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Financial instruments and risk management - Financial instruments (Details) - CAD
CAD in Thousands
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Credit facilities    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Basis points 1.00%  
Outstanding balance, long-term debt CAD 32,000 CAD 39,600
Level 2 | Carrying Amount    
Financial Instruments, Financial Liabilities, Balance Sheet Groupings [Abstract]    
Capital lease obligations [1] 66,969 61,400
Level 2 | Fair Value    
Financial Instruments, Financial Liabilities, Balance Sheet Groupings [Abstract]    
Capital lease obligations [1] 61,872 57,741
Level 1 | Carrying Amount    
Financial Instruments, Financial Liabilities, Balance Sheet Groupings [Abstract]    
Convertible debentures [2] 40,000 0
Level 1 | Fair Value    
Financial Instruments, Financial Liabilities, Balance Sheet Groupings [Abstract]    
Convertible debentures [2] CAD 38,700 CAD 0
[1] The fair values of amounts due under capital leases are based on management estimates which are determined by discounting cash flows required under the instruments at the interest rates currently estimated to be available for instruments with similar terms.
[2] The fair value of the Convertible Debentures is based upon the period end closing market price.