Accumulated Other Comprehensive Income (Loss) Accumulated Other Comprehensive Income (Loss) (Details 1) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Other income (expense), net	$ (58.1)	$ (15.4)	$ (169.4)	$ (41.3)
Income tax benefit (expense)	337.0	330.1	1,047.0	904.5
Product, net	(2,539.6)	(2,391.7)	(7,315.0)	(6,762.6)
Net income attributable to Biogen Inc.	1,032.9	965.6	3,053.6	2,715.4
Reclassification out of Accumulated Other Comprehensive Income
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net income attributable to Biogen Inc.	(5.3)	42.9	(1.3)	117.7
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	0.0	(1.3)	(0.6)	(1.6)
Income tax benefit (expense)	0.0	0.5	0.2	0.6
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	0.1	0.0	0.2	0.0
Income tax benefit (expense)	0.0	(0.2)	0.0	(0.6)
Product, net	(5.2)	(43.9)	(0.7)	(119.3)
Operating expenses	$ (0.2)	$ 0.0	$ (0.4)	$ 0.0