Divestitures, Assets Held for Sale, and Acquisitions Acquisitions (Details) $ in Thousands	3 Months Ended	6 Months Ended			9 Months Ended
	Mar. 31, 2017 USD ($)	Jun. 30, 2017 USD ($)	Jun. 30, 2017 USD ($) a	Jun. 30, 2016 USD ($)	Jun. 30, 2017 USD ($)
Business Acquisition [Line Items]
Payments to Acquire Oil and Gas Property and Equipment			$ 88,140	$ 17,751
Other Howard and Martin Counties Trades [Member]
Business Acquisition [Line Items]
Non-monetary Trade, Acreage Acquired | a			6,550
Non-monetary Trade, Acreage Exchanged | a			5,700
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			3,400
Payments to Acquire Oil and Gas Property and Equipment			$ 72,300
QStar Acquisition 2016 [Member]
Business Acquisition [Line Items]
Payments to Acquire Oil and Gas Property and Equipment			$ 7,100
Acquisition of Oil And Gas Property And Equipment, Consideration Transferred		$ 1,600,000