Supplementary Information (Costs Incurred For Oil And Natural Gas Producing Activities) (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Disclosure Supplementary Information Costs Incurred For Oil And Natural Gas Producing Activities [Abstract]
Proved	$ 162	$ 1	$ 301	$ 257
Unproved	472	14	865	172
Exploration	252	177	725	513
Development	175	97	478	287
Total costs incurred for oil and natural gas properties	$ 1,061	$ 289	$ 2,369	$ 1,229