Exploratory Well Costs (Capitalized Exploratory Costs And the Number Of Projects For Which Exploratory Costs Have Been Capitalized) (Details) $ in Millions	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)
Capitalized Exploratory Well Costs [Abstract]
One year or less	$ 495	$ 660	$ 509
More than one year	3	0	0
Total	$ 498	$ 660	$ 509
Number of projects with exploratory well costs that have been suspended for a period greater than one year (a)	1	0	0