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Segmented information - Information on Operations by Geographic Area (Detail) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Segment Reporting Information [Line Items]    
Revenue $ 1,647,387 $ 1,521,938
Property, plant and equipment 6,393,558 6,304,897
Intangible assets 54,994 51,103
Canada    
Segment Reporting Information [Line Items]    
Revenue 70,358 73,406
Property, plant and equipment 415,979 453,323
Intangible assets 23,994 27,624
United States    
Segment Reporting Information [Line Items]    
Revenue 1,577,029 1,448,532
Property, plant and equipment 5,977,579 5,851,574
Intangible assets $ 31,000 $ 23,479