Capitalized Exploratory Well Costs and Undeveloped Leasehold Costs - Exploratory Well Costs (Details) $ in Millions	12 Months Ended
	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2018 USD ($) project	Dec. 31, 2017 USD ($) project	Dec. 31, 2016 USD ($) project
Increase (Decrease) in Capitalized Exploratory Well Costs that are Pending Determination of Proved Reserves [Roll Forward]
Capitalized Exploratory Well Costs, Beginning of Period	$ 520	$ 768	$ 1,353
Additions to Capitalized Exploratory Well Costs Pending Determination of Proved Reserves	7	20	84
Divestitures and Other	(168)	0	(143)
Reclassified to Proved Oil and Gas Properties Based on Determination of Proved Reserves or to Assets Held for Sale	(1)	(203)	(1)
Capitalized Exploratory Well Costs Charged to Expense	(4)	(65)	(525)
Capitalized Exploratory Well Costs, End of Period	354	520	768
Exploratory Wells Drilled [Line Items]
Ownership interest sold						35.00%
Exploratory Well Costs Capitalized for a Period of One Year or Less				$ 6	$ 10	$ 69
Exploratory Well Costs Capitalized for a Period Greater Than One Year Since Commencement of Drilling				348	510	699
Balance at End of Period	$ 520	$ 520	$ 768	$ 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				7	8	10
Cyprus Block 12
Exploratory Wells Drilled [Line Items]
Ownership interest sold				35.00%