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Income taxes - Summary of Change in Net Deferred Tax Position (Detail) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Net deferred tax asset (liability)    
Beginning balance $ (549,127) $ (443,501)
Deferred income tax liability related to Integra acquisition 0 (126,903)
Deferred income tax recovery in the income statement 118,839 19,849
Deferred tax recovery in other comprehensive loss 359 1,428
Ending balance $ (429,929) $ (549,127)