Supplemental Oil and Gas Disclosures (Unaudited) (Schedule of Analysis of Changes in Standardized Measure) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Increase (Decrease) in Standardized Measure of Discounted Future Net Cash Flows Relating to Proved Oil and Gas Reserves [Roll Forward]
Standardized measure, beginning of year	$ 3,700	$ 5,999	$ 5,562
Sales and transfers of natural gas and oil produced, net of production costs	(478)	(923)	(1,564)
Net changes in prices and production costs	(2,720)	(3,510)	2,162
Extensions, discoveries, and other additions, net of future production and development costs	81	234	335
Increase Due to Purchases of Minerals in Place	443	0	0
Sales of reserves in place	0	(2)	(2,022)
Revisions of previous quantity estimates	(987)	152	361
Net change in income taxes	35	491	(304)
Changes in estimated future development costs	1,241	621	(166)
Previously estimated development costs incurred during the year	624	704	536
Changes in production rates (timing) and other	(466)	(718)	521
Accretion of discount	374	652	578
Standardized measure, end of year	$ 1,847	$ 3,700	$ 5,999