Capitalized Exploratory Well Costs and Undeveloped Leasehold Costs - Changes in Capitalized Exploratory Well Costs (Details) $ in Millions	3 Months Ended
	Mar. 31, 2018 USD ($)
Capitalized Exploratory Well Costs [Roll Forward]
Capitalized Exploratory Well Costs, Beginning of Period	$ 520
Additions to Capitalized Exploratory Well Costs Pending Determination of Proved Reserves	3
Reclassified to Assets Held for Sale	(159)
Reclassified to Proved Oil and Gas Properties Based on Determination of Proved Reserves	(1)
Capitalized Exploratory Well Costs, End of Period	$ 363