Accumulated Other Comprehensive Loss - Schedule of Reclassifications of Recognized Transactions out of AOCI (Details) - USD ($) $ in Millions	3 Months Ended				6 Months Ended
	Sep. 30, 2019	Jun. 30, 2019	Sep. 30, 2018	Jun. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Other (loss) income, net	$ (1.4)		$ (0.2)		$ 1.3	$ (10.0)
Net income	108.9	$ 50.7	96.3	$ 35.7	159.6	132.0
Reclassification out of Accumulated Other Comprehensive Income
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net income	(0.2)		(0.2)		(0.4)	(6.1)
Reclassification out of Accumulated Other Comprehensive Income | Unrealized losses on available-for-sale debt securities
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Other (loss) income, net	0.0		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	$ (0.2)		$ (0.2)		$ (0.4)	$ (0.5)