Supplemental Oil and Gas Disclosures (Standardized Measure of Discounted Future Cash Flows Relating to Proved Reserves Disclosure) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Supplemental Oil and Gas Disclosures [Abstract]
Future cash inflows	$ 36,576	$ 9,064	$ 11,887
Future production costs	(18,390)	(5,880)	(7,376)
Future development costs	(4,676)	(485)	(792)
Future income tax expense	(1,342)
Future net cash flows	12,168	2,699	3,719
10% annual discount for estimated timing of cash flows	(6,606)	(1,034)	(1,302)
Standardized measure of discounted future net cash flows	$ 5,562	$ 1,665	$ 2,417	$ 7,543
Annual discount for estimated timing of cash flows percent	10.00%	10.00%	10.00%