Comprehensive Income (Loss) - Schedule of Reclassifications of Recognized Transactions out of AOCI (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Other income (loss)	$ (0.2)	$ 5.8	$ (10.0)	$ 10.1
Net income	96.3	189.2	132.0	359.7
Reclassification out of accumulated other comprehensive income
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net income	0.2	0.0	6.1	0.0
Reclassification out of accumulated other comprehensive income | Unrealized holding gains (losses) available-for-sale debt securities
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Other income (loss)	0.0	0.0	5.6	0.0
Reclassification out of accumulated other comprehensive income | Amortization of actuarial loss
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Other income (loss)	$ 0.2	$ 0.0	$ 0.5	$ 0.0