Supplemental Oil And Gas Disclosures (Composition Of Net Unevaluated Costs Excluded From Amortization) (Details) - USD ($) $ in Millions	12 Months Ended			120 Months Ended	156 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2016
Supplemental Oil And Gas Disclosures [Abstract]
Property acquisition costs	$ 22	$ 213	$ 1,501	$ 54	$ 1,790
Exploration and development costs	55	64	24	16	159
Capitalized interest	70	55	10	21	156
Net unevaluated costs excluded from amortization	$ 147	$ 332	$ 1,535	$ 91	$ 2,105