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
Feb. 13, 2017
Oct. 20, 2016
May 26, 2016
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