Retirement Benefits - Schedule of Fair Value of Pension Plan Assets and Liabilities by Asset Category and by Level (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	$ 272.5	$ 369.4
Fair value of pension plan assets, Receivables	1.0	1.8
Fair value of pension plan assets, Payables	(8.7)	(2.5)
Fair value of pension plan assets, Total	931.2	925.1
Cash and cash equivalents
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	28.2	31.3
Private assets
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	69.0	70.7
Hedge funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	153.7	79.3
Registered investment companies
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	10.5
CCTs
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	39.3	219.4
Equity securities | Domestic equity securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	440.9	377.2
Equity securities | International equity securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	39.9	16.2
Derivatives | Written options
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets		(0.1)
Derivatives | Short sales
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets		(0.1)
Fixed income
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	666.4	556.4
Fixed income | Short sales
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets		(0.2)
Fixed income | Corporate debt securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	87.1	33.8
Fixed income | Asset-backed securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	33.8	71.5
Fixed income | Foreign bonds
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	0.1
Fixed income | U.S. government securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	10.7
Fixed income | Municipal bonds
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets		26.3
Fixed income | Real estate investments
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets		0.5
Fixed income | Private assets
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	25.7
Quoted Prices in Active Markets for Identical Assets (Level 1) | Cash and cash equivalents
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	28.2	31.3
Quoted Prices in Active Markets for Identical Assets (Level 1) | Equity securities | Domestic equity securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	437.1	373.8
Quoted Prices in Active Markets for Identical Assets (Level 1) | Equity securities | International equity securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	39.9	16.2
Quoted Prices in Active Markets for Identical Assets (Level 1) | Derivatives | Written options
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets		(0.1)
Quoted Prices in Active Markets for Identical Assets (Level 1) | Derivatives | Short sales
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets		(0.1)
Quoted Prices in Active Markets for Identical Assets (Level 1) | Fixed income
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	505.2	421.1
Quoted Prices in Active Markets for Identical Assets (Level 1) | Fixed income | Short sales
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets		0.0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Fixed income | Corporate debt securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	0.0	0.0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Fixed income | Asset-backed securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	0.0	0.0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Fixed income | Foreign bonds
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	0.0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Fixed income | U.S. government securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	0.0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Fixed income | Municipal bonds
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets		0.0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Fixed income | Real estate investments
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets		0.0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Fixed income | Private assets
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	0.0
Significant Other Observable Inputs (Level 2) | Cash and cash equivalents
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	0.0	0.0
Significant Other Observable Inputs (Level 2) | Equity securities | Domestic equity securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	0.0	1.2
Significant Other Observable Inputs (Level 2) | Equity securities | International equity securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	0.0	0.0
Significant Other Observable Inputs (Level 2) | Derivatives | Written options
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets		0.0
Significant Other Observable Inputs (Level 2) | Derivatives | Short sales
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets		0.0
Significant Other Observable Inputs (Level 2) | Fixed income
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	120.4	125.8
Significant Other Observable Inputs (Level 2) | Fixed income | Short sales
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets		(0.2)
Significant Other Observable Inputs (Level 2) | Fixed income | Corporate debt securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	75.8	27.0
Significant Other Observable Inputs (Level 2) | Fixed income | Asset-backed securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	33.8	71.5
Significant Other Observable Inputs (Level 2) | Fixed income | Foreign bonds
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	0.1
Significant Other Observable Inputs (Level 2) | Fixed income | U.S. government securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	10.7
Significant Other Observable Inputs (Level 2) | Fixed income | Municipal bonds
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets		26.3
Significant Other Observable Inputs (Level 2) | Fixed income | Real estate investments
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets		0.0
Significant Other Observable Inputs (Level 2) | Fixed income | Private assets
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	0.0
Significant Unobservable Inputs (Level 3)
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets, Total	40.8	9.5	$ 1.2
Significant Unobservable Inputs (Level 3) | Cash and cash equivalents
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	0.0	0.0
Significant Unobservable Inputs (Level 3) | Real estate investments
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets, Total	0.0	0.5	0.7
Significant Unobservable Inputs (Level 3) | Private assets
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets, Total	25.7	0.0
Significant Unobservable Inputs (Level 3) | Equity securities | Domestic equity securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	3.8	2.2
Fair value of pension plan assets, Total	3.8	2.2	0.5
Significant Unobservable Inputs (Level 3) | Equity securities | International equity securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	0.0	0.0
Significant Unobservable Inputs (Level 3) | Derivatives | Written options
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets		0.0
Significant Unobservable Inputs (Level 3) | Derivatives | Short sales
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets		0.0
Significant Unobservable Inputs (Level 3) | Fixed income
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	40.8	9.5
Significant Unobservable Inputs (Level 3) | Fixed income | Short sales
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets		0.0
Significant Unobservable Inputs (Level 3) | Fixed income | Corporate debt securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	11.3	6.8
Fair value of pension plan assets, Total	11.3	6.8	$ 0.0
Significant Unobservable Inputs (Level 3) | Fixed income | Asset-backed securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	0.0	0.0
Significant Unobservable Inputs (Level 3) | Fixed income | Foreign bonds
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	0.0
Significant Unobservable Inputs (Level 3) | Fixed income | U.S. government securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	0.0
Significant Unobservable Inputs (Level 3) | Fixed income | Municipal bonds
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets		0.0
Significant Unobservable Inputs (Level 3) | Fixed income | Real estate investments
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets		$ 0.5
Significant Unobservable Inputs (Level 3) | Fixed income | Private assets
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	$ 25.7