Benefit Plans and Other Postretirement Benefits, OPEB benefit obligation (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Amounts recognized in the balance sheet as of December 31:
Accrued pension and postretirement benefit obligations	$ 272.0	$ 288.4
Accumulated other comprehensive loss, net	(201.0)	(469.0)
Other Postretirement Benefits
Change in projected benefit obligation:
Projected benefit obligation at beginning of year	13.6	13.2
Interest cost	0.4	0.4	$ 0.4
Benefits paid	(0.9)	(0.9)
Actuarial (gain) loss		0.9
Projected benefit obligation at end of year	13.1	13.6	$ 13.2
Amounts recognized in the balance sheet as of December 31:
Other accrued expenses	1.2	1.2
Accrued pension and postretirement benefit obligations	11.9	12.4
Underfunded status at end of year	13.1	13.6
Accumulated other comprehensive loss, net	$ (3.2)	$ (3.6)
Weighted average assumptions used to determine projected benefit obligations:
Benefit obligation discount rate (as a percent)	3.29%	3.65%