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Pension and other post-retirement benefits - Additional Information (Detail) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2023
Dec. 31, 2022
Dec. 31, 2021
Employee Benefits Disclosure [Line Items]      
Defined contribution pension plan cost $ 14,521 $ 12,126  
Accumulated benefit obligation for pension plan 827,559 815,589  
Pension benefits      
Employee Benefits Disclosure [Line Items]      
Defined benefit plan, plan assets, amount 610,828 569,255 $ 648,864
Expected employer contributions for next year 23,248    
OPEB      
Employee Benefits Disclosure [Line Items]      
Defined benefit plan, plan assets, amount 193,782 172,167 $ 192,375
Expected employer contributions for next year 3,583    
Private Equity Funds | Level 3      
Employee Benefits Disclosure [Line Items]      
Defined benefit plan, plan assets, amount $ 26,381 $ 21,904