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Summary of Significant Accounting Policies (Details) (USD $)
In Millions, unless otherwise specified
12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Business (Textual)      
Interest in subsidiary (less than given percentage) 100.00%biib_InterestInSubsidiary    
Payment terms of accounts receivable arising from product sales 30 to 90 days    
Estimated useful lives of leasehold improvements Lesser of the useful life or the term of the respective lease    
Purchase price of common stock under ESPP 85% of the lower of (i) the market value per share of the common stock on the participant's entry date into an offering period or (ii) the market value per share of the common stock on the purchase date    
Percentage of market value per share of common stock 85.00%biib_PercentageOfMarketValuePerShareOfCommonStock    
Compensation expense over purchase period The fair value of the look-back provision plus the 15% discount    
Discount rate recognized in compensation expense 15.00%biib_DiscountRateRecognizedInCompensationExpense    
Advertising costs $ 92.9us-gaap_AdvertisingExpense $ 72.7us-gaap_AdvertisingExpense $ 54.3us-gaap_AdvertisingExpense
Minimum | Building      
Property, Plant and Equipment      
Property, plant and equipment, useful life 15 years    
Minimum | Furniture and Fixtures      
Property, Plant and Equipment      
Property, plant and equipment, useful life 5 years    
Minimum | Machinery and Equipment      
Property, Plant and Equipment      
Property, plant and equipment, useful life 5 years    
Minimum | Computer Equipment      
Property, Plant and Equipment      
Property, plant and equipment, useful life 3 years    
Maximum | Building      
Property, Plant and Equipment      
Property, plant and equipment, useful life 40 years    
Maximum | Furniture and Fixtures      
Property, Plant and Equipment      
Property, plant and equipment, useful life 7 years    
Maximum | Machinery and Equipment      
Property, Plant and Equipment      
Property, plant and equipment, useful life 20 years    
Maximum | Computer Equipment      
Property, Plant and Equipment      
Property, plant and equipment, useful life 5 years    
Distributor One      
Property, Plant and Equipment      
Percentage receivables of wholesale distributor accounted in consolidated receivables 34.40%biib_PercentageReceivablesOfWholesaleDistributorAccountedInConsolidatedReceivables
/ biib_MajorDistributorAxis
= biib_DistributorOneMember
34.50%biib_PercentageReceivablesOfWholesaleDistributorAccountedInConsolidatedReceivables
/ biib_MajorDistributorAxis
= biib_DistributorOneMember
 
Distributor Two      
Property, Plant and Equipment      
Percentage receivables of wholesale distributor accounted in consolidated receivables 23.30%biib_PercentageReceivablesOfWholesaleDistributorAccountedInConsolidatedReceivables
/ biib_MajorDistributorAxis
= biib_DistributorTwoMember
15.70%biib_PercentageReceivablesOfWholesaleDistributorAccountedInConsolidatedReceivables
/ biib_MajorDistributorAxis
= biib_DistributorTwoMember