Capitalized Exploratory Well Costs and Undeveloped Leasehold Costs - Rollforward of Undeveloped Lease Costs (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Proved Developed and Undeveloped Reserves [Roll Forward]
Undeveloped Leasehold Costs, Beginning of Period	$ 2,922	$ 2,197
Additions to Undeveloped Leasehold Costs	47	1,859
Transfers to Proved Properties	(453)	(174)
Assets Sold	(142)	(884)
Impairment	(1)	(62)
Other	0	(14)
Undeveloped Leasehold Costs, Net of Impairment, End of Period	$ 2,373	$ 2,922