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Income taxes - Income Tax Expense (Recovery) Attributable to Income (Loss) (Detail) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Income Tax Expenses [Line Items]    
Income tax expenses, current $ 4,888 $ 16,431
Deferred Income Tax Expense (Benefit) 59,695 53,686
Income tax expense 64,583 70,117
Canada    
Income Tax Expenses [Line Items]    
Income tax expenses, current 6,336 6,695
Deferred Income Tax Expense (Benefit) 61,440 17,607
Income tax expense 67,776 24,302
United States    
Income Tax Expenses [Line Items]    
Income tax expenses, current (1,448) 9,736
Deferred Income Tax Expense (Benefit) (1,745) 36,079
Income tax expense $ (3,193) $ 45,815