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
May 26, 2016
May 16, 2016
Jan. 18, 2016
Howard County, Texas [Member] | Big Star Transaction [Member]      
Business Acquisition [Line Items]      
Oil and natural gas properties $ 96,194    
Asset retirement obligations (8)    
Net assets acquired 329,573    
Howard County, Texas [Member] | Big Star Transaction [Member] | Unevaluated Oil and Gas Properties [Member]      
Business Acquisition [Line Items]      
Oil and natural gas properties $ 233,387    
Casselman-Bohannon Fields [Member]      
Business Acquisition [Line Items]      
Oil and natural gas properties     $ 5,527
Net assets acquired     10,183
Casselman-Bohannon Fields [Member] | Unevaluated Oil and Gas Properties [Member]      
Business Acquisition [Line Items]      
Oil and natural gas properties     $ 4,656
Western Reagan County, Texas [Member] | AMI Transaction [Member]      
Business Acquisition [Line Items]      
Oil and natural gas properties   $ 15,951  
Asset retirement obligations   (8)  
Net assets acquired   33,012  
Western Reagan County, Texas [Member] | AMI Transaction [Member] | Unevaluated Oil and Gas Properties [Member]      
Business Acquisition [Line Items]      
Oil and natural gas properties   $ 17,069