Quarterly report pursuant to Section 13 or 15(d)

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

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Acquisitions (Fair Value of Net Assets Acquired ) (Details) - USD ($)
$ in Thousands
Aug. 31, 2018
Feb. 13, 2017
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  
Ameredev    
Business Acquisition [Line Items]    
Asset retirement obligations   $ (168)
Net assets acquired   646,559
Ameredev | Evaluated oil and natural gas properties    
Business Acquisition [Line Items]    
Oil and natural gas properties   137,368
Ameredev | Unevaluated oil and natural gas properties    
Business Acquisition [Line Items]    
Oil and natural gas properties   $ 509,359