Supplemental Oil And Gas Disclosures (Schedule Of Principal Sources Of Changes In Standardized Measure Of Discounted Future Net Cash Flows) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Reserve Quantities [Line Items]
Sales and transfers of oil and natural gas produced, net of production costs	$ (239,953)	$ (173,920)	$ (218,115)
Net changes in prices and production costs	236,126	(94,026)	(1,356,333)
Revisions in quantity estimates and changes in production timing	87,239	(51,979)	(213,945)
Extensions, discoveries, and improved recovery, less related costs	102,965	84,738	95,671
Changes in estimated future development costs	(5,194)	70,976	227,857
Previously estimated cost incurred during the period	36,044	16,602	59,117
Purchases of minerals in place	51,686	2,652	0
Sales of minerals in place	(1,447)	(17,248)	(3,338)
Accretion of discount	57,517	69,069	209,979
Net change in income taxes	(33,389)	44,241	562,838
Other-net	(2,634)	(22,381)	(209,989)
Net change	288,960	(71,276)	(846,258)
Beginning of year	518,210	589,486	1,435,744
End of year	$ 807,170	$ 518,210	$ 589,486