Supplemental Oil and Gas Disclosures (Schedule of Analysis of Changes in Standardized Measure) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Supplemental Oil and Gas Disclosures [Abstract]
Standardized measure, beginning of year	$ 1,665	$ 2,417	$ 7,543
Sales and transfers of natural gas and oil produced, net of production costs	(1,191)	(574)	(1,082)
Net changes in prices and production costs	1,963	(415)	(8,075)
Extensions, discoveries, and other additions, net of future production and development costs	1,715	45	162
Acquisition of reserves in place			28
Sales of reserves in place		(10)	(244)
Revisions of previous quantity estimates	1,721	(140)	(1,385)
Net change in income taxes	(222)		1,915
Changes in estimated future development costs	(6)	71	2,007
Previously estimated development costs incurred during the year	55	114	875
Changes in production rates (timing) and other	(304)	(85)	(273)
Accretion of discount	166	242	946
Standardized measure, end of year	$ 5,562	$ 1,665	$ 2,417