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Note 14 - Asset Retirement Obligation (Tables)
 12 Months Ended
Dec. 31, 2018
Notes Tables  
Schedule of Asset Retirement Obligations [Table Text Block]
   
Year Ended December 31,
  
   
2018
   
2017
 
   
(Dollars in thousands)
  
Balance at beginning of year
   $
27,110
     $
23,348
  
Revision in estimated cash flows
    
(258
 )    
1,888
  
Liabilities incurred and acquired
    
10,149
      
—
  
Accretion expense
    
2,474
      
1,874
  
Balance at end of year
   $
39,475
     $
27,110