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Pension and other postretirement benefits - Weighted average assumptions used in determining benefit obligations (Detail)
12 Months Ended
Dec. 31, 2021
Dec. 31, 2020
Pension    
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]    
Discount rate 2.80% 2.50%
Rate of compensation increase 2.80% 2.80%
Other Benefits    
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]    
Discount rate 2.70% 2.30%
Initial healthcare trend rate 6.00% 6.40%
Ultimate healthcare trend rate 4.50% 4.50%