Pension and Other Postretirement Benefits - Expected Contribution Cash Flows (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Defined Benefit Plan Disclosure [Line Items]
Employer contributions	$ 1.8	$ 1.8
Defined Benefit Pension Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Employer contributions	1.8	1.8
Expected contributions in next twelve months	1.8
Defined Benefit Plan, Expected Future Benefit Payment [Abstract]
2019	20.6
2020	20.8
2021	20.9
2022	21.1
2023	21.1
2024-2028	100.6
Other Postretirement Benefit Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Employer contributions	6.1	$ 6.5
Expected contributions in next twelve months	7.5
Defined Benefit Plan, Expected Future Benefit Payment [Abstract]
2019	7.5
2020	7.4
2021	7.6
2022	7.6
2023	7.6
2024-2028	$ 37.0