Supplemental Oil and Gas Disclosures - Standardized Measure of Discounted Future Net Cash Flows (Detail) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Discounted Future Net Cash Flows Relating to Proved Oil and Gas Reserves [Line Items]
Future cash inflows	$ 18,012	$ 20,963
Future production costs	8,407	7,615
Future development costs	1,469	2,345
Future income tax provisions	772	1,366
Future net cash flows	7,364	9,637
Less 10 percent annual discount for estimated timing of cash flows	(3,233)	(4,446)
Standardized measure of discounted future net cash inflows	$ 4,131	$ 5,191	$ 3,161	$ 1,038