Supplemental Oil and Gas Disclosures (Unaudited) (Standardized Measure of Discounted Future Cash Flows Relating to Proved Reserves Disclosure) (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Oil and Gas Exploration and Production Industries Disclosures [Abstract]
Future cash inflows	$ 17,997	$ 27,003	$ 34,523
Future production costs	(11,969)	(14,981)	(15,347)
Future development costs	(1,924)	(3,246)	(4,095)
Future income tax expense	0	(476)	(2,079)
Future net cash flows	4,104	8,300	13,002
10% annual discount for estimated timing of cash flows	(2,257)	(4,600)	(7,003)
Standardized measure of discounted future net cash flows	$ 1,847	$ 3,700	$ 5,999	$ 5,562