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Divestitures, Assets Held for Sale, and Acquisitions Acquisitions (Details)
$ in Thousands
3 Months Ended 4 Months Ended 6 Months Ended
Mar. 31, 2017
USD ($)
a
Mar. 31, 2016
USD ($)
Apr. 26, 2017
a
Mar. 31, 2017
USD ($)
Business Acquisition [Line Items]        
Payments to Acquire Oil and Gas Property and Equipment | $ $ 75,105 $ 15,044    
Subsequent Event [Member] | Other Howard County Trades [Member]        
Business Acquisition [Line Items]        
Non-monetary Trade, Acreage Acquired | a     3,020  
Non-monetary Trade, Acreage Exchanged | a     2,310  
Rock Oil Acquisition 2016 [Member]        
Business Acquisition [Line Items]        
Payments to Acquire Businesses, Gross | $ $ 7,400     $ 998,400
Other Midland Basin Acquisition 2017 [Member]        
Business Acquisition [Line Items]        
Net Acres Acquired | a 2,900      
Payments to Acquire Oil and Gas Property and Equipment | $ $ 59,600