Supplemental Oil and Gas Disclosures (Unaudited) (Composition of Net Unevaluated Costs Excluded from Amortization) (Details) - USD ($) $ in Millions	12 Months Ended			168 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Capitalized Costs of Unproved Properties Excluded from Amortization, Period Cost [Abstract]
Property acquisition costs	$ 116	$ 44	$ 34	$ 1,022
Exploration and development costs	17	17	14	20
Capitalized interest	62	47	33	46
Net unevaluated costs excluded from amortization	195	108	$ 81	$ 1,088
Capitalized Costs of Unproved Properties Excluded from Amortization, Cumulative [Abstract]
Property acquisition costs	1,216
Exploration and development costs	68
Capitalized interest	188
Net unevaluated costs excluded from amortization, cumulative	$ 1,472	$ 1,506