Retirement Benefits - Deferred Compensation Plan (Details) - Supplemental Employee Retirement Plan - USD ($) $ in Millions	Mar. 31, 2019	Mar. 31, 2018
Defined Benefit Plan Disclosure [Line Items]
Plan assets	$ 6.0	$ 3.5
Plan liabilities	6.1	3.5
Level 1
Defined Benefit Plan Disclosure [Line Items]
Plan assets	2.3	1.6
Plan liabilities	6.1	3.5
Level 2
Defined Benefit Plan Disclosure [Line Items]
Plan assets	3.7	1.9
Plan liabilities	0.0	0.0
Level 3
Defined Benefit Plan Disclosure [Line Items]
Plan assets	0.0	0.0
Plan liabilities	0.0	0.0
Mutual Funds
Defined Benefit Plan Disclosure [Line Items]
Plan assets	2.3	1.6
Mutual Funds | Level 1
Defined Benefit Plan Disclosure [Line Items]
Plan assets	2.3	1.6
Mutual Funds | Level 2
Defined Benefit Plan Disclosure [Line Items]
Plan assets	0.0	0.0
Mutual Funds | Level 3
Defined Benefit Plan Disclosure [Line Items]
Plan assets	0.0	0.0
Corporate owned life insurance
Defined Benefit Plan Disclosure [Line Items]
Plan assets	3.7	1.9
Corporate owned life insurance | Level 1
Defined Benefit Plan Disclosure [Line Items]
Plan assets	0.0	0.0
Corporate owned life insurance | Level 2
Defined Benefit Plan Disclosure [Line Items]
Plan assets	3.7	1.9
Corporate owned life insurance | Level 3
Defined Benefit Plan Disclosure [Line Items]
Plan assets	$ 0.0	$ 0.0