Benefit Plans and Other PostretirementBenefits, OPEB benefit expense (Details) - Other Postretirement Benefits - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Components of net pension expense:
Service cost			$ 0.1
Interest cost	$ 0.4	$ 0.4	0.4
Amortization of net actuarial losses	0.5	0.7	0.3
Net pension expense	$ 0.9	$ 1.1	$ 0.8
Weighted average assumptions used to determine net periodic benefit cost/expense:
Discount rate (as a percent)	3.65%	3.71%	3.50%