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PROPERTY, PLANT AND EQUIPMENT - Property, plant and equipment are presented at cost, net of accumulated depreciation (Details) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Property, Plant and Equipment [Line Items]    
Gross property, plant and equipment $ 38,613 $ 36,804
Less: accumulated depreciation (24,395) (23,470)
Net property, plant and equipment 14,218 13,334
Land    
Property, Plant and Equipment [Line Items]    
Gross property, plant and equipment 878 878
Buildings    
Property, Plant and Equipment [Line Items]    
Gross property, plant and equipment $ 19,611 18,314
Buildings | Minimum    
Property, Plant and Equipment [Line Items]    
Estimated Useful Lives 7 years  
Buildings | Maximum    
Property, Plant and Equipment [Line Items]    
Estimated Useful Lives 40 years  
Operating equipment and property    
Property, Plant and Equipment [Line Items]    
Gross property, plant and equipment $ 10,360 10,070
Operating equipment and property | Minimum    
Property, Plant and Equipment [Line Items]    
Estimated Useful Lives 3 years  
Operating equipment and property | Maximum    
Property, Plant and Equipment [Line Items]    
Estimated Useful Lives 15 years  
Furniture and fixtures    
Property, Plant and Equipment [Line Items]    
Gross property, plant and equipment $ 1,488 1,535
Furniture and fixtures | Minimum    
Property, Plant and Equipment [Line Items]    
Estimated Useful Lives 5 years  
Furniture and fixtures | Maximum    
Property, Plant and Equipment [Line Items]    
Estimated Useful Lives 7 years  
Vehicles    
Property, Plant and Equipment [Line Items]    
Gross property, plant and equipment $ 6,276 $ 6,007
Vehicles | Minimum    
Property, Plant and Equipment [Line Items]    
Estimated Useful Lives 5 years  
Vehicles | Maximum    
Property, Plant and Equipment [Line Items]    
Estimated Useful Lives 10 years