Supplemental Information on Oil and Gas Operations (Unaudited) (Schedule Of Principal Changes In The Standardized Measure Of Discounted Future Net Cash Flows Attributable To Proved Reserves) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Supplemental Information On Oil And Gas Operations [Abstract]
Standardized measure of discounted future net cash flows, beginning balance	$ 5,398	$ 7,150	$ 5,954
Net changes in prices and production costs	(3,277)	(2,323)	1,533
Oil, gas and NGL sales, net of production costs	(1,572)	(2,612)	(2,932)
Changes in estimated future development costs	402	303	(273)
Extensions and discoveries, net of future development costs	988	1,690	2,944
Purchase of reserves	23	43
Sales of reserves in place	(7)	(481)	(120)
Revisions of quantity estimates	147	(359)	(152)
Previously estimated development costs incurred during the period	537	857	787
Accretion of discount	285	506	648
Net change in income taxes and other	548	624	(1,239)
Standardized measure of discounted future net cash flows, ending balance	$ 3,472	$ 5,398	$ 7,150