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Pension and other postretirement benefits - Weighted average assumptions used in determining net periodic benefit cost (Detail)
12 Months Ended
Dec. 31, 2024
Dec. 31, 2023
Dec. 31, 2022
Pension      
Defined Benefit Plan Disclosure [Line Items]      
Discount rate 4.70% 5.10% 2.80%
Rate of return 4.80% 7.30% 5.60%
Other Benefits      
Defined Benefit Plan Disclosure [Line Items]      
Discount rate 5.00% 5.10% 2.70%
Initial healthcare trend rate 6.90% 6.10% 6.40%
Ultimate healthcare trend rate 4.50% 4.50% 4.50%