Supplemental Oil And Gas Disclosures (Schedule Of Analysis Of Changes In Standardized Measure) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Supplemental Oil And Gas Disclosures [Abstract]
Standardized measure, beginning of year	$ 2,417	$ 7,543	$ 3,736
Sales and transfers of natural gas and oil produced, net of production costs	(574)	(1,082)	(2,084)
Net changes in prices and production costs	(415)	(8,075)	1,192
Extensions, discoveries, and other additions, net of future production and development costs	45	162	1,049
Acquisition of reserves in place		28	1,897
Sales of reserves in place	(10)	(244)
Revisions of previous quantity estimates	(140)	(1,385)	622
Accretion of discount	242	946	513
Net change in income taxes		1,915	(522)
Changes in estimated future development costs	71	2,007	110
Previously estimated development costs incurred during the year	114	875	815
Changes in production rates (timing) and other	(85)	(273)	215
Standardized measure, end of year	$ 1,665	$ 2,417	$ 7,543