Consolidated Statements of Comprehensive Income - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Statement of Comprehensive Income [Abstract]
Net income attributable to Biogen Inc.	$ 1,556.1	$ 4,000.6	$ 5,888.5
Other comprehensive income:
Unrealized gains (losses) on securities available for sale, net of tax	(3.6)	(2.8)	8.2
Unrealized gains (losses) on cash flow hedges, net of tax	232.8	(186.8)	(26.9)
Gains (losses) on net investment hedges, net of tax	34.0	(33.6)	21.6
Unrealized gains (losses) on pension benefit obligation, net of tax	21.5	(33.5)	(1.5)
Currency translation adjustment	(92.4)	92.9	103.8
Total other comprehensive income (loss), net of tax	192.3	(163.8)	105.2
Comprehensive income attributable to Biogen Inc.	1,748.4	3,836.8	5,993.7
Comprehensive income (loss) attributable to noncontrolling interests, net of tax	172.1	60.9	(0.4)
Comprehensive income	$ 1,920.5	$ 3,897.7	$ 5,993.3