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Accumulated Other Comprehensive Income (Loss) Accumulated Other Comprehensive Income (Loss) (Details 1) (USD $)
In Thousands, unless otherwise specified
 3 Months Ended 9 Months Ended
Sep. 30, 2014
Sep. 30, 2013
Sep. 30, 2014
Sep. 30, 2013
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]        
Other income (expense) $ (16,290) $ (4,640) $ (17,030) $ (29,525)
Income tax benefit (expense) 274,774 186,105 721,709 410,753
Product, net 2,117,366 1,453,554 5,916,423 3,935,251
Net income attributable to Biogen Idec Inc. 856,858 487,616 2,051,320 1,405,035
Reclassification out of Accumulated Other Comprehensive Income
        
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]        
Net income attributable to Biogen Idec Inc. 2,700 (7,200) (800) (3,500)
Accumulated Net Unrealized Investment Gain (Loss) | Reclassification out of Accumulated Other Comprehensive Income
        
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]        
Other income (expense) (100) 200 10,100 4,500
Income tax benefit (expense) 0 (100) (3,600) (1,600)
Accumulated Net Gain (Loss) from Designated or Qualifying Cash Flow Hedges | Reclassification out of Accumulated Other Comprehensive Income
        
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]        
Income tax benefit (expense) (100) 0 (200) (300)
Product, net $ 2,900 $ (7,300) $ (7,100) $ (6,100)