Accumulated Other Comprehensive Income (Loss) Accumulated Other Comprehensive Income (Loss) (Details 1) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Other income (expense), net	$ 115.1	$ (44.0)	$ 39.6	$ (150.6)
Income tax expense (benefit)	369.8	383.8	956.0	892.6
Product revenues	(3,439.0)	(3,077.8)	(9,926.6)	(8,966.9)
Net income attributable to Biogen Inc.	1,444.4	1,226.1	3,483.9	2,836.5
Reclassification out of Accumulated Other Comprehensive Income
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net income attributable to Biogen Inc.	(8.7)	(18.8)	(56.8)	(15.3)
Accumulated net unrealized gains (losses) on securities available-for-sale | Reclassification out of Accumulated Other Comprehensive Income
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Other income (expense), net	(0.1)	(0.9)	(7.7)	(1.7)
Income tax expense (benefit)	0.0	0.3	1.6	0.6
Accumulated net unrealized gains (losses) on 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	0.1	0.2	0.2
Income tax expense (benefit)	0.0	0.0	0.2	0.0
Product revenues	(8.4)	(18.8)	(51.7)	(15.1)
Operating expenses	$ 0.3	$ 0.5	$ 0.6	$ 0.7