Pension and Postretirement Plans - Expected Future Benefit Payments (Details) $ in Millions	Dec. 31, 2017 USD ($)
Defined Benefit Plan, Expected Future Prescription Drug Subsidy Receipt [Abstract]
2018	$ (0.5)
2019	(0.4)
2020	(0.4)
2021	(0.4)
2022	(0.3)
Years 2023 - 2027	(1.2)
Pension Plan
Defined Benefit Plan, Expected Future Benefit Payment [Abstract]
2018	39.8
2019	38.1
2020	38.6
2021	37.1
2022	35.6
Years 2023 - 2027	155.9
Other Postretirement Benefits Plan
Defined Benefit Plan, Expected Future Benefit Payment [Abstract]
2018	11.0
2019	9.5
2020	8.4
2021	8.1
2022	7.7
Years 2023 - 2027	$ 32.0