Consolidated Statements of Comprehensive Income - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Statement of Comprehensive Income [Abstract]
Net income attributable to Biogen Inc.	$ 2,539.1	$ 3,702.8	$ 3,547.0
Other comprehensive income:
Unrealized gains (losses) recognized during the period, net of tax	(3.5)	(10.6)	(1.7)
Less: reclassification adjustment for (gains) losses included in net income, net of tax	12.7	0.6	1.3
Unrealized gains (losses) on securities available for sale, net of tax	9.2	(10.0)	(0.4)
Unrealized gains (losses) recognized during the period, net of tax	(193.8)	51.6	110.8
Less: reclassification adjustment for (gains) losses included in net income, net of tax	31.5	(4.0)	(172.3)
Unrealized gains (losses) on cash flow hedges, net of tax	(162.3)	47.6	(61.5)
Unrealized gains (losses) on pension benefit obligation, net of tax	(4.1)	5.1	(6.2)
Currency translation adjustment	158.7	(138.6)	(96.4)
Total other comprehensive income (loss), net of tax	1.5	(95.9)	(164.5)
Comprehensive income attributable to Biogen Inc.	2,540.6	3,606.9	3,382.5
Comprehensive income (loss) attributable to noncontrolling interests, net of tax	131.0	(7.1)	46.2
Comprehensive income	$ 2,671.6	$ 3,599.8	$ 3,428.7