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Segmented information - Information on Operations by Geographic Area (Detail) - USD ($)
$ in Thousands
3 Months Ended 6 Months Ended
Jun. 30, 2019
Jun. 30, 2018
Jun. 30, 2019
Jun. 30, 2018
Segment Reporting Information [Line Items]        
Revenue $ 343,578 $ 366,143 $ 820,803 $ 860,357
Canada        
Segment Reporting Information [Line Items]        
Revenue 21,633 17,616 40,164 36,902
United States        
Segment Reporting Information [Line Items]        
Revenue $ 321,945 $ 348,527 $ 780,639 $ 823,455