Retirement Benefits - Fair Value Pension Plan Assets (Details) - USD ($)	Dec. 31, 2018	Dec. 31, 2017
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 670,600,000	$ 738,800,000
Investments Priced Using Net Asset Value	86,100,000	72,400,000
Employer stock included in plan assets	0	0
Cash and cash equivalents
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	47,000,000	47,900,000
US Large-Cap
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	8,200,000	15,600,000
US Mid-Cap and Small-Cap Growth
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	3,100,000	3,100,000
International Large-cap
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	8,600,000	30,900,000
Emerging Markets
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	13,300,000	8,600,000
Common Pooled Equity Funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	12,700,000
U.S. Treasuries
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	378,000,000	402,200,000
Corporate Bonds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	300,000	10,600,000
Asset Backed Securities and Other
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	22,800,000	75,600,000
Common Pooled Fixed Income Fund
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	61,800,000
Investments Priced Using Net Asset Value	4,200,000
Assets
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	8,200,000	4,600,000
(Liabilities)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	(7,900,000)	(1,300,000)
Alternative Investment Funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	96,800,000	123,000,000
Investments Priced Using Net Asset Value	81,900,000	72,400,000
Common Pooled Fund
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	17,700,000	18,000,000
Quoted Prices in Active Markets for Identical Assets (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	88,900,000	159,400,000
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	47,000,000	47,900,000
Quoted Prices in Active Markets for Identical Assets (Level 1) | US Large-Cap
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	8,200,000	15,600,000
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	3,100,000	3,100,000
Quoted Prices in Active Markets for Identical Assets (Level 1) | International Large-cap
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	8,600,000	30,900,000
Quoted Prices in Active Markets for Identical Assets (Level 1) | Emerging Markets
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	8,100,000	8,600,000
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
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
Quoted Prices in Active Markets for Identical Assets (Level 1) | Corporate Bonds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	300,000	300,000
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) | Common Pooled Fixed Income Fund
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Assets
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	8,000,000	1,500,000
Quoted Prices in Active Markets for Identical Assets (Level 1) | (Liabilities)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	(6,900,000)	100,000
Quoted Prices in Active Markets for Identical Assets (Level 1) | Alternative Investment Funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	11,600,000	50,600,000
Quoted Prices in Active Markets for Identical Assets (Level 1) | Common Pooled Fund
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	900,000	800,000
Quoted Prices in Active Markets for Similar Assets (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	495,600,000	507,000,000
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
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) | Emerging Markets
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	5,200,000	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	12,700,000
Quoted Prices in Active Markets for Similar Assets (Level 2) | U.S. Treasuries
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	378,000,000	402,200,000
Quoted Prices in Active Markets for Similar Assets (Level 2) | Corporate Bonds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	10,300,000
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	22,800,000	75,600,000
Quoted Prices in Active Markets for Similar Assets (Level 2) | Common Pooled Fixed Income Fund
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	57,600,000
Quoted Prices in Active Markets for Similar Assets (Level 2) | Assets
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	200,000	3,100,000
Quoted Prices in Active Markets for Similar Assets (Level 2) | (Liabilities)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	(1,000,000)	(1,400,000)
Quoted Prices in Active Markets for Similar Assets (Level 2) | Alternative Investment Funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	3,300,000	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	16,800,000	17,200,000
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
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
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) | 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) | Common Pooled Equity Funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	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
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
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) | Common Pooled Fixed Income Fund
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0
Unobservable inputs for which little or no market data exists (Level 3) | 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) | (Liabilities)
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
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