Condensed Consolidated Statements of Comprehensive Income (Unaudited) - USD ($) $ in Millions	3 Months Ended
Mar. 31, 2020	Mar. 31, 2019
Statement of Comprehensive Income [Abstract]
Net income attributable to Biogen Inc.	$ 1,399.1	$ 1,408.8
Other comprehensive income:
Unrealized gains (losses) on securities available for sale, net of tax	(7.8)	6.9
Unrealized gains (losses) on cash flow hedges, net of tax	33.8	16.9
Gains (losses) on net investment hedges	23.0	14.0
Unrealized gains (losses) on pension benefit obligation, net of tax	0.8	0.6
Currency translation adjustment	(63.9)	(17.8)
Total other comprehensive income (loss), net of tax	(14.1)	20.6
Comprehensive income	1,385.0	1,429.4
Comprehensive income (loss) attributable to noncontrolling interests, net of tax	(5.9)	0.0
Comprehensive income attributable to Biogen Inc.	$ 1,379.1	$ 1,429.4

Amount after tax of increase (decrease) in equity from transactions and other events and circumstances from net income and other comprehensive income, attributable to parent entity. Excludes changes in equity resulting from investments by owners and distributions to owners.

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