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Benefit Plans - Postretirement assumptions and trends (Details) - Postemployment Retirement Benefits
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Weighted average assumptions used to determine the company's obligations      
Discount rate (as a percent) 3.69% 4.18%  
Weighted average assumptions used to determine the company's net periodic benefit cost      
Discount rate (as a percent) 4.42% 5.49% 4.93%
US      
Assumptions used in measuring benefit obligation      
2020 increase in per capita cost (as a percent) 5.90%    
Ultimate trend (as a percent) 4.50%    
Year ultimate trend reached 2028    
Canada      
Assumptions used in measuring benefit obligation      
2020 increase in per capita cost (as a percent) 5.83%    
Ultimate trend (as a percent) 4.00%    
Year ultimate trend reached 2040    
Brazil      
Assumptions used in measuring benefit obligation      
2020 increase in per capita cost (as a percent) 7.07%    
Ultimate trend (as a percent) 7.07%    
Year ultimate trend reached 2020