Supplemental Oil and Gas Disclosures (Composition of Net Unevaluated Costs Excluded from Amortization) (Details) - USD ($) $ in Millions	12 Months Ended			144 Months Ended	180 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2018
Supplemental Oil and Gas Disclosures [Abstract]
Property acquisition costs	$ 49	$ 70	$ 12	$ 1,346	$ 1,477
Exploration and development costs	42	23	6	23	94
Capitalized interest	77	45	28	34	184
Net unevaluated costs excluded from amortization	$ 168	$ 138	$ 46	$ 1,403	$ 1,755