Annual report pursuant to Section 13 and 15(d)

Acquisitions and Dispositions (Fair Value of Net Assets Acquired ) (Details)

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Acquisitions and Dispositions (Fair Value of Net Assets Acquired ) (Details) - USD ($)
$ in Thousands
Aug. 31, 2018
Feb. 13, 2017
Oct. 20, 2016
May 26, 2016
Cimarex Energy Company        
Business Acquisition [Line Items]        
Asset retirement obligations $ (570)      
Net assets acquired 539,519      
Cimarex Energy Company | Evaluated oil and natural gas properties        
Business Acquisition [Line Items]        
Oil and natural gas properties 253,089      
Cimarex Energy Company | Unevaluated oil and natural gas properties        
Business Acquisition [Line Items]        
Oil and natural gas properties $ 287,000      
Delaware Basin        
Business Acquisition [Line Items]        
Asset retirement obligations   $ (168)    
Net assets acquired   646,559    
Delaware Basin | Evaluated oil and natural gas properties        
Business Acquisition [Line Items]        
Oil and natural gas properties   137,368    
Delaware Basin | Unevaluated oil and natural gas properties        
Business Acquisition [Line Items]        
Oil and natural gas properties   $ 509,359    
Plymouth Transaction        
Business Acquisition [Line Items]        
Asset retirement obligations     $ (20)  
Net assets acquired     339,687  
Plymouth Transaction | Evaluated oil and natural gas properties        
Business Acquisition [Line Items]        
Oil and natural gas properties     65,043  
Plymouth Transaction | Unevaluated oil and natural gas properties        
Business Acquisition [Line Items]        
Oil and natural gas properties     $ 274,664  
Big Star Transaction        
Business Acquisition [Line Items]        
Asset retirement obligations       $ (8)
Net assets acquired       329,573
Big Star Transaction | Evaluated oil and natural gas properties        
Business Acquisition [Line Items]        
Oil and natural gas properties       96,194
Big Star Transaction | Unevaluated oil and natural gas properties        
Business Acquisition [Line Items]        
Oil and natural gas properties       $ 233,387