Retirement Benefits - Fair Value Pension Plan Assets (Details) - Pension Benefits - USD ($) $ in Millions	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 765.6	$ 805.1	$ 743.4
Cash and cash equivalents
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	8.9	26.6
Equity Mutual Funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	31.7	29.8
Common Pooled Equity Funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	123.3	153.9
U.S. Treasuries
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	55.0	52.0
State and Local Municipal Bonds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	6.2	9.3
Sovereign Debt
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	7.1	9.3
Corporate Bonds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	141.6	145.1
Fixed Income Mutual Funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	55.5	59.9
Common Pooled Fixed Income Funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	306.3	279.4
Alternative Investment Funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	7.1	14.3
Common Pooled Fund
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	22.9	25.5
Quoted Prices in Active Markets for Identical Assets (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	96.6	116.8
Quoted Prices in Active Markets for Identical Assets (Level 1) | Cash and cash equivalents
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	8.9	26.6
Quoted Prices in Active Markets for Identical Assets (Level 1) | Equity Mutual Funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	31.7	29.8
Quoted Prices in Active Markets for Identical Assets (Level 1) | Common Pooled Equity Funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0.0	0.0
Quoted Prices in Active Markets for Identical Assets (Level 1) | U.S. Treasuries
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0.0	0.0
Quoted Prices in Active Markets for Identical Assets (Level 1) | State and Local Municipal Bonds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0.0	0.0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Sovereign Debt
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0.0	0.0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Corporate Bonds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0.0	0.0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Fixed Income Mutual Funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	55.5	59.9
Quoted Prices in Active Markets for Identical Assets (Level 1) | Common Pooled Fixed Income Funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0.0	0.0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Alternative Investment Funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0.0	0.0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Common Pooled Fund
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0.5	0.5
Quoted Prices in Active Markets for Similar Assets (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	630.8	674.0
Quoted Prices in Active Markets for Similar Assets (Level 2) | Cash and cash equivalents
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0.0	0.0
Quoted Prices in Active Markets for Similar Assets (Level 2) | Equity Mutual Funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0.0	0.0
Quoted Prices in Active Markets for Similar Assets (Level 2) | Common Pooled Equity Funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	123.3	153.9
Quoted Prices in Active Markets for Similar Assets (Level 2) | U.S. Treasuries
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	55.0	52.0
Quoted Prices in Active Markets for Similar Assets (Level 2) | State and Local Municipal Bonds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	6.2	9.3
Quoted Prices in Active Markets for Similar Assets (Level 2) | Sovereign Debt
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	7.1	9.3
Quoted Prices in Active Markets for Similar Assets (Level 2) | Corporate Bonds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	141.6	145.1
Quoted Prices in Active Markets for Similar Assets (Level 2) | Fixed Income Mutual Funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0.0	0.0
Quoted Prices in Active Markets for Similar Assets (Level 2) | Common Pooled Fixed Income Funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	275.2	279.4
Quoted Prices in Active Markets for Similar Assets (Level 2) | Alternative Investment Funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0.0	0.0
Quoted Prices in Active Markets for Similar Assets (Level 2) | Common Pooled Fund
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	22.4	25.0
Unobservable inputs for which little or no market data exists (Level 3)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0.0	0.0
Unobservable inputs for which little or no market data exists (Level 3) | Cash and cash equivalents
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0.0	0.0
Unobservable inputs for which little or no market data exists (Level 3) | Equity Mutual Funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0.0	0.0
Unobservable inputs for which little or no market data exists (Level 3) | Common Pooled Equity Funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0.0	0.0
Unobservable inputs for which little or no market data exists (Level 3) | U.S. Treasuries
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0.0	0.0
Unobservable inputs for which little or no market data exists (Level 3) | State and Local Municipal Bonds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0.0	0.0
Unobservable inputs for which little or no market data exists (Level 3) | Sovereign Debt
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0.0	0.0
Unobservable inputs for which little or no market data exists (Level 3) | Corporate Bonds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0.0	0.0
Unobservable inputs for which little or no market data exists (Level 3) | Fixed Income Mutual Funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0.0	0.0
Unobservable inputs for which little or no market data exists (Level 3) | Common Pooled Fixed Income Funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0.0	0.0
Unobservable inputs for which little or no market data exists (Level 3) | Alternative Investment Funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0.0	0.0
Unobservable inputs for which little or no market data exists (Level 3) | Common Pooled Fund
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0.0	0.0
Investments Priced Using Net Asset Value
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	38.2	14.3
Investments Priced Using Net Asset Value | Cash and cash equivalents
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0.0	0.0
Investments Priced Using Net Asset Value | Equity Mutual Funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0.0	0.0
Investments Priced Using Net Asset Value | Common Pooled Equity Funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0.0	0.0
Investments Priced Using Net Asset Value | U.S. Treasuries
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0.0	0.0
Investments Priced Using Net Asset Value | State and Local Municipal Bonds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0.0	0.0
Investments Priced Using Net Asset Value | Sovereign Debt
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0.0	0.0
Investments Priced Using Net Asset Value | Corporate Bonds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0.0	0.0
Investments Priced Using Net Asset Value | Fixed Income Mutual Funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0.0	0.0
Investments Priced Using Net Asset Value | Common Pooled Fixed Income Funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	31.1	0.0
Investments Priced Using Net Asset Value | Alternative Investment Funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	7.1	14.3
Investments Priced Using Net Asset Value | Common Pooled Fund
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 0.0	$ 0.0