Supplemental Financial Information - Comprehensive Income (Schedule Of Reclassifications Of Recognized Transactions Out Of AOCI) (Details) - USD ($) $ in Millions	12 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Mar. 31, 2017
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Other (loss) income, net	$ (2.2)	$ (5.8)	$ 1.3
Net income	355.9	255.4	164.6
Reclassification out of Accumulated Other Comprehensive Income
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net income	(6.6)	(16.0)	(1.5)
Reclassification out of Accumulated Other Comprehensive Income | Unrealized Holding Gains (Losses) Available-for-sale Securities
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Other (loss) income, net	(5.6)	(15.2)	(1.5)
Reclassification out of Accumulated Other Comprehensive Income | Minimum Pension Liability
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Other (loss) income, net	$ (1.0)	$ (0.8)	$ 0.0