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Pension and other post-retirement benefits - Projected Benefit Obligations, Fair Value of Plan Assets, and Funded Status (Detail) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2023
Dec. 31, 2022
Change in plan assets    
Non-current assets (note 11) $ 48,477 $ 26,482
Pension benefits    
Change in projected benefit obligation    
Projected benefit obligation, beginning of year 628,135 765,618
Projected benefit obligation assumed from business combination 0 87,933
Plan settlements (3,226) (112)
Service cost 11,954 16,309
Interest cost 33,687 24,787
Actuarial loss (gain) 20,172 (198,074)
Contributions from retirees 0 0
Plan amendments 0 0
Medicare Part D 0 0
Benefits paid (42,801) (68,197)
Foreign exchange (53) (129)
Projected benefit obligation, end of year 647,868 628,135
Change in plan assets    
Fair value of plan assets, beginning of year 569,255 648,864
Plan assets acquired in business combination 0 74,532
Actual return on plan assets 65,272 (109,118)
Employer contributions 22,326 23,296
Plan settlements (3,226) (112)
Contributions from retirees 0 0
Medicare Part D subsidy receipts 0 0
Benefits paid (42,801) (68,197)
Foreign exchange 2 (10)
Fair value of plan assets, end of year 610,828 569,255
Unfunded status (37,040) (58,880)
Non-current assets (note 11) 12,598 12,264
Current liabilities (1,416) (1,907)
Non-current liabilities (48,222) (69,237)
Net amount recognized (37,040) (58,880)
OPEB    
Change in projected benefit obligation    
Projected benefit obligation, beginning of year 217,330 292,646
Projected benefit obligation assumed from business combination 0 5,195
Plan settlements 0 0
Service cost 3,253 6,277
Interest cost 11,510 9,146
Actuarial loss (gain) (10,913) (82,991)
Contributions from retirees 2,189 2,220
Plan amendments 0 (2,452)
Medicare Part D 355 367
Benefits paid (14,226) (13,078)
Foreign exchange 0 0
Projected benefit obligation, end of year 209,498 217,330
Change in plan assets    
Fair value of plan assets, beginning of year 172,167 192,375
Plan assets acquired in business combination 0 8,577
Actual return on plan assets 22,620 (30,105)
Employer contributions 10,677 11,811
Plan settlements 0 0
Contributions from retirees 2,189 2,220
Medicare Part D subsidy receipts 355 367
Benefits paid (14,226) (13,078)
Foreign exchange 0 0
Fair value of plan assets, end of year 193,782 172,167
Unfunded status (15,716) (45,163)
Non-current assets (note 11) 35,879 14,218
Current liabilities (3,164) (3,039)
Non-current liabilities (48,431) (56,342)
Net amount recognized $ (15,716) $ (45,163)