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Pension and other post-retirement benefits - Additional Information (Detail) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2021
Dec. 31, 2020
Dec. 31, 2019
Employee Benefits Disclosure [Line Items]      
Defined contribution pension plan cost $ 10,836 $ 9,672  
Accumulated benefit obligation for pension plan 1,008,754 1,080,685  
Pension benefits      
Employee Benefits Disclosure [Line Items]      
Defined benefit plan, plan assets, amount 648,864 629,157 $ 407,074
Expected employer contributions for next year 21,305    
OPEB      
Employee Benefits Disclosure [Line Items]      
Defined benefit plan, plan assets, amount 192,375 $ 176,616 $ 158,873
Expected employer contributions for next year $ 12,208