Accumulated Other Comprehensive Income (Loss) Accumulated Other Comprehensive Income (Loss) (Details 1) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Other income (expense), net	$ (10,889)	$ 4,861	$ (25,875)	$ (740)
Income tax benefit (expense)	292,501	268,521	574,382	446,935
Product, net	(2,198,566)	(2,056,292)	(4,370,888)	(3,799,057)
Net income attributable to Biogen Inc.	927,276	714,506	1,749,817	1,194,462
Reclassification out of Accumulated Other Comprehensive Income
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net income attributable to Biogen Inc.	40,100	700	74,800	(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), net	(200)	10,100	(300)	10,200
Income tax benefit (expense)	100	(3,600)	100	(3,600)
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)	(200)	(500)	(400)	(100)
Product, net	$ (40,400)	$ (5,300)	$ (75,400)	$ (10,000)