Annual report pursuant to Section 13 and 15(d)

Acquisitions and Dispositions (Schedule of Acquisition Fair Value of Net Assets ) (Details)

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Acquisitions and Dispositions (Schedule of Acquisition Fair Value of Net Assets ) (Details) - USD ($)
$ in Thousands
Nov. 09, 2015
Oct. 08, 2014
Mar. 31, 2014
Jun. 30, 2013
Central Midland Basin Texas [Member]        
Business Acquisition [Line Items]        
Asset retirement obligations $ (37) $ (140)    
Net assets acquired 29,800 210,205    
Central Midland Basin Texas [Member] | Oil and Gas Properties [Member]        
Business Acquisition [Line Items]        
Oil and natural gas properties 24,926 91,895    
Central Midland Basin Texas [Member] | Unevaluated Oil and Gas Properties [Member]        
Business Acquisition [Line Items]        
Oil and natural gas properties $ 4,911 $ 118,450    
Southern Midland Basin Texas [Member]        
Business Acquisition [Line Items]        
Asset retirement obligations     $ (124) $ (25)
Net assets acquired     8,200 11,000
Southern Midland Basin Texas [Member] | Oil and Gas Properties [Member]        
Business Acquisition [Line Items]        
Oil and natural gas properties     930 9,025
Southern Midland Basin Texas [Member] | Unevaluated Oil and Gas Properties [Member]        
Business Acquisition [Line Items]        
Oil and natural gas properties     $ 7,394 $ 2,000