CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME - USD ($) $ in Millions	12 Months Ended
Mar. 31, 2020	Mar. 31, 2019	Mar. 31, 2018
Statement of Comprehensive Income [Abstract]
Net income	$ 570.6	$ 355.9	$ 255.4
Available-for-sale securities:
Unrealized holding losses, net of tax effect	0.0	(5.6)	(13.6)
Reclassification of realized transactions, net of tax effect	0.0	5.6	15.2
Defined benefit plans:
Actuarial gains (losses) related to defined benefit pension plans, net of tax (provision) benefit	1.4	2.9	(5.6)
Reclassification of realized transactions, net of tax effect	0.8	1.0	0.8
Change in net foreign currency translation adjustment	(1.8)	(5.3)	0.0
Other comprehensive income (loss), net of tax effect	0.4	(1.4)	(3.2)
Comprehensive income	$ 571.0	$ 354.5	$ 252.2

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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