Employee benefits - Discount rate and assumed health care cost trend rates (Details)	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Pension Plans | Minimum
Weighted average assumptions used to determine net periodic benefit cost for the years ended December 31:
Discount rate for benefit obligation	5.02%	5.34%	2.79%
Discount rate for interest cost	4.95%	5.23%	2.30%
Expected return on plan asset	5.60%	5.90%	4.30%
Weighted average assumptions used to determine benefit obligation at December 31:
Discount rate for benefit obligation	5.54%	5.02%
Pension Plans | Maximum
Weighted average assumptions used to determine net periodic benefit cost for the years ended December 31:
Discount rate for benefit obligation	5.05%	5.37%	2.83%
Discount rate for interest cost	4.96%	5.24%	2.33%
Expected return on plan asset	6.60%	6.50%	5.40%
Weighted average assumptions used to determine benefit obligation at December 31:
Discount rate for benefit obligation	5.57%	5.05%
OPEB Plan
Weighted average assumptions used to determine net periodic benefit cost for the years ended December 31:
Discount rate for benefit obligation	5.10%	5.42%	2.94%
Discount rate for service cost	5.37%	5.66%	3.21%
Discount rate for interest cost	4.99%	5.28%	2.51%
Initial health care cost trend rates	7.25%	7.50%	4.75%
Ultimate health care cost trend rate	4.50%	4.50%	4.50%
Year that the ultimate trend rate is reached	2035	2035	2023
Weighted average assumptions used to determine benefit obligation at December 31:
Discount rate for benefit obligation	5.65%	5.10%
Initial health care cost trend rates	7.00%	7.25%
Ultimate health care cost trend rate	4.50%	4.50%
Year that the ultimate trend rate is reached	2035	2035