Supplemental Oil and Gas Disclosures (Composition of Net Unevaluated Costs Excluded from Amortization) (Details) - USD ($) $ in Millions	12 Months Ended			132 Months Ended	168 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2017
Supplemental Oil and Gas Disclosures [Abstract]
Property acquisition costs	$ 80	$ 18	$ 145	$ 1,295	$ 1,538
Exploration and development costs	67	7	32	14	120
Capitalized interest	67	41	33	18	159
Net unevaluated costs excluded from amortization	$ 214	$ 66	$ 210	$ 1,327	$ 1,817