Comprehensive Income (Schedule of Changes in the Components of AOCI) (Details) - USD ($) $ in Millions	12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Mar. 31, 2016
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Net other comprehensive loss	$ (3.2)	$ (11.0)	$ (14.1)
Accumulated Net Unrealized Investment Gain (Loss)
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Stockholders' Equity Attributable to Parent	0.3	0.3
Other comprehensive loss before reclassifications	(13.6)	(1.5)
Amounts reclassified from accumulated other comprehensive income (loss)	15.2	1.5
Net other comprehensive loss	1.6	0.0
Stockholders' Equity Attributable to Parent	1.9	0.3	0.3
Accumulated Defined Benefit Plans Adjustment
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Stockholders' Equity Attributable to Parent	(5.3)	0.0
Other comprehensive loss before reclassifications	(5.6)	(5.3)
Amounts reclassified from accumulated other comprehensive income (loss)	0.8	0.0
Net other comprehensive loss	(4.8)	(5.3)
Stockholders' Equity Attributable to Parent	(10.1)	(5.3)	0.0
Accumulated Translation Adjustment
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Stockholders' Equity Attributable to Parent	(9.4)	(3.7)
Other comprehensive loss before reclassifications	0.0	(5.7)
Amounts reclassified from accumulated other comprehensive income (loss)	0.0	0.0
Net other comprehensive loss	0.0	(5.7)
Stockholders' Equity Attributable to Parent	(9.4)	(9.4)	(3.7)
Accumulated Other Comprehensive Income
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Stockholders' Equity Attributable to Parent	(14.4)	(3.4)
Other comprehensive loss before reclassifications	(19.2)	(12.5)
Amounts reclassified from accumulated other comprehensive income (loss)	16.0	1.5
Net other comprehensive loss	(3.2)	(11.0)
Stockholders' Equity Attributable to Parent	$ (17.6)	$ (14.4)	$ (3.4)