Derivative And Financial Instruments (Effect On Statement Of Operations) (Details) - Commodity Contract - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Embedded Derivative [Line Items]
Net gains (losses) on derivatives	$ (1,937)	$ 6,055
Oil reserves
Embedded Derivative [Line Items]
Net gains (losses) on derivatives	(1,939)	5,495	$ 3,284
Natural gas
Embedded Derivative [Line Items]
Net gains (losses) on derivatives	$ 2	$ 560	$ 663