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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, 2021
Dec. 31, 2020
Change in plan assets    
Non-current assets (note 11) $ 11,963 $ 10,662
Pension benefits    
Change in projected benefit obligation    
Projected benefit obligation, beginning of year 834,913 564,970
Projected benefit obligation assumed from business combination 0 195,231
Plan Settlements (1,294) 0
Service cost 14,673 15,450
Interest cost 20,676 19,281
Actuarial loss (gain) (36,597) 76,618
Contributions from retirees 0 171
Plan amendments 237 (191)
Medicare Part D 0 0
Benefits paid (66,800) (37,020)
Foreign exchange (190) 403
Projected benefit obligation, end of year 765,618 834,913
Change in plan assets    
Fair value of plan assets, beginning of year 629,157 407,074
Plan assets acquired in business combination 0 179,600
Actual return on plan assets 58,721 52,876
Employer contributions 29,058 26,099
Plan Settlements (1,294) 0
Contributions from retirees 0 171
Medicare Part D subsidy receipts 0 0
Benefits paid (66,800) (37,020)
Foreign exchange 22 357
Fair value of plan assets, end of year 648,864 629,157
Unfunded status (116,754) (205,756)
Non-current assets (note 11) 84 488
Current liabilities (1,902) (1,989)
Non-current liabilities (114,936) (204,255)
Net amount recognized (116,754) (205,756)
OPEB    
Change in projected benefit obligation    
Projected benefit obligation, beginning of year 306,524 219,217
Projected benefit obligation assumed from business combination 0 44,950
Plan Settlements 0 0
Service cost 7,307 6,175
Interest cost 8,048 7,695
Actuarial loss (gain) (18,977) 34,507
Contributions from retirees 2,040 2,037
Plan amendments 310 0
Medicare Part D 373 377
Benefits paid (12,979) (8,434)
Foreign exchange 0 0
Projected benefit obligation, end of year 292,646 306,524
Change in plan assets    
Fair value of plan assets, beginning of year 176,616 158,873
Plan assets acquired in business combination 0 0
Actual return on plan assets 15,200 21,219
Employer contributions 11,178 2,583
Plan Settlements 0 0
Contributions from retirees 1,988 1,998
Medicare Part D subsidy receipts 372 377
Benefits paid (12,979) (8,434)
Foreign exchange 0 0
Fair value of plan assets, end of year 192,375 176,616
Unfunded status (100,271) (129,908)
Non-current assets (note 11) 11,879 10,174
Current liabilities (699) (2,835)
Non-current liabilities (111,451) (137,247)
Net amount recognized $ (100,271) $ (129,908)