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, 2017	Dec. 31, 2016	Dec. 31, 2015
Capitalized costs:
Proved properties	$ 5,712,813	$ 5,446,305	$ 5,401,618
Unproved properties not being amortized	296,764	314,867	337,099
Capitalized costs gross	6,009,577	5,761,172	5,738,717
Accumulated depreciation, depletion, amortization, and impairment	(4,996,696)	(4,900,304)	(4,631,404)
Net capitalized costs	1,012,881	860,868	1,107,313
Costs incurred:
Unproved properties acquired	47,029	21,675	41,777
Proved properties acquired	47,638	564	179
Exploration	14,811	17,325	19,222
Development	160,941	80,582	208,845
Asset retirement obligation	(3,613)	(30,906)	(5,693)
Total costs incurred	$ 266,806	$ 89,240	$ 264,330