Derivative Instruments (Schedule of Cash Received for Derivative Settlements) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Schedule Of Cash Received For Derivatives [Line Items]
Cash paid for commodity derivative settlements, net	$ (2,638)	$ (14,365)
Cash paid for settlements of contingent consideration arrangements, net	(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	(320)	(12,123)
Natural Gas Liquids
Schedule Of Cash Received For Derivatives [Line Items]
Cash paid for commodity derivative settlements, net	623	(432)
Natural Gas
Schedule Of Cash Received For Derivatives [Line Items]
Cash paid for commodity derivative settlements, net	(300)	52
Deferred Premiums on Derivative Instruments
Schedule Of Cash Received For Derivatives [Line Items]
Cash paid for commodity derivative settlements, net	(2,641)	(1,862)
ExL Acquisition
Schedule Of Cash Received For Derivatives [Line Items]
Cash paid for settlements of contingent consideration arrangements, net	(50,000)	0
Niobrara Divestiture
Schedule Of Cash Received For Derivatives [Line Items]
Receipt For Contingent Consideration Asset, Financing Activities	5,000	0
Utica Shale Divestiture
Schedule Of Cash Received For Derivatives [Line Items]
Receipt For Contingent Consideration Asset, Financing Activities	$ 5,000	$ 0