Pension and Other Post-Retirement Benefits - Expected Benefit Plan Payments (Details) $ in Millions	Dec. 31, 2017 USD ($)
Qualified Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
2018	$ 27.4
2019	27.7
2020	27.7
2021	28.0
2022	28.1
Years 2023-2027	142.5
Total	281.4
Nonqualified Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
2018	3.0
2019	3.6
2020	3.9
2021	3.9
2022	4.4
Years 2023-2027	20.3
Total	39.1
Other Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
2018	3.4
2019	3.4
2020	3.2
2021	3.0
2022	2.8
Years 2023-2027	11.4
Total	$ 27.2