Accumulated Other Comprehensive Income (Loss) Accumulated Other Comprehensive Income (Loss) (Details 1) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Other income (expense), net	$ (15,413)	$ (16,290)	$ (41,288)	$ (17,030)
Income tax benefit (expense)	330,093	274,774	904,475	721,709
Product, net	2,391,717	2,117,366	6,762,605	5,916,423
Net income attributable to Biogen Inc.	965,622	856,858	2,715,439	2,051,320
Reclassification out of Accumulated Other Comprehensive Income
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net income attributable to Biogen Inc.	42,900	2,700	117,700	(800)
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	(1,300)	(100)	(1,600)	10,100
Income tax benefit (expense)	500	0	600	(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]
Other income (expense), net	0	0	0	0
Income tax benefit (expense)	(200)	(100)	(600)	(200)
Product, net	$ 43,900	$ 2,900	$ 119,300	$ 7,100