Supplemental Oil and Gas Information (Unaudited) -Standardized Measure of Discounted Future Net Cash Flows (Detail) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Extractive Industries [Abstract]
Future cash inflows	$ 2,410,260	$ 5,146,288
Future production costs	(1,589,945)	(2,917,262)
Future development costs	(375,146)	(567,423)
Future income tax expense	0	0
Future net cash flows for estimated timing of cash flows	445,169	1,661,603
10% annual discount for estimated timing of cash flows	(147,358)	(745,042)
Standardized measure of discounted future net cash flows	$ 297,811	$ 916,561	$ 1,113,029