Consolidated Statements of Comprehensive Income - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Statement of Comprehensive Income [Abstract]
Net income attributable to Biogen Inc.	$ 5,888.5	$ 4,430.7	$ 2,539.1
Other comprehensive income:
Unrealized gains (losses) recognized during the period, net of tax	11.8	(10.6)	(3.5)
Less: reclassification adjustment for (gains) losses included in net income, net of tax	(3.6)	6.7	12.7
Unrealized gains (losses) on securities available for sale, net of tax	8.2	(3.9)	9.2
Unrealized gains (losses) recognized during the period, net of tax	88.1	97.4	(193.8)
Less: reclassification adjustment for (gains) losses included in net income, net of tax	(115.0)	41.8	31.5
Unrealized gains (losses) on cash flow hedges, net of tax	(26.9)	139.2	(162.3)
Gains (Losses) on net investment hedge recognized during period	28.6	5.0	0.0
Less: Reclassification of net investment hedge forward points amortization	(7.0)	(1.5)	0.0
Gains (losses) on net investment hedges	21.6	3.5	0.0
Unrealized gains (losses) on pension benefit obligation, net of tax	(1.5)	5.5	(4.1)
Currency translation adjustment	103.8	(67.8)	158.7
Total other comprehensive income (loss), net of tax	105.2	76.5	1.5
Comprehensive income attributable to Biogen Inc.	5,993.7	4,507.2	2,540.6
Comprehensive income (loss) attributable to noncontrolling interests, net of tax	0.4	(42.9)	(131.0)
Comprehensive income	$ 5,993.3	$ 4,550.1	$ 2,671.6