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Note 9 - Asset Retirement Obligations (Tables)
 12 Months Ended
Dec. 31, 2017
Notes Tables  
Schedule of Change in Asset Retirement Obligation [Table Text Block]
   
2017
   
2016
 
Balance at January 1
   $
3,708
     $
3,429
  
Asset retirement obligation
    
228
      
159
  
Accretion expense
    
196
      
182
  
Settlement of obligations
    
(44
 )    
(62
 )
Balance at December 31
   $
4,088
     $
3,708