Supplemental Oil and Gas Disclosures (Standardized Measure of Discounted Future Cash Flows Relating to Proved Reserves Disclosure) (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Supplemental Oil and Gas Disclosures [Abstract]
Future cash inflows	$ 34,523	$ 36,576	$ 9,064
Future production costs	(15,347)	(18,390)	(5,880)
Future development costs	(4,095)	(4,676)	(485)
Future income tax expense	(2,079)	(1,342)
Future net cash flows	13,002	12,168	2,699
10% annual discount for estimated timing of cash flows	(7,003)	(6,606)	(1,034)
Standardized measure of discounted future net cash flows	$ 5,999	$ 5,562	$ 1,665	$ 2,417