Retirement Benefits - Accumulated Other Comprehensive Income (Details) - USD ($) $ in Millions	Mar. 31, 2020	Mar. 31, 2019
Defined Benefit Plan Disclosure [Line Items]
Unrecognized prior service credit	$ (1.2)	$ (1.5)
Unrecognized actuarial loss (gain)	55.4	50.9
Accumulated other comprehensive loss (income), gross	54.2	49.4
Deferred income tax (benefit) provision	(13.6)	(12.1)
Accumulated other comprehensive loss (income), net	40.6	37.3
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Unrecognized prior service credit	(0.1)	(0.1)
Unrecognized actuarial loss (gain)	56.5	52.6
Accumulated other comprehensive loss (income), gross	56.4	52.5
Deferred income tax (benefit) provision	(14.1)	(12.9)
Accumulated other comprehensive loss (income), net	42.3	39.6
Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Unrecognized prior service credit	(1.1)	(1.4)
Unrecognized actuarial loss (gain)	(1.1)	(1.7)
Accumulated other comprehensive loss (income), gross	(2.2)	(3.1)
Deferred income tax (benefit) provision	0.5	0.8
Accumulated other comprehensive loss (income), net	$ (1.7)	$ (2.3)