Comprehensive Income (Loss) - Schedule of Reclassifications of Recognized Transactions out of AOCI (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2018	Dec. 31, 2017
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Other income (loss)	$ (2.5)	$ (2.9)	$ (12.4)	$ 7.4
Net income	49.2	(251.1)	181.2	108.7
Reclassification out of accumulated other comprehensive income
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net income	(0.3)	(0.2)	(6.4)	(0.6)
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)	$ (0.3)	$ (0.2)	$ (0.8)	$ (0.6)