Divestitures, Assets Held for Sale, and Acquisitions Acquisitions (Details) $ in Thousands	3 Months Ended	9 Months Ended			12 Months Ended
	Mar. 31, 2017 USD ($)	Sep. 30, 2017 USD ($)	Sep. 30, 2017 USD ($) a	Sep. 30, 2016 USD ($)	Sep. 30, 2017 USD ($)
Acquisition [Line Items]
Cash consideration paid to acquire oil and gas properties			$ 87,389	$ 21,853
Non-monetary trade, value attributed to properties assigned				$ 733
Other Howard and Martin Counties Trades [Member]
Acquisition [Line Items]
Non-monetary trade, acreage acquired | a			7,425
Non-monetary trade, acreage exchanged | a			6,725
Non-monetary trade, value attributed to properties assigned			$ 283,651
Rock Oil Acquisition 2016 [Member]
Acquisition [Line Items]
Cash consideration paid to acquire a business	$ 7,400				$ 1,000,000
Other Howard and Martin Counties Acquisitions 2017 [Member]
Acquisition [Line Items]
Net acres acquired | a			3,400
Cash consideration paid to acquire oil and gas properties			$ 72,200
QStar Acquisition 2016 [Member]
Acquisition [Line Items]
Cash consideration paid to acquire oil and gas properties			$ 7,300
Total consideration paid to aquire oil and gas properties		$ 1,600,000