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Note 6 - Property and Equipment (Tables)
 12 Months Ended
Dec. 31, 2018
Notes Tables  
Property, Plant and Equipment [Table Text Block]
   
Estimated
    
December 31,
 
   
Useful life
    
2017
    
2018
  
   
Years
    
(In thousands)
  
Oil and gas properties
  
(1)
     $
923,237
     $
1,091,905
  
Equipment and other
  
3-39
      
15,648
      
15,369
  
Drilling rig
  
15
      
23,488
      
24,084
  
   
 
      
962,373
      
1,131,358
  
Accumulated depreciation, depletion, amortization and impairment
  
 
      
(724,606
 )    
(768,140
 )
Net Property and Equipment
  
 
     $
237,767
     $
363,218