Supplemental Information on Oil and Gas Exploration and Production Activities (Unaudited) - Standardized Measure of Discounted Future Net Cash Flows Relating to Proved Oil and Gas Reserves (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Extractive Industries [Abstract]
Future cash flows	$ 4,079	$ 10,451	$ 14,483
Future development and abandonment costs	(652)	(2,058)	(2,438)
Future production costs	(2,325)	(4,513)	(5,068)
Future production taxes	(137)	(333)	(456)
Future income taxes	0	0	(943)
Future net cash flows	965	3,547	5,578
10% discount to reflect timing of cash flows	(425)	(1,844)	(2,596)
Standardized measure of discounted future net cash flows	$ 540	$ 1,703	$ 2,982