Derivative And Financial Instruments (Changes In Fair Value) (Details) $ in Thousands	1 Months Ended	3 Months Ended		6 Months Ended		12 Months Ended
	Aug. 31, 2017 USD ($)	Jun. 30, 2018 USD ($)	Jun. 30, 2017 USD ($)	Jun. 30, 2018 USD ($) item	Jun. 30, 2017 USD ($)	Dec. 31, 2017 USD ($)
Derivative Instruments Gain Loss [Line Items]
Early termination of hedges	$ 3,600
Commodity Contract
Derivative Instruments Gain Loss [Line Items]
Beginning fair value of commodity derivatives				$ 1,231	$ 6,436	$ 6,436
Net gains (losses) on derivatives		$ (3,715)	$ 3,213	(5,653)	9,268
Ending fair value of commodity derivatives		(3,676)		$ (3,676)		1,231
Number of counterparties | item				4
Oil reserves | Commodity Contract
Derivative Instruments Gain Loss [Line Items]
Net gains (losses) on derivatives		(3,717)	3,048	$ (5,657)	8,543	3,284
Net settlements paid (received) on derivative contracts						(6,422)
Net settlements paid (received) on derivative contracts				771
Natural gas | Commodity Contract
Derivative Instruments Gain Loss [Line Items]
Net gains (losses) on derivatives		$ 2	$ 165	4	$ 725	663
Net settlements paid (received) on derivative contracts				$ (25)		$ (2,730)