Derivative And Financial Instruments (Effect On Statement Of Operations) (Details) - Commodity Contract - USD ($) $ in Thousands	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Embedded Derivative [Line Items]
Net gains (losses) on derivatives	$ (3,715)	$ 3,213	$ (5,653)	$ 9,268
Oil reserves
Embedded Derivative [Line Items]
Net gains (losses) on derivatives	(3,717)	3,048	(5,657)	8,543	$ 3,284
Natural gas
Embedded Derivative [Line Items]
Net gains (losses) on derivatives	$ 2	$ 165	$ 4	$ 725	$ 663