Retirement Benefits - Schedule of Estimated Future Benefit Payments (Details) $ in Millions	Dec. 31, 2016 USD ($)
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Estimated future Net Benefit Payments, 2017	$ 121.0
Estimated future Net Benefit Payments, 2018	118.5
Estimated future Net Benefit Payments, 2019	115.6
Estimated future Net Benefit Payments, 2020	112.5
Estimated future Net Benefit Payments, 2021	109.3
Estimated future Net Benefit Payments, Years 2022 - 2026	495.0
Medical and Life Insurance Benefits
Defined Benefit Plan Disclosure [Line Items]
Estimated future Net Benefit Payments, 2017	5.2
Estimated future Net Benefit Payments, 2018	5.0
Estimated future Net Benefit Payments, 2019	4.6
Estimated future Net Benefit Payments, 2020	4.3
Estimated future Net Benefit Payments, 2021	3.9
Estimated future Net Benefit Payments, Years 2022 - 2026	15.0
Estimated future Gross Benefit Payments, 2017	5.4
Estimated future Gross Benefit Payments, 2018	5.2
Estimated future Gross Benefit Payments, 2019	4.8
Estimated future Gross Benefit Payments, 2020	4.5
Estimated future Gross Benefit Payments, 2021	4.1
Estimated future Gross Benefit Payments, Years 2022 - 2026	15.6
Estimated future Medicare D Subsidy, 2017	0.2
Estimated future Medicare D Subsidy, 2018	0.2
Estimated future Medicare D Subsidy, 2019	0.2
Estimated future Medicare D Subsidy, 2020	0.2
Estimated future Medicare D Subsidy, 2021	0.2
Estimated future Medicare D Subsidy, Years 2022 - 2026	$ 0.6