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Employee benefits - Discount rate and assumed health care cost trend rates (Details)
 12 Months Ended
Dec. 31, 2021
Dec. 31, 2020
Dec. 31, 2019
Pension Plans | Minimum      
Weighted average assumptions used to determine net periodic benefit cost for the years ended December 31:      
Discount rate for benefit obligation 2.41% 3.22% 4.20%
Discount rate for interest cost 1.76% 2.81% 3.87%
Expected return on plan asset 4.60% 5.00% 5.30%
Weighted average assumptions used to determine benefit obligation at December 31:      
Discount rate for benefit obligation 2.79% 2.41%  
Pension Plans | Maximum      
Weighted average assumptions used to determine net periodic benefit cost for the years ended December 31:      
Discount rate for benefit obligation 2.48% 3.27% 4.23%
Discount rate for interest cost 1.80% 2.83% 3.90%
Expected return on plan asset 5.50% 5.80% 6.00%
Weighted average assumptions used to determine benefit obligation at December 31:      
Discount rate for benefit obligation 2.83% 2.48%  
OPEB Plan      
Weighted average assumptions used to determine net periodic benefit cost for the years ended December 31:      
Discount rate for benefit obligation 2.65% 3.38% 4.30%
Discount rate for service cost 3.09% 3.72% 4.49%
Discount rate for interest cost 2.03% 2.98% 3.99%
Initial health care cost trend rates 5.00% 5.00% 5.00%
Ultimate health care cost trend rate 4.50% 5.00% 5.00%
Year that the ultimate trend rate is reached 2023 2020 2019
Weighted average assumptions used to determine benefit obligation at December 31:      
Discount rate for benefit obligation 2.94% 2.65%  
Initial health care cost trend rates 4.75% 5.00%  
Ultimate health care cost trend rate 4.50% 4.50%  
Year that the ultimate trend rate is reached 2023 2023