Capitalized Exploratory Well Costs and Undeveloped Leasehold Costs (Details) $ in Millions	3 Months Ended	9 Months Ended
	Sep. 30, 2017 USD ($)	Sep. 30, 2017 USD ($)	Sep. 30, 2016 USD ($)	Sep. 30, 2017 USD ($)	Jun. 30, 2017 MMBoe	Apr. 24, 2017 USD ($)	Dec. 31, 2016 USD ($)
Capitalized Exploratory Well Costs [Roll Forward]
Capitalized Exploratory Well Costs, December 31, 2016		$ 768
Additions to Capitalized Exploratory Well Costs Pending Determination of Proved Reserves		10
Reclassified to Proved Oil and Gas Properties Based on Determination of Proved Reserves		(203)
Capitalized Exploratory Well Costs, September 30, 2017	$ 575	575
Proved undeveloped reserves (MMBOEs) | MMBoe					551
Exploratory Well Costs Capitalized for a Period of One Year or Less				$ 11			$ 69
Exploratory Well Costs Capitalized for a Period Greater Than One Year Since Commencement of Drilling				564			699
Balance at End of Period	575	768		$ 575			$ 768
Capitalized undeveloped leasehold cost		3,000
Impairment of undeveloped oil and gas leasehold	33	51	$ 81
Clayton Williams Energy
Capitalized Exploratory Well Costs [Roll Forward]
Capitalized undeveloped leasehold cost		1,600
Onshore US
Capitalized Exploratory Well Costs [Roll Forward]
Capitalized undeveloped leasehold cost		1,100
Eagle Ford
Capitalized Exploratory Well Costs [Roll Forward]
Capitalized undeveloped leasehold cost		149
Deepwater Gulf of Mexico
Capitalized Exploratory Well Costs [Roll Forward]
Capitalized undeveloped leasehold cost		56
Other Int'l
Capitalized Exploratory Well Costs [Roll Forward]
Capitalized undeveloped leasehold cost		53
Gulf of Mexico and Falkland Islands
Capitalized Exploratory Well Costs [Roll Forward]
Impairment of undeveloped oil and gas leasehold	$ 31	$ 49
Clayton Williams Energy
Capitalized Exploratory Well Costs [Roll Forward]
Properties acquired						$ 1,571