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, 2016 USD ($) $ / MMcfe	Dec. 31, 2015 USD ($) $ / MMcfe	Dec. 31, 2014 USD ($) $ / MMcfe
Supplemental Information on Oil and Gas Producing Activities (Unaudited)
Weighted average price of equivalent reserves (in dollar per share) | $ / MMcfe	2.39	2.66	5.02
Changes in Standardized Measure of Discounted Future Net Cash Flow
Sales of oil and gas, net of productions costs	$ (756)	$ (601)	$ (1,158)
Net changes in prices and production costs	(1,540)	(9,416)	(184)
Development costs incurred during the period	733	769	564
Net changes in future development costs	212	671	(102)
Extensions, discoveries and other additions	673	861	5,759
Acquisitions	66		42
Divestitures	(7)
Revisions of previous quantity estimates	461	(1,167)	(828)
Accretion of discount	363	1,132	600
Net change in income taxes	12	3,284	(2,198)
Other changes	(163)	65	630
Net increase (decrease)	54	(4,402)	3,125
Beginning of period	3,233	7,635	4,510
End of period	$ 3,287	$ 3,233	$ 7,635