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Cost of sales
 12 Months Ended
Dec. 31, 2017
Disclosure of cost of sales [abstract]  
Cost of sales
15.   Cost of sales
 
  
Year ended
    
Year ended
  
 
  
December 31,
    
December 31,
  
 
  
2017
    
2016
  
 
            
Salaries and employee benefits
  
$
22,314
    
$
23,430
  
Raw materials and consumables
    
14,252
      
15,065
  
Utilities
    
3,902
      
4,113
  
Other costs
    
2,318
      
1,768
  
Changes in inventories
    
(2,748
)
    
1,769
  
 
  
$
40,038
    
$
46,145