Derivative Instruments (Schedule of Cash Received for Derivative Settlements) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Schedule Of Cash Received For Derivatives [Line Items]
Cash paid for commodity derivative settlements, net	$ (4,522)	$ (24,083)	$ (7,160)	$ (38,448)
Cash paid for settlements of contingent consideration arrangements, net	0	0	(40,000)	0
Receipt For Contingent Consideration Asset, Financing Activities			10,000
Crude Oil
Schedule Of Cash Received For Derivatives [Line Items]
Cash paid for commodity derivative settlements, net	(3,698)	(21,210)	(4,018)	(33,333)
Natural Gas Liquids
Schedule Of Cash Received For Derivatives [Line Items]
Cash paid for commodity derivative settlements, net	0	(756)	623	(1,188)
Natural Gas
Schedule Of Cash Received For Derivatives [Line Items]
Cash paid for commodity derivative settlements, net	1,925	488	1,625	540
Deferred Premiums on Derivative Instruments
Schedule Of Cash Received For Derivatives [Line Items]
Cash paid for commodity derivative settlements, net	(2,749)	(2,605)	(5,390)	(4,467)
ExL Acquisition
Schedule Of Cash Received For Derivatives [Line Items]
Cash paid for settlements of contingent consideration arrangements, net	0	0	(50,000)	0
Niobrara Divestiture
Schedule Of Cash Received For Derivatives [Line Items]
Receipt For Contingent Consideration Asset, Financing Activities	0	0	5,000	0
Utica Shale Divestiture
Schedule Of Cash Received For Derivatives [Line Items]
Receipt For Contingent Consideration Asset, Financing Activities	$ 0	$ 0	$ 5,000	$ 0