Accumulated Other Comprehensive Loss - Schedule of Reclassifications of Recognized Transactions out of AOCI (Details) - USD ($) $ in Millions	3 Months Ended						9 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Dec. 31, 2019	Dec. 31, 2018
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Other loss, net	$ (1.5)			$ (2.5)			$ (0.2)	$ (12.4)
Net income	311.1	$ 108.9	$ 50.7	49.2	$ 96.3	$ 35.7	470.7	181.2
Reclassification out of Accumulated Other Comprehensive Income
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net income	(0.2)			(0.3)			(0.6)	(6.4)
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, 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, net	$ (0.2)			$ (0.3)			$ (0.6)	$ (0.8)