Retirement Benefits - Fair Value Measurements (Details) - USD ($) $ in Millions	Mar. 31, 2020	Mar. 31, 2019	Mar. 31, 2018
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 446.9	$ 480.2
Quoted Prices in Active  Market (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	22.8	20.2
Significant Other Observable  Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0.0	0.0
Significant Unobservable Inputs (Level 3)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	32.6	29.9
Fair Value Measured at Net Asset Value Per Share
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	391.5	430.1
Cash and cash equivalents
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	11.1	20.2
Cash and cash equivalents | Quoted Prices in Active  Market (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	11.1	20.2
Cash and cash equivalents | Significant Other Observable  Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0.0	0.0
Cash and cash equivalents | Significant Unobservable Inputs (Level 3)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		0.0
Cash and cash equivalents | Fair Value Measured at Net Asset Value Per Share
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0.0	0.0
Fixed income funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	272.4	304.6
Fixed income funds | Quoted Prices in Active  Market (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0.0	0.0
Fixed income funds | Significant Other Observable  Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0.0	0.0
Fixed income funds | Significant Unobservable Inputs (Level 3)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0.0	0.0
Fixed income funds | Fair Value Measured at Net Asset Value Per Share
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	272.4	304.6
US equity funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	67.9	54.3
US equity funds | Quoted Prices in Active  Market (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	11.7	0.0
US equity funds | Significant Other Observable  Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0.0	0.0
US equity funds | Significant Unobservable Inputs (Level 3)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0.0	0.0
US equity funds | Fair Value Measured at Net Asset Value Per Share
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	56.2	54.3
International equity funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	35.7	33.4
International equity funds | Quoted Prices in Active  Market (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0.0	0.0
International equity funds | Significant Other Observable  Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0.0	0.0
International equity funds | Significant Unobservable Inputs (Level 3)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0.0	0.0
International equity funds | Fair Value Measured at Net Asset Value Per Share
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	35.7	33.4
Balanced funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	5.6	6.4
Balanced funds | Quoted Prices in Active  Market (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0.0	0.0
Balanced funds | Significant Other Observable  Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0.0	0.0
Balanced funds | Significant Unobservable Inputs (Level 3)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0.0	0.0
Balanced funds | Fair Value Measured at Net Asset Value Per Share
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	5.6	6.4
Alternative Investments
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	21.6	31.4
Alternative Investments | Quoted Prices in Active  Market (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0.0	0.0
Alternative Investments | Significant Other Observable  Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0.0	0.0
Alternative Investments | Significant Unobservable Inputs (Level 3)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0.0	0.0
Alternative Investments | Fair Value Measured at Net Asset Value Per Share
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	21.6	31.4
Insurance contracts
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	32.6	29.9
Insurance contracts | Quoted Prices in Active  Market (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0.0	0.0
Insurance contracts | Significant Other Observable  Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0.0	0.0
Insurance contracts | Significant Unobservable Inputs (Level 3)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	32.6	29.9	$ 30.2
Insurance contracts | Fair Value Measured at Net Asset Value Per Share
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 0.0	$ 0.0