Retirement Benefits - Fair Value Pension Plan Assets (Details) - Pension Benefits - USD ($) $ in Millions	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 543.1	$ 515.4	$ 765.6
Cash and cash equivalents
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	18.0	9.8
Equity Mutual Funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	22.9	21.5
Common Pooled Equity Funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	98.5	84.5
U.S. Treasuries
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	42.4	47.2
State and Local Municipal Bonds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	7.6	6.4
Sovereign Debt
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	6.7	4.9
Corporate Bonds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	120.3	105.5
Fixed Income Mutual Funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	47.0	41.8
Common Pooled Fixed Income Funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	176.7	174.2
Alternative Investment Funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0.8	2.1
Common Pooled Fund
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	2.2	17.5
Quoted Prices in Active Markets for Identical Assets (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	82.4	67.2
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	12.1	3.5
Quoted Prices in Active Markets for Identical Assets (Level 1) | Equity Mutual Funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	22.9	21.5
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	47.0	41.8
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	0.4	0.4
Quoted Prices in Active Markets for Similar Assets (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	459.9	420.6
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	5.9	6.3
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	98.5	84.5
Quoted Prices in Active Markets for Similar Assets (Level 2) | U.S. Treasuries
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	42.4	47.2
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	7.6	6.4
Quoted Prices in Active Markets for Similar Assets (Level 2) | Sovereign Debt
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	6.7	4.9
Quoted Prices in Active Markets for Similar Assets (Level 2) | Corporate Bonds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	120.3	105.5
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	176.7	148.7
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	1.8	17.1
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	0.8	27.6
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	0.0	25.5
Investments Priced Using Net Asset Value | Alternative Investment Funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0.8	2.1
Investments Priced Using Net Asset Value | Common Pooled Fund
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 0.0	$ 0.0