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Significant accounting policies - Stock-based compensation (Details) - CAD
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Number of trading days used to determine weighted average trading price of common shares 30 days  
Restricted Share Unit (RSU)    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Award service period 2 years  
Award vesting period 3 years  
Compensation expense recognition period 3 years  
Phantom Share Units (PSUs)    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Award vesting period 3 years  
Compensation expense recognition period 3 years  
General and administrative expenses    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Stock-based compensation expenses CAD 3,995,000 CAD 6,030,000
General and administrative expenses | Liability classified restricted share unit plan    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Stock-based compensation expenses CAD 0 CAD 52,000