Supplemental Information on Oil and Gas Producing Activities (Unaudited) - Changes in standardized measure of discounted future net cash flow (Detail) $ in Thousands	12 Months Ended
	Dec. 31, 2019 USD ($) $ / MMcfe	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($) $ / MMcfe	Dec. 31, 2016 USD ($) $ / MMcfe
Supplemental Information on Oil and Gas Producing Activities (Unaudited)
Weighted average price of equivalent reserves (in dollar per share) | $ / MMcfe	3.56		3.23	2.39
Changes in Standardized Measure of Discounted Future Net Cash Flow
Sales of oil and gas, net of productions costs		$ (2,051)	$ (1,469)	$ (756)
Net changes in prices and production costs		707	3,918	(1,540)
Development costs incurred during the period		755	627	733
Net changes in future development costs		37	229	212
Extensions, discoveries and other additions		1,925	1,448	673
Acquisitions			258	66
Divestitures				(7)
Revisions of previous quantity estimates		(53)	734	461
Accretion of discount		1,018	368	363
Net change in income taxes		(563)	(1,159)	12
Other changes		76	386	(163)
Net increase (decrease)		1,851	5,340	54
Beginning of period	$ 10,478	8,627	3,287	3,233
End of period		$ 10,478	$ 8,627	$ 3,287