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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, 2020
Dec. 31, 2019
Change in plan assets    
Non-current assets (note 11) $ 10,662 $ 8,437
Pension Plans    
Change in projected benefit obligation    
Projected benefit obligation, beginning of year 564,970 484,707
Projected benefit obligation assumed from business combination 195,231 20,196
Modifications to plans (191) (7,705)
Service cost 15,450 12,351
Interest cost 19,281 20,222
Actuarial (gain) loss 76,618 65,443
Contributions from retirees 171 0
Medicare Part D 0 0
Benefits paid (37,020) (30,244)
Foreign exchange 403 0
Projected benefit obligation, end of year 834,913 564,970
Change in plan assets    
Fair value of plan assets, beginning of year 407,074 339,099
Plan assets acquired in business combination 179,600 8,004
Actual return on plan assets 52,876 68,025
Employer contributions 26,099 22,190
Contributions from retirees 171 0
Medicare Part D subsidy receipts 0 0
Benefits paid (37,020) (30,244)
Foreign exchange 357 0
Fair value of plan assets, end of year 629,157 407,074
Unfunded status (205,756) (157,896)
Non-current assets (note 11) 488 0
Current liabilities (1,989) (1,415)
Non-current liabilities (204,255) (156,481)
Net amount recognized (205,756) (157,896)
Other Postretirement Benefit Plans, Defined Benefit    
Change in projected benefit obligation    
Projected benefit obligation, beginning of year 219,217 168,325
Projected benefit obligation assumed from business combination 44,950 11,646
Modifications to plans 0 0
Service cost 6,175 4,587
Interest cost 7,695 7,575
Actuarial (gain) loss 34,507 33,605
Contributions from retirees 2,037 1,913
Medicare Part D 377 414
Benefits paid (8,434) (8,848)
Foreign exchange 0 0
Projected benefit obligation, end of year 306,524 219,217
Change in plan assets    
Fair value of plan assets, beginning of year 158,873 115,542
Plan assets acquired in business combination 0 15,688
Actual return on plan assets 21,219 25,464
Employer contributions 2,583 8,628
Contributions from retirees 1,998 1,913
Medicare Part D subsidy receipts 377 414
Benefits paid (8,434) (8,776)
Foreign exchange 0 0
Fair value of plan assets, end of year 176,616 158,873
Unfunded status (129,908) (60,344)
Non-current assets (note 11) 10,174 8,437
Current liabilities (2,835) (1,168)
Non-current liabilities (137,247) (67,613)
Net amount recognized $ (129,908) $ (60,344)