Employee Benefits - Assumptions Used to Determine Benefit Obligation (Details)	Dec. 31, 2020	Dec. 31, 2019
Salaried VEBA
Defined Benefit Plan Disclosure [Line Items]
Discount rate	2.05%	2.95%
Expected long-term return on plan assets	5.50%	5.50%
Canadian pension plan | Canadian pension plan
Defined Benefit Plan Disclosure [Line Items]
Discount rate	2.50%	3.10%
Expected long-term return on plan assets	4.45%	4.45%
Rate of compensation increase	3.00%	3.00%