Divestitures, Assets Held for Sale, and Acquisitions Acquisitions (Details) $ in Thousands	3 Months Ended	6 Months Ended
	Jun. 30, 2018 USD ($) a	Jun. 30, 2018 USD ($)	Jun. 30, 2017 USD ($) a
Business Acquisition [Line Items]
Payments to Acquire Oil and Gas Property and Equipment		$ 24,615	$ 88,140
Noncash or Part Noncash Acquisition, Net Nonmonetary Assets Acquired (Liabilities Assumed)		$ 0
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
Noncash or Part Noncash Acquisition, Net Nonmonetary Assets Acquired (Liabilities Assumed)			$ 279,750
Gain or loss recognized on trade			0
Other Martin County Acquisition 2018 [Member]
Business Acquisition [Line Items]
Payments to Acquire Oil and Gas Property and Equipment	$ 24,600
Net Acres Acquired | a	720
Other Howard and Martin Counties Acquisitions 2017 [Member]
Business Acquisition [Line Items]
Payments to Acquire Oil and Gas Property and Equipment			$ 72,300
Net Acres Acquired | a			3,400