Supplemental Oil And Gas Disclosures (Schedule Of Capitalized Costs And Costs Incurred On Oil And Gas Properties) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Capitalized costs:
Proved properties	$ 5,446,305	$ 5,401,618	$ 4,990,753
Unproved properties not being amortized	314,867	337,099	485,568
Capitalized costs gross	5,761,172	5,738,717	5,476,321
Accumulated depreciation, depletion, amortization, and impairment	(4,900,304)	(4,631,404)	(2,786,678)
Net capitalized costs	860,868	1,107,313	2,689,643
Costs incurred:
Unproved properties acquired	21,675	41,777	76,041
Proved properties acquired	564	179	5,723
Exploration	17,325	19,222	68,811
Development	80,582	208,845	615,252
Asset retirement obligation	(30,906)	(5,693)	(37,687)
Total costs incurred	$ 89,240	$ 264,330	$ 728,140