Retirement Benefits - Accumulated Other Comprehensive Income (Details) - USD ($) $ in Millions	Mar. 31, 2019	Mar. 31, 2018
Defined Benefit Plan Disclosure [Line Items]
Unrecognized prior service credit	$ (1.5)	$ (1.4)
Unrecognized actuarial loss (gain)	50.9	44.5
Accumulated other comprehensive loss (income), gross	49.4	43.1
Deferred income tax (benefit) provision	(12.1)	(14.9)
Accumulated other comprehensive loss (income), net	37.3	28.2
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Unrecognized prior service credit	(0.1)	(0.2)
Unrecognized actuarial loss (gain)	52.6	46.2
Accumulated other comprehensive loss (income), gross	52.5	46.0
Deferred income tax (benefit) provision	(12.9)	(15.9)
Accumulated other comprehensive loss (income), net	39.6	30.1
Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Unrecognized prior service credit	(1.4)	(1.2)
Unrecognized actuarial loss (gain)	(1.7)	(1.7)
Accumulated other comprehensive loss (income), gross	(3.1)	(2.9)
Deferred income tax (benefit) provision	0.8	1.0
Accumulated other comprehensive loss (income), net	$ (2.3)	$ (1.9)