Supplemental Oil and Gas Disclosures (Standardized Measure of Discounted Future Cash Flows Relating to Proved Reserves Disclosure) (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Oil and Gas Exploration and Production Industries Disclosures [Abstract]
Future cash inflows	$ 27,003	$ 34,523	$ 36,576
Future production costs	(14,981)	(15,347)	(18,390)
Future development costs	(3,246)	(4,095)	(4,676)
Future income tax expense	(476)	(2,079)	(1,342)
Future net cash flows	8,300	13,002	12,168
10% annual discount for estimated timing of cash flows	(4,600)	(7,003)	(6,606)
Standardized measure of discounted future net cash flows	$ 3,700	$ 5,999	$ 5,562	$ 1,665