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Pension and other post-retirement benefits - Benefit Obligations in Excess of Plan Assets (Detail) - USD ($)
$ in Thousands
Dec. 31, 2023
Dec. 31, 2022
Pension benefits    
Defined Benefit Plan Disclosure [Line Items]    
Accumulated benefit obligation $ 425,842 $ 413,041
Fair value of plan assets 393,857 364,229
Projected benefit obligation 507,612 489,140
Fair value of plan assets 458,497 417,994
OPEB    
Defined Benefit Plan Disclosure [Line Items]    
Accumulated benefit obligation 71,089 198,463
Fair value of plan assets 18,793 139,368
Projected benefit obligation 71,089 198,463
Fair value of plan assets $ 18,793 $ 139,368