Employee Benefits, Assumptions used to determine benefit obligations (Details)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Salaried VEBA
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Assumptions Used Calculating Benefit Obligation, Discount Rate	3.90%	3.20%
Defined Benefit Plan, Assumptions Used Calculating Benefit Obligation, Expected Long-term Rate of Return on Plan Assets	5.50%	5.50%
Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Expected Long-term Rate of Return on Plan Assets	5.50%	7.75%	7.75%
Canadian pension plan | Canadian pension plan
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Assumptions Used Calculating Benefit Obligation, Discount Rate	3.90%	3.40%
Defined Benefit Plan, Assumptions Used Calculating Benefit Obligation, Expected Long-term Rate of Return on Plan Assets	4.45%	4.45%
Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Expected Long-term Rate of Return on Plan Assets	4.45%	4.45%	4.45%
Defined Benefit Plan, Assumptions Used Calculating Benefit Obligation, Rate of Compensation Increase	3.00%	3.00%