Retirement Benefits - Schedule of Estimated Future Benefit Payments (Details) $ in Millions	Dec. 31, 2017 USD ($)
Pension Benefits
Expected Benefit Payments
2018	$ 116.2
2019	113.9
2020	110.7
2021	107.2
2022	103.6
Years 2023 - 2027	463.2
Medical and Life Insurance Benefits
Expected Benefit Payments
2018	4.8
2019	4.5
2020	4.2
2021	3.9
2022	3.5
Years 2023 - 2027	12.7
Gross Benefit Payments
2018	5.0
2019	4.7
2020	4.4
2021	4.0
2022	3.6
Years 2023 - 2027	13.1
Medicare D Subsidy
2018	0.2
2019	0.2
2020	0.2
2021	0.1
2022	0.1
Years 2023 - 2027	$ 0.4