Employee Benefits - Assumptions Used to Determine Net Periodic Benefit Cost (Income) (Details)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Salaried VEBA
Defined Benefit Plan Disclosure [Line Items]
Discount rate	2.95%	3.90%	3.20%
Expected long-term return on plan assets	5.50%	5.50%	5.50%
Canadian pension plan | Canadian pension plan
Defined Benefit Plan Disclosure [Line Items]
Discount rate	3.10%	3.90%	3.40%
Expected long-term return on plan assets	4.45%	4.45%	4.45%
Rate of compensation increase	3.00%	3.00%	3.00%