Retirement Benefits - Fair Value Pension Plan Assets (Details) - USD ($)	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Defined Benefit Plan Disclosure [Line Items]
Employer stock included in plan assets	$ 0	$ 40,000,000
Pension Plan
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	738,800,000	705,100,000	$ 757,600,000
Pension Plan | Fair Value, Measurements, Recurring
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	738,800,000	705,100,000
Investments Priced Using Net Asset Value	72,400,000	86,500,000
Pension Plan | Fair Value, Measurements, Recurring | Cash and cash equivalents
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	47,900,000	48,300,000
Pension Plan | Fair Value, Measurements, Recurring | US Large-Cap
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	15,600,000	29,500,000
Pension Plan | Fair Value, Measurements, Recurring | US Mid-Cap and Small-Cap Growth
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	3,100,000	42,600,000
Pension Plan | Fair Value, Measurements, Recurring | International Large-cap
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	30,900,000	27,600,000
Pension Plan | Fair Value, Measurements, Recurring | Emerging Markets
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	8,600,000	5,900,000
Pension Plan | Fair Value, Measurements, Recurring | US Treasuries
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	402,200,000	334,500,000
Pension Plan | Fair Value, Measurements, Recurring | Corporate Bonds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	10,600,000	21,000,000
Pension Plan | Fair Value, Measurements, Recurring | Asset Backed Securities and Other
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	75,600,000	45,000,000
Pension Plan | Fair Value, Measurements, Recurring | Equity/Debt Futures
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	4,600,000	2,100,000
Pension Plan | Fair Value, Measurements, Recurring | Derivative Liability
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	(1,300,000)	(1,100,000)
Pension Plan | Fair Value, Measurements, Recurring | Alternative Investment Funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	123,000,000	133,500,000
Investments Priced Using Net Asset Value	72,400,000	86,500,000
Pension Plan | Fair Value, Measurements, Recurring | Common Pooled Fund
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	18,000,000	16,200,000
Pension Plan | Fair Value, Measurements, Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	159,400,000	202,000,000
Pension Plan | Fair Value, Measurements, Recurring | 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	47,900,000	48,300,000
Pension Plan | Fair Value, Measurements, Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1) | US Large-Cap
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	15,600,000	29,500,000
Pension Plan | Fair Value, Measurements, Recurring | 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	42,600,000
Pension Plan | Fair Value, Measurements, Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1) | International Large-cap
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	30,900,000	27,600,000
Pension Plan | Fair Value, Measurements, Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1) | Emerging Markets
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	8,600,000	5,900,000
Pension Plan | Fair Value, Measurements, Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1) | US Treasuries
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Pension Plan | Fair Value, Measurements, Recurring | 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
Pension Plan | Fair Value, Measurements, Recurring | 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
Pension Plan | Fair Value, Measurements, Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1) | Equity/Debt Futures
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	1,500,000	500,000
Pension Plan | Fair Value, Measurements, Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1) | Derivative Liability
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	100,000	(500,000)
Pension Plan | Fair Value, Measurements, Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1) | Alternative Investment Funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	50,600,000	47,000,000
Pension Plan | Fair Value, Measurements, Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1) | Common Pooled Fund
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	800,000	800,000
Pension Plan | Fair Value, Measurements, Recurring | Quoted Prices in Active Markets for Similar Assets (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	507,000,000	416,500,000
Pension Plan | Fair Value, Measurements, Recurring | 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
Pension Plan | Fair Value, Measurements, Recurring | 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
Pension Plan | Fair Value, Measurements, Recurring | 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
Pension Plan | Fair Value, Measurements, Recurring | 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
Pension Plan | Fair Value, Measurements, Recurring | Quoted Prices in Active Markets for Similar Assets (Level 2) | Emerging Markets
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Pension Plan | Fair Value, Measurements, Recurring | Quoted Prices in Active Markets for Similar Assets (Level 2) | US Treasuries
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	402,200,000	334,500,000
Pension Plan | Fair Value, Measurements, Recurring | Quoted Prices in Active Markets for Similar Assets (Level 2) | Corporate Bonds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	10,300,000	20,600,000
Pension Plan | Fair Value, Measurements, Recurring | 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	75,600,000	45,000,000
Pension Plan | Fair Value, Measurements, Recurring | Quoted Prices in Active Markets for Similar Assets (Level 2) | Equity/Debt Futures
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	3,100,000	1,600,000
Pension Plan | Fair Value, Measurements, Recurring | Quoted Prices in Active Markets for Similar Assets (Level 2) | Derivative Liability
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	(1,400,000)	(600,000)
Pension Plan | Fair Value, Measurements, Recurring | 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
Pension Plan | Fair Value, Measurements, Recurring | Quoted Prices in Active Markets for Similar Assets (Level 2) | Common Pooled Fund
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	17,200,000	15,400,000
Pension Plan | Fair Value, Measurements, Recurring | Unobservable inputs for which little or no market data exists (Level 3)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	100,000
Pension Plan | Fair Value, Measurements, Recurring | 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
Pension Plan | Fair Value, Measurements, Recurring | 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
Pension Plan | Fair Value, Measurements, Recurring | 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
Pension Plan | Fair Value, Measurements, Recurring | 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
Pension Plan | Fair Value, Measurements, Recurring | 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
Pension Plan | Fair Value, Measurements, Recurring | Unobservable inputs for which little or no market data exists (Level 3) | US Treasuries
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Pension Plan | Fair Value, Measurements, Recurring | 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	100,000
Pension Plan | Fair Value, Measurements, Recurring | 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
Pension Plan | Fair Value, Measurements, Recurring | Unobservable inputs for which little or no market data exists (Level 3) | Equity/Debt Futures
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Pension Plan | Fair Value, Measurements, Recurring | 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
Pension Plan | Fair Value, Measurements, Recurring | 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
Pension Plan | Fair Value, Measurements, Recurring | 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