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Income taxes - Additional Information (Detail) - CAD
CAD in Thousands
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Income Tax Disclosure [Abstract]    
Canadian enacted statutory rate 26.50% 26.50%
Valuation allowance for deferred tax assets CAD 15,486 CAD 21,656
Deferred taxes 85,286 CAD 28,675
Deferred income taxes, undistributed earnings of foreign subsidiaries 188,348  
Tax Cuts And Jobs Act Of 2017, Incomplete Accounting, Change In Tax Rate, Deferred Tax Asset, Provisional Income Tax Expense 22,390  
Tax Cuts And Jobs Act Of 2017, Incomplete Accounting, Change In Tax Rate, Deferred Tax Liability, Provisional Income Tax Benefit CAD 411,409