Derivative Instruments (Schedule of Cash Received for Derivative Settlements) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Schedule Of Cash Received For Derivatives [Line Items]
Cash (paid) received for derivative settlements, net	$ (24,083)	$ (261)	$ (38,448)	$ 1,258
Crude Oil
Schedule Of Cash Received For Derivatives [Line Items]
Cash (paid) received for derivative settlements, net	(21,210)	409	(33,333)	3,441
Natural Gas Liquids
Schedule Of Cash Received For Derivatives [Line Items]
Cash (paid) received for derivative settlements, net	(756)	0	(1,188)	0
Natural Gas
Schedule Of Cash Received For Derivatives [Line Items]
Cash (paid) received for derivative settlements, net	488	(104)	540	(1,253)
Deferred Premiums on Derivative Instruments
Schedule Of Cash Received For Derivatives [Line Items]
Cash (paid) received for derivative settlements, net	$ (2,605)	$ (566)	$ (4,467)	$ (930)