Supplemental Oil and Gas Disclosures (Composition of Net Unevaluated Costs Excluded from Amortization) (Details) - USD ($) $ in Millions	12 Months Ended			156 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Capitalized Costs of Unproved Properties Excluded from Amortization, Period Cost [Abstract]
Property acquisition costs	$ 45	$ 40	$ 32	$ 1,106
Exploration and development costs	53	23	16	12
Capitalized interest	67	47	27	38
Net unevaluated costs excluded from amortization	165	110	$ 75	$ 1,156
Capitalized Costs of Unproved Properties Excluded from Amortization, Cumulative [Abstract]
Property acquisition costs	1,223
Exploration and development costs	104
Capitalized interest	179
Net unevaluated costs excluded from amortization, cumulative	$ 1,506	$ 1,755