Employee Benefits Employee Benefits, Benefit obligations and funded status of our Canadian pension and the VEBAs (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Change in plan assets:
Fair value of plan assets at beginning of year	$ 69.6
Fair value of plan assets at end of year	72.3	$ 69.6
Accrued Annual Veba Contribution	15.7	20.0
Net liabilities of Salaried VEBA	31.9	28.6
Long-term portion of contingent contribution to Union VEBA	0.0	12.8
VEBAs
Change in benefit obligation:
Obligation at beginning of year	86.8	77.9
Defined Benefit Plan, Benefit Obligation, Foreign Currency Translation Gain (Loss)	0.0	0.0
Defined Benefit Plan, Service Cost	0.0	0.0
Defined Benefit Plan, Interest Cost	3.0	2.9
Defined Benefit Plan, Benefit Obligation, Increase (Decrease) for Plan Amendment	7.3	8.4
Defined Benefit Plan, Benefit Obligation, Actuarial Gain (Loss)	0.5	(4.1)
Benefits paid by VEBA	(6.6)	(6.5)
Obligation at end of year	90.0	86.8
Change in plan assets:
Fair value of plan assets at beginning of year	58.2	58.9
Defined Benefit Plan, Plan Assets, Increase (Decrease) for Actual Return (Loss)	3.6	2.9
Defined Benefit Plan, Plan Assets, Contributions by Employer	2.9	2.9
Benefits paid by VEBA	(6.6)	(6.5)
Fair value of plan assets at end of year	58.1	58.2
Defined Benefit Plan, Funded (Unfunded) Status of Plan	(31.9)	(28.6)
Salaried VEBA
Change in plan assets:
Actuarial (gain) loss due to projected lower benefit utilization	(1.0)	(0.4)
Actuarial (Gain) Loss Due To Change In Discount Rate	3.0	2.2
Actuarial (Gain) Loss Due To Changes In Census Data	(2.5)	2.3
Accrued Annual Veba Contribution	2.9	2.9
Defined Benefit Plan, Funded (Unfunded) Status of Plan	(31.9)	(28.6)
Union VEBA
Change in plan assets:
Accrued Annual Veba Contribution	12.8	17.1
Canadian pension plan
Change in plan assets:
Defined Benefit Plan, Benefit Obligation, Contributions by Plan Participant	0.1	0.0
Canadian pension plan | Canadian pension plan
Change in benefit obligation:
Obligation at beginning of year	7.0	6.1
Defined Benefit Plan, Benefit Obligation, Foreign Currency Translation Gain (Loss)	0.5	0.2
Defined Benefit Plan, Service Cost at Balance Sheet Exchange Rate	0.3	0.3
Defined Benefit Plan, Interest Cost at Balance Sheet Exchange Rate	0.3	0.3
Defined Benefit Plan, Benefit Obligation, Actuarial Gain (Loss)	(0.6)	(0.3)
Obligation at end of year	8.5	7.0
Change in plan assets:
Fair value of plan assets at beginning of year	6.1	5.7
Defined Benefit Plan, Plan Assets, Foreign Currency Translation Gain (Loss)	0.5	0.2
Defined Benefit Plan, Plan Assets, Increase (Decrease) for Actual Return (Loss)	0.5	0.1
Defined Benefit Plan, Plan Assets, Contributions by Employer	0.4	0.3
Fair value of plan assets at end of year	7.3	6.1
Defined Benefit Plan, Funded (Unfunded) Status of Plan	(1.2)	(0.9)
Defined Benefit Plan, Plan Assets, Benefits Paid	$ 0.3	$ 0.2