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Income and other taxes - Schedule of components of income and other taxes (Details) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Reconciliation of accounting profit multiplied by applicable tax rates [abstract]    
Consolidated income before income taxes $ 192,713 $ 68,896
Canadian federal and provincial income tax rates 27.00% 26.00%
Income tax expense at statutory rates $ 52,033 $ 17,913
Effects of different foreign statutory tax rates and tax holidays 4,624 (23,358)
Non-deductible expenditures 32,507 15,218
Losses for which no tax benefit has been recorded 29,551 19,880
Benefit of optional tax deductions (9,779) 0
Withholding tax 8,616 5,372
Minimum tax 4,960 4,780
Change due to foreign exchange 18,111 (30,607)
Change in accruals for tax audits 0 (649)
Changes in estimates of deferred tax assets 6,759 131
Non-deductible portion of gains 0 (500)
Amounts under provided for in prior years 210 (850)
Income tax expense 147,592 7,330
Current tax expense (income) and adjustments for current tax of prior periods 109,200 27,500
Deferred income tax (expense) recovery (Note 16) $ (38,392) $ 20,170