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Financial instruments and risk management - Trade receivables (Details) - CAD
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Accounts, Notes, Loans and Financing Receivable [Line Items]    
Not past due CAD 42,882,000 CAD 34,263,000
Past due 1-30 days 2,566,000 2,956,000
Past due 31-60 days 0 2,000,000
More than 61 days 268,000 55,000
Total 45,716,000 39,274,000
Allowance for Doubtful Accounts Receivable [Roll Forward]    
Allowance for doubtful accounts CAD 0 CAD 0
Minimum    
Accounts, Notes, Loans and Financing Receivable [Line Items]    
Payment terms 15 days  
Maximum    
Accounts, Notes, Loans and Financing Receivable [Line Items]    
Payment terms 60 days