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Note 5 - Asset Retirement Obligations (Tables)
 12 Months Ended
Dec. 31, 2018
Notes Tables  
Schedule of Change in Asset Retirement Obligation [Table Text Block]
   
Year Ended December 31,
  
(In thousands)
  
2018
    
2017
  
Balance at beginning of year
   $
12,347
     $
10,129
  
Additional asset retirement obligations incurred
    
131
      
2,034
  
Accretion expense
    
494
      
405
  
Revisions to estimates
    
(194
 )    
(221
 )
Balance at end of year
   $
12,778
     $
12,347