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Financial instruments and risk management - Trade receivables (Details) - CAD ($)
 12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Accounts, Notes, Loans and Financing Receivable [Line Items]    
Not past due $ 35,409,000 $ 60,326,000
Past due 1-30 days 10,380,000 6,649,000
Past due 31-60 days 5,000 728,000
More than 61 days 44,000 768,000
Total 45,838,000 68,471,000
Accounts Receivable, Allowance for Credit Loss [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