Supplemental Oil and Gas Disclosures (Capitalized Costs Relating to Oil and Gas Producing Activities Disclosure) (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Supplemental Oil and Gas Disclosures [Abstract]
Proved properties	$ 22,073	$ 20,548
Unproved properties	1,817	2,105
Total capitalized costs	23,890	22,653
Less: Accumulated depreciation, depletion and amortization	(19,287)	(18,897)
Net capitalized costs	$ 4,603	$ 3,756