Supplemental Information on Oil and Gas Producing Activities (Unaudited) - Discounted future cash flows (Detail) MMcfe in Thousands, Bcfe in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2018 USD ($) Bcfe	Dec. 31, 2017 USD ($) Bcfe	Dec. 31, 2016 USD ($) MMcfe	Dec. 31, 2015 USD ($)
Supplemental Information on Oil and Gas Producing Activities (Unaudited)
Increase in proved reserves due to ethane recovery		345	1,359
Increase (decrease) in proved reserves due to development plan revisions | Bcfe	742	188
Increase in proved reserves due to additions to development plan | Bcfe	1,722	2,092
Decrease in proved reserves due to reclassifications related to five-year rule	2,464	2,280	(2,478)
Increase (decrease) in proved reserves resulting from price revisions	18		(47)
Increase (decrease) in proved reserves due to performance revisions	115	132	762
Future cash inflows computation period	12 months
Percentage of net cash inflows that were discounted at annual rate	10.00%
Annual net cash inflows
Period of unweighted first day of the month average prices used to compute future cash inflows	12 months
Future cash inflows	$ 64,199	$ 55,824	$ 36,800
Future production costs	(30,007)	(26,375)	(21,275)
Future development costs	(3,453)	(3,312)	(3,902)
Future net cash flows before income tax	30,739	26,137	11,623
Future income tax expense	(5,505)	(4,104)	(1,042)
Future net cash flows	25,234	22,033	10,581
10% annual discount for estimated timing of cash flows	(14,756)	(13,406)	(7,294)
Standardized Measure of Discounted Future Net Cash Flows Relating to Proved Oil and Gas Reserves, Total	$ 10,478	$ 8,627	$ 3,287	$ 3,233