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
Dec. 31, 2016
Oct. 20, 2016
May 26, 2016
Dec. 31, 2015
Dec. 31, 2014
Oct. 08, 2014
Central Midland Basin Transaction [Member]            
Business Acquisition [Line Items]            
Asset retirement obligations           $ (140)
Net assets acquired           210,205
Central Midland Basin Transaction [Member] | Evaluated Oil and Natural Gas Properties [Member]            
Business Acquisition [Line Items]            
Oil and natural gas properties           91,895
Central Midland Basin Transaction [Member] | Unevaluated Oil and Natural Gas Properties [Member]            
Business Acquisition [Line Items]            
Oil and natural gas properties           $ 118,450
Howard County, Texas [Member] | Big Star Transaction [Member]            
Business Acquisition [Line Items]            
Asset retirement obligations     $ (8)      
Net assets acquired     329,573      
Howard County, Texas [Member] | Big Star Transaction [Member] | Evaluated Oil and Natural Gas Properties [Member]            
Business Acquisition [Line Items]            
Oil and natural gas properties     96,194      
Howard County, Texas [Member] | Big Star Transaction [Member] | Unevaluated Oil and Natural Gas Properties [Member]            
Business Acquisition [Line Items]            
Oil and natural gas properties     233,387      
Howard County, Texas [Member] | Big Star Transaction [Member] | May 26, 2016 Acquisitions [Member]            
Business Acquisition [Line Items]            
Net assets acquired     $ 329,573      
Howard County, Texas [Member] | Plymouth Transaction [Member]            
Business Acquisition [Line Items]            
Asset retirement obligations   $ (20)        
Net assets acquired   339,687        
Howard County, Texas [Member] | Plymouth Transaction [Member] | Evaluated Oil and Natural Gas Properties [Member]            
Business Acquisition [Line Items]            
Oil and natural gas properties   65,043        
Howard County, Texas [Member] | Plymouth Transaction [Member] | Unevaluated Oil and Natural Gas Properties [Member]            
Business Acquisition [Line Items]            
Oil and natural gas properties   $ 274,664        
Central Midland Basin [Member]            
Business Acquisition [Line Items]            
Net assets acquired $ 73,240     $ 29,800 $ 8,200