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Financial instruments and risk management - Trade receivables (Details) - CAD ($)
 12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Accounts, Notes, Loans and Financing Receivable [Line Items]    
Not past due $ 60,326,000 $ 42,882,000
Past due 1-30 days 6,649,000 2,566,000
Past due 31-60 days 728,000 0
More than 61 days 768,000 268,000
Total 68,471,000 45,716,000
Allowance for Doubtful Accounts Receivable [Roll Forward]    
Allowance for doubtful accounts $ 0 $ 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