Supplemental Oil And Gas Disclosures (Results Of Operations For Producing Activities) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Supplemental Oil and Gas Disclosures [Abstract]
Revenues	$ 347,285	$ 282,742	$ 371,335
Production costs	(107,332)	(108,822)	(152,560)
Depreciation, depletion, amortization, and impairment	(96,392)	(268,901)	(1,844,726)
Results of operations, income before income taxes	143,561	(94,981)	(1,625,951)
Income tax (expense) benefit	(56,376)	32,696	612,496
Results of operations for producing activities (excluding corporate overhead and financing costs)	$ 87,185	$ (62,285)	$ (1,013,455)