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Employee benefits - Discount rate and assumed health care cost trend rates (Details)
 12 Months Ended
Dec. 31, 2023
Dec. 31, 2022
Dec. 31, 2021
Pension Plans | Minimum      
Weighted average assumptions used to determine net periodic benefit cost for the years ended December 31:      
Discount rate for benefit obligation 5.34% 2.79% 2.41%
Discount rate for interest cost 5.23% 2.30% 1.76%
Expected return on plan asset 5.90% 4.30% 4.60%
Weighted average assumptions used to determine benefit obligation at December 31:      
Discount rate for benefit obligation 5.02% 5.34%  
Pension Plans | Maximum      
Weighted average assumptions used to determine net periodic benefit cost for the years ended December 31:      
Discount rate for benefit obligation 5.37% 2.83% 2.48%
Discount rate for interest cost 5.24% 2.33% 1.80%
Expected return on plan asset 6.50% 5.40% 5.50%
Weighted average assumptions used to determine benefit obligation at December 31:      
Discount rate for benefit obligation 5.05% 5.37%  
OPEB Plan      
Weighted average assumptions used to determine net periodic benefit cost for the years ended December 31:      
Discount rate for benefit obligation 5.42% 2.94% 2.65%
Discount rate for service cost 5.66% 3.21% 3.09%
Discount rate for interest cost 5.28% 2.51% 2.03%
Initial health care cost trend rates 7.50% 4.75% 5.00%
Ultimate health care cost trend rate 4.50% 4.50% 4.50%
Year that the ultimate trend rate is reached 2035 2023 2023
Weighted average assumptions used to determine benefit obligation at December 31:      
Discount rate for benefit obligation 5.10% 5.42%  
Initial health care cost trend rates 7.25% 7.50%  
Ultimate health care cost trend rate 4.50% 4.50%  
Year that the ultimate trend rate is reached 2035 2035