Capitalized Exploratory Well Costs and Undeveloped Leasehold Costs - Exploratory Well Costs (Details) $ in Millions	12 Months Ended
	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2017 USD ($) project	Dec. 31, 2016 USD ($) project	Dec. 31, 2015 USD ($) project
Exploratory Wells Drilled [Line Items]
Ownership interest sold				35.00%	35.00%
Increase (Decrease) in Capitalized Exploratory Well Costs that are Pending Determination of Proved Reserves [Roll Forward]
Capitalized Exploratory Well Costs, Beginning of Period	$ 768	$ 1,353	$ 1,337
Additions to Capitalized Exploratory Well Costs Pending Determination of Proved Reserves	20	84	123
Divestitures and Other	0	(143)	0
Reclassified to Proved Oil and Gas Properties Based on Determination of Proved Reserves or to Assets Held for Sale	(203)	(1)	(19)
Capitalized Exploratory Well Costs Charged to Expense	(65)	(525)	(88)
Capitalized Exploratory Well Costs, End of Period	520	768	1,353
Exploratory Well Costs Capitalized for a Period of One Year or Less				$ 10	$ 69	$ 95
Exploratory Well Costs Capitalized for a Period Greater Than One Year Since Commencement of Drilling				510	699	1,258
Capitalized Exploratory Well Costs	$ 768	$ 1,353	$ 1,337	$ 520	$ 768	$ 1,353
Number of Projects with Exploratory Well Costs That Have Been Capitalized for a Period Greater Than One Year Since Commencement of Drilling | project				8	10	14
Cyprus Block 12
Exploratory Wells Drilled [Line Items]
Ownership interest sold				35.00%