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Pension and other postretirement benefits - Weighted average assumptions used in determining benefit obligations (Detail)
12 Months Ended
Dec. 31, 2023
Dec. 31, 2022
Pension    
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]    
Discount rate 4.70% 5.10%
Rate of compensation increase 3.00% 3.00%
Other Benefits    
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]    
Discount rate 5.00% 5.10%
Initial healthcare trend rate 6.60% 5.90%
Ultimate healthcare trend rate 4.50% 4.50%