Retirement Benefits - Schedule of Fair Value of Pension Plan Assets and Liabilities by Asset Category and by Level (Details) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	$ 369.4	$ 398.4
Fair value of pension plan assets, Receivables	1.8	7.3
Fair value of pension plan assets, Payables	(2.5)	(10.5)
Fair value of pension plan assets, Total	925.1	931.4
Cash and cash equivalents
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	31.3	101.6
Private assets
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	70.7	53.5
Hedge funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	79.3	98.2
Common/collective trusts (CCTs)
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	219.4	246.7
Cash and cash equivalents | Domestic equity securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	377.2	340.2
Cash and cash equivalents | International equity securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	16.2	32.4
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)	(58.1)
Fixed income:
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	556.4	536.2
Fixed income: | Short sales
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	(0.2)	(3.7)
Fixed income: | Corporate debt securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	33.8	29.2
Fixed income: | Asset-backed securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	71.5	93.9
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	0.7
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	31.3	101.6
Quoted Prices in Active Markets for Identical Assets (Level 1) | Cash and cash equivalents | Domestic equity securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	373.8	332.7
Quoted Prices in Active Markets for Identical Assets (Level 1) | Cash and cash equivalents | International equity securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	16.2	31.3
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)	(58.1)
Quoted Prices in Active Markets for Identical Assets (Level 1) | Fixed income:
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	421.1	405.0
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	(2.5)
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: | 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	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) | Cash and cash equivalents | Domestic equity securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	1.2	7.0
Significant Other Observable Inputs (Level 2) | Cash and cash equivalents | International equity securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	0.0	1.1
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	0.0
Significant Other Observable Inputs (Level 2) | Fixed income:
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	125.8	130.0
Significant Other Observable Inputs (Level 2) | Fixed income: | Short sales
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	(0.2)	(1.2)
Significant Other Observable Inputs (Level 2) | Fixed income: | Corporate debt securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	27.0	29.2
Significant Other Observable Inputs (Level 2) | Fixed income: | Asset-backed securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	71.5	93.9
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	0.0
Significant Unobservable Inputs (Level 3)
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets, Total	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	0.5	0.7
Significant Unobservable Inputs (Level 3) | Cash and cash equivalents | Domestic equity securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	2.2	0.5
Fair value of pension plan assets, Total	2.2	0.5
Significant Unobservable Inputs (Level 3) | Cash and cash equivalents | 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	0.0
Significant Unobservable Inputs (Level 3) | Fixed income:
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	9.5	1.2
Significant Unobservable Inputs (Level 3) | Fixed income: | Short sales
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	0.0	0.0
Significant Unobservable Inputs (Level 3) | Fixed income: | Corporate debt securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	6.8	0.0
Fair value of pension plan assets, Total	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: | 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	$ 0.7