Pension and Postretirement Plans - Assumptions (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Defined Benefit Plan, Assumed Health Care Cost Trend Rates [Abstract]
Healthcare cost trend	6.50%	6.50%
Rate to which the cost trend is assumed to decline (ultimate trend rate)	4.50%	4.50%
Year the rates reach the ultimate trend rate	2022	2021
Defined Benefit Plan, Effect of One-Percentage Point Change in Assumed Health Care Cost Trend Rate [Abstract]
Service and interest costs for current year	$ 0.2
Service and interest costs for current year	(0.1)
Postretirement benefit obligation at December 31	3.2
Postretirement benefit obligation at December 31	$ (2.9)
Pension Plan
Defined Benefit Plan, Weighted Average Assumptions Used in Calculating Net Periodic Benefit Cost [Abstract]
Discount rate	4.10%	3.80%	3.40%	[1]
Expected long-term rate of return	7.25%	7.50%	7.75%
Future compensation growth rate	0.00%	0.00%	0.00%
Defined Benefit Plan, Weighted Average Assumptions Used in Calculating Benefit Obligation [Abstract]
Discount rate	3.60%	4.00%
Future compensation growth rate	0.00%	0.00%
Other Postretirement Benefits Plan
Defined Benefit Plan, Weighted Average Assumptions Used in Calculating Net Periodic Benefit Cost [Abstract]
Discount rate	4.00%	3.70%	3.40%
Expected long-term rate of return	0.00%	0.00%	0.00%
Future compensation growth rate	0.00%	0.00%	0.00%
Defined Benefit Plan, Weighted Average Assumptions Used in Calculating Benefit Obligation [Abstract]
Discount rate	3.60%	4.00%
Future compensation growth rate	0.00%	0.00%

[1] Discount rate used for the remeasurement of the non-management pension plan in April 2015 was consistent with the discount rate previously established.