Annual report pursuant to Section 13 and 15(d)

Acquisitions and Dispositions (Narrative) (Details)

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Acquisitions and Dispositions (Narrative) (Details)
12 Months Ended
Jun. 05, 2017
USD ($)
a
Feb. 13, 2017
USD ($)
a
Oct. 20, 2016
USD ($)
a
May 26, 2016
USD ($)
a
$ / shares
shares
Dec. 31, 2017
USD ($)
Dec. 31, 2016
USD ($)
Dec. 31, 2015
USD ($)
Sep. 30, 2017
May 19, 2017
USD ($)
Oct. 03, 2016
USD ($)
Business Acquisition [Line Items]                    
Acquisition deposit         $ 900,000 $ 46,138,000        
Proceeds from sales of working interest         $ 20,525,000 24,562,000 $ 377,000      
6.125% senior unsecured notes due 2024                    
Business Acquisition [Line Items]                    
Debt instrument principal amount                 $ 200,000,000 $ 400,000,000
Debt instrument, interest rate, stated (as a percent)         6.125%     6.125% 6.125% 6.125%
Delaware Basin                    
Business Acquisition [Line Items]                    
Gas and oil area, developed and undeveloped, gross | a 7,031 29,175                
Gas and oil area, developed and undeveloped, net | a 2,488 16,688                
Consideration transferred $ 52,500,000 $ 646,559,000                
Working interest   82.00%                
Net revenue interest   75.00%                
Acquisition deposit           46,138,000        
Plymouth Transaction                    
Business Acquisition [Line Items]                    
Gas and oil area, developed and undeveloped, gross | a     6,904              
Gas and oil area, developed and undeveloped, net | a     5,952              
Consideration transferred     $ 339,687,000              
Working interest     82.00%              
Net revenue interest     62.00%              
Big Star Transaction                    
Business Acquisition [Line Items]                    
Gas and oil area, developed and undeveloped, gross | a       17,298            
Gas and oil area, developed and undeveloped, net | a       14,089            
Consideration transferred     $ 329,573,000 $ 220,000,000            
Working interest     81.00%              
Net revenue interest     61.00%              
Shares of common stock issued in acquisition | shares       9,333,333            
Sales price of common stock | $ / shares       $ 11.74            
Midland Basin                    
Business Acquisition [Line Items]                    
Consideration transferred           73,240,000 $ 29,800,000      
Proceeds from sales of working interest           $ 23,045,000