Accumulated Other Comprehensive Income (Loss) Accumulated Other Comprehensive Income (Loss) (Details 1) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Other income (expense), net	$ (41.0)	$ (38.0)
Income tax expense (benefit)	322.5	239.2
Product, net	2,523.5	2,380.1
Net income attributable to Biogen Inc.	1,172.9	747.6
Reclassification out of Accumulated Other Comprehensive Income
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net income attributable to Biogen Inc.	(37.4)	6.3
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	(7.6)	(0.7)
Income tax expense (benefit)	1.6	0.3
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	0.1
Income tax expense (benefit)	0.1	0.0
Product, net	32.9	6.7
Operating expenses	$ 1.3	$ 0.1