Supplemental Oil and Gas Disclosures (Schedule of Analysis of Changes in Standardized Measure) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Supplemental Oil and Gas Disclosures [Abstract]
Standardized measure, beginning of year	$ 5,562	$ 1,665	$ 2,417
Sales and transfers of natural gas and oil produced, net of production costs	(1,564)	(1,191)	(574)
Net changes in prices and production costs	2,162	1,963	(415)
Extensions, discoveries, and other additions, net of future production and development costs	335	1,715	45
Sales of reserves in place	(2,022)		(10)
Revisions of previous quantity estimates	361	1,721	(140)
Net change in income taxes	(304)	(222)
Changes in estimated future development costs	(166)	(6)	71
Previously estimated development costs incurred during the year	536	55	114
Changes in production rates (timing) and other	521	(304)	(85)
Accretion of discount	578	166	242
Standardized measure, end of year	$ 5,999	$ 5,562	$ 1,665