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Accounts Receivable (Details Textual)
3 Months Ended 3 Months Ended
Mar. 31, 2015
USD ($)
Mar. 31, 2014
USD ($)
Dec. 31, 2014
USD ($)
Jun. 30, 2014
Italy
USD ($)
Dec. 31, 2011
Italy
Mar. 31, 2015
Italy
EUR (€)
Dec. 31, 2014
Italy
EUR (€)
Accounts Receivable (Textual) [Abstract]              
Payment terms of accounts receivable arising from product sales 30 to 90 days            
Minimum term for accounts receivable 30 days            
Maximum term for accounts receivable 90 days            
Net accounts receivable balances overdue more than one year $ 11,400,000biib_AccountsReceivableOutstandingForGreaterThanOneYear   $ 11,100,000biib_AccountsReceivableOutstandingForGreaterThanOneYear        
Reimbursement limit stated in the resolution         24 months    
Product, net 2,172,322,000us-gaap_SalesRevenueGoodsNet 1,742,765,000us-gaap_SalesRevenueGoodsNet   53,500,000us-gaap_SalesRevenueGoodsNet
/ us-gaap_StatementGeographicalAxis
= country_IT
     
Proposed settlement             35,600,000us-gaap_LossContingencyEstimateOfPossibleLoss
/ us-gaap_StatementGeographicalAxis
= country_IT
Amount reserved for this matter           € 75,000,000us-gaap_DeferredCreditsAndOtherLiabilitiesCurrent
/ us-gaap_StatementGeographicalAxis
= country_IT