Retirement Benefits - Fair Value Pension Plan Assets (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 743.4	$ 670.6
Cash and cash equivalents
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	16.6	47.0
US Large-Cap
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		8.2
US Mid-Cap and Small-Cap Growth
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		3.1
International Large-cap
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		8.6
Equity Mutual Funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	31.6
Common Pooled Equity Funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	144.0	12.7
Emerging Markets
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		13.3
U.S. Treasuries
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	64.5	378.0
State and Local Municipal Bonds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	9.4
Sovereign Debt
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	7.6
Corporate Bonds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	122.9	0.3
Fixed Income Mutual Funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	54.2
Common Pooled Fixed Income Funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	252.3	61.8
Asset Backed Securities and Other
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		22.8
Derivative Assets
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		8.2
Derivative Liability
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		(7.9)
Alternative Investment Funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	18.1	96.8
Common Pooled Fund
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	22.2	17.7
Quoted Prices in Active Markets for Identical Assets (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	103.0	88.9
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	16.6	47.0
Quoted Prices in Active Markets for Identical Assets (Level 1) | US Large-Cap
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		8.2
Quoted Prices in Active Markets for Identical Assets (Level 1) | US Mid-Cap and Small-Cap Growth
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		3.1
Quoted Prices in Active Markets for Identical Assets (Level 1) | International Large-cap
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		8.6
Quoted Prices in Active Markets for Identical Assets (Level 1) | Equity Mutual Funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	31.6
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) | Emerging Markets
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		8.1
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
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
Quoted Prices in Active Markets for Identical Assets (Level 1) | Corporate Bonds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0.1	0.3
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	54.2
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) | Asset Backed Securities and Other
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		0.0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Derivative Assets
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		8.0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Derivative Liability
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		(6.9)
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	11.6
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	0.9
Quoted Prices in Active Markets for Similar Assets (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	623.2	495.6
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	0.0	0.0
Quoted Prices in Active Markets for Similar Assets (Level 2) | US Large-Cap
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		0.0
Quoted Prices in Active Markets for Similar Assets (Level 2) | US Mid-Cap and Small-Cap Growth
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		0.0
Quoted Prices in Active Markets for Similar Assets (Level 2) | International Large-cap
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		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
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	144.0	12.7
Quoted Prices in Active Markets for Similar Assets (Level 2) | Emerging Markets
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		5.2
Quoted Prices in Active Markets for Similar Assets (Level 2) | U.S. Treasuries
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	64.5	378.0
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	9.4
Quoted Prices in Active Markets for Similar Assets (Level 2) | Sovereign Debt
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	7.6
Quoted Prices in Active Markets for Similar Assets (Level 2) | Corporate Bonds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	122.8	0.0
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
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	252.3	57.6
Quoted Prices in Active Markets for Similar Assets (Level 2) | Asset Backed Securities and Other
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		22.8
Quoted Prices in Active Markets for Similar Assets (Level 2) | Derivative Assets
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		0.2
Quoted Prices in Active Markets for Similar Assets (Level 2) | Derivative Liability
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		(1.0)
Quoted Prices in Active Markets for Similar Assets (Level 2) | Alternative Investment Funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0.9	3.3
Quoted Prices in Active Markets for Similar Assets (Level 2) | Common Pooled Fund
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	21.7	16.8
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) | US Large-Cap
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		0.0
Unobservable inputs for which little or no market data exists (Level 3) | US Mid-Cap and Small-Cap Growth
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		0.0
Unobservable inputs for which little or no market data exists (Level 3) | International Large-cap
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		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
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) | Emerging Markets
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		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
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
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
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) | Asset Backed Securities and Other
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		0.0
Unobservable inputs for which little or no market data exists (Level 3) | Derivative Assets
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		0.0
Unobservable inputs for which little or no market data exists (Level 3) | Derivative Liability
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		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	17.2	86.1
Investments Priced Using Net Asset Value | Common Pooled Fixed Income Funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		4.2
Investments Priced Using Net Asset Value | Alternative Investment Funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 17.2	$ 81.9