Property and equipment (Tables)
 12 Months Ended
Dec. 31, 2016
Property, Plant and Equipment [Abstract]  
Components of Property and Equipment

Major classes of property and equipment consist of the following at December 31:

 

     2016      2015  

Furniture and fixtures

    $ 2,518      $ 2,518  

Automobiles and trucks

      9,793        10,689  

Machinery and equipment

      53,757        55,963  

Office and computer equipment

      20,817        19,993  

Building and improvements

      25,085        25,534  
  

 

 

     

 

 

  
     111,970        114,697  

Less accumulated depreciation and amortization

      79,415        75,041  
  

 

 

     

 

 

  
     32,555        39,656  

Land

      8,792        9,306  
  

 

 

     

 

 

  
   $ 41,347      $ 48,962