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Equity (Details Textual) (USD $)
In Thousands, except Share data, unless otherwise specified
 12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Feb. 28, 2011
Class of Stock [Line Items]        
Preferred stock, shares authorized 8,000,000us-gaap_PreferredStockSharesAuthorized      
Equity (Textual)        
Common stock shares authorized for repurchase 1,300,000us-gaap_StockRepurchaseProgramNumberOfSharesAuthorizedToBeRepurchased     20,000,000us-gaap_StockRepurchaseProgramNumberOfSharesAuthorizedToBeRepurchased
Repurchase of common stock, shares 2,900,000us-gaap_TreasuryStockSharesAcquired 2,000,000us-gaap_TreasuryStockSharesAcquired    
Repurchase of common stock, value $ 886,779us-gaap_PaymentsForRepurchaseOfCommonStock $ 400,309us-gaap_PaymentsForRepurchaseOfCommonStock $ 984,715us-gaap_PaymentsForRepurchaseOfCommonStock  
Series A        
Class of Stock [Line Items]        
Preferred stock, shares authorized 1,750,000us-gaap_PreferredStockSharesAuthorized
/ us-gaap_StatementClassOfStockAxis
= us-gaap_SeriesAPreferredStockMember		      
Series X junior participating        
Class of Stock [Line Items]        
Preferred stock, shares authorized 1,000,000us-gaap_PreferredStockSharesAuthorized
/ us-gaap_StatementClassOfStockAxis
= biib_SeriesXPreferredStockMember		      
Undesignated        
Class of Stock [Line Items]        
Preferred stock, shares authorized 5,250,000us-gaap_PreferredStockSharesAuthorized
/ us-gaap_StatementClassOfStockAxis
= biib_UndesignatedPreferredStockMember