Accumulated Other Comprehensive Loss - Schedule of Reclassifications of Recognized Transactions out of AOCI (Details) - USD ($) $ in Millions	12 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Mar. 31, 2019
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Other (loss) income, net	$ (3.8)	$ 3.2	$ (2.2)
Net income	349.4	570.6	355.9
Reclassification out of Accumulated Other Comprehensive Income
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net income	(1.1)	(0.8)	(6.6)
Reclassification out of Accumulated Other Comprehensive Income | Unrealized losses on available-for-sale securities
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Other (loss) income, net	0.0	0.0	(5.6)
Reclassification out of Accumulated Other Comprehensive Income | Amortization of actuarial loss
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Other (loss) income, net	$ (1.1)	$ (0.8)	$ (1.0)