Accumulated Other Comprehensive Income (Loss) (Details 1) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Other income (expense)									$ (123.7)	$ (25.8)	$ (34.9)
Income tax expense									1,161.6	989.9	601.0
Product, net	$ (2,425.9)	$ (2,391.7)	$ (2,198.6)	$ (2,172.3)	$ (2,287.0)	$ (2,117.3)	$ (2,056.3)	$ (1,742.8)	(9,188.5)	(8,203.4)	(5,542.3)
Net income attributable to Biogen Inc.	$ 831.6	$ 965.6	$ 927.3	$ 822.5	$ 883.5	$ 856.9	$ 714.5	$ 480.0	3,547.0	2,934.8	1,862.3
Reclassification out of Accumulated Other Comprehensive Income [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net income attributable to Biogen Inc.									171.0	12.7	(3.3)
Accumulated Net Unrealized Investment Gain (Loss) [Member] | Reclassification out of Accumulated Other Comprehensive Income [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Other income (expense)									(2.0)	9.9	15.9
Income tax expense									0.7	(3.5)	(5.5)
Accumulated Net Gain (Loss) from Designated or Qualifying Cash Flow Hedges [Member] | Reclassification out of Accumulated Other Comprehensive Income [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Other income (expense)									(0.1)	0.0	0.0
Income tax expense									(0.8)	(0.5)	(0.5)
Product, net									$ (173.2)	$ (6.8)	$ (13.2)