Accumulated Other Comprehensive Income (Loss) (Details 1) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Other income (expense)									$ (217.4)	$ (123.7)	$ (25.8)
Income tax expense									1,237.3	1,161.6	989.9
Product, net	$ 2,502.9	$ 2,539.6	$ 2,466.0	$ 2,309.4	$ 2,425.9	$ 2,391.7	$ 2,198.6	$ 2,172.3	9,817.9	9,188.5	8,203.4
Net income attributable to Biogen Inc.	$ 649.2	$ 1,032.9	$ 1,049.8	$ 970.9	$ 831.6	$ 965.6	$ 927.3	$ 822.5	3,702.8	3,547.0	2,934.8
Reclassification out of Accumulated Other Comprehensive Income [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net income attributable to Biogen Inc.									3.4	171.0	12.7
Unrealized Gains (Losses) on Securities Available for Sale | Reclassification out of Accumulated Other Comprehensive Income [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Other income (expense)									(0.9)	(2.0)	9.9
Income tax expense									0.3	0.7	(3.5)
Unrealized Gains (Losses) on Cash Flow Hedges | Reclassification out of Accumulated Other Comprehensive Income [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Other income (expense)									0.2	(0.1)	0.0
Income tax expense									0.0	(0.8)	(0.5)
Product, net									5.3	173.2	6.8
Operating Expenses									$ (1.5)	$ 0.0	$ 0.0