/ biib_FacilityLocationAxis
= biib_CambridgeLeasesMember