Significant accounting policies (Tables)
 12 Months Ended
Dec. 31, 2017
Text block1 [abstract]  
Summary of Basis Depreciation of Property, Plant and Equipment

Property, plant, and equipment are stated at cost less accumulated depreciation. The assets are depreciated over their estimated useful lives using the following methods and rates:

 

     Method    Rate  

Building structures

   

Straight-line

      15 to 20 years  

Furniture and equipment

   

Straight-line

      5 years  

Computer equipment

   

Straight-line

      3 years  

Security equipment

   

Straight-line

      5 years  

Production equipment

   

Straight-line

      7 years  

Road

   

Straight-line

      25 years  

Leasehold improvements

   

Straight-line

      5 to 10 years