Comprehensive Income (Loss) (Schedule of Reclassifications of Recognized Transactions out of AOCI) (Details) - USD ($) $ in Millions	3 Months Ended
	Jun. 30, 2018	Jun. 30, 2017
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Other income (loss)	$ (9.8)	$ 4.5
Net income	35.7	170.6
Unrealized holding gains (losses) on available-for-sale securities | Reclassification out of Accumulated Other Comprehensive Income
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Other income (loss)	5.6	0.0
Net income	5.9	0.2
Amortization of actuarial loss | Reclassification out of Accumulated Other Comprehensive Income
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Other income (loss)	$ 0.3	$ 0.2