Capitalized Exploratory Well Costs and Undeveloped Leasehold Costs - Aging of Capitalized Exploratory Well Costs (Details) $ in Millions	Dec. 31, 2019 USD ($) project	Dec. 31, 2018 USD ($) project	Dec. 31, 2017 USD ($) project	Dec. 31, 2016 USD ($)
Capitalized Exploratory Well Costs [Abstract]
Exploratory Well Costs Capitalized for a Period of One Year or Less	$ 22	$ 6	$ 10
Exploratory Well Costs Capitalized for a Period Greater Than One Year Since Commencement of Drilling	258	348	510
Balance at End of Period	$ 280	$ 354	$ 520	$ 768
Number of Projects with Exploratory Well Costs That Have Been Capitalized for a Period Greater Than One Year Since Commencement of Drilling | project	5	7	8