Supplemental Information on Oil and Gas Producing Activities (Unaudited) - Discounted future cash flows (Detail) $ in Thousands	12 Months Ended
	Dec. 31, 2016 USD ($) MMcfe	Dec. 31, 2015 USD ($) MMcfe bbl	Dec. 31, 2014 USD ($) MMcfe item	Dec. 31, 2013 USD ($)
Supplemental Information on Oil and Gas Producing Activities (Unaudited)
Increase in proved reserves due to ethane recovery | MMcfe	1,359,000	1,091,000
Ethane recovery quantity assumption | bbl		11,500
Decrease in proved reserves due to reclassifications related to five-year rule | MMcfe	(2,478,000)	(2,332,000)	(1,417,000)
Increase (decrease) in proved reserves resulting from price revisions | MMcfe	(47,000)	(202,000)	2.000
Increase (decrease) in proved reserves due to performance revisions | MMcfe	762,000	(358,000)	361,000
Number of horizontal producing wells acquired | item			5
Number of dry gas locations | item			191
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	$ 36,800	$ 35,179	$ 63,632
Future production costs	(21,275)	(17,393)	(21,722)
Future development costs	(3,902)	(5,217)	(8,212)
Future net cash flows before income tax	11,623	12,569	33,698
Future income tax expense	(1,042)	(1,708)	(10,726)
Future net cash flows	10,581	10,861	22,972
10% annual discount for estimated timing of cash flows	(7,294)	(7,628)	(15,337)
Standardized Measure of Discounted Future Net Cash Flows Relating to Proved Oil and Gas Reserves, Total	$ 3,287	$ 3,233	$ 7,635	$ 4,510