Supplemental Oil and Gas Disclosures (Unaudited) - Estimates of Future Cash Flows from Proved Developed Oil and Natural Gas Reserves (Detail) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Discounted Future Net Cash Flows Relating to Proved Oil and Gas Reserves [Line Items]
Standardized measure of discounted future net cash flows	$ 10,985	$ 67,498	$ 165,616	$ 136,853
Consolidated Entities
Discounted Future Net Cash Flows Relating to Proved Oil and Gas Reserves [Line Items]
Future cash inflows	24,304	216,588	665,657
Future Net Cash Flows Relating to Proved Oil and Gas Reserves, Production Costs	(2,988)	(93,623)	(271,735)
Future income tax expenses	(3,926)	(22,218)	(106,002)
Future net cash flows	17,390	100,747	287,920
10% annual discount for estimated timing of cash flows	(7,077)	(33,951)	(124,079)
Standardized measure of discounted future net cash flows	10,313	66,796	163,841	135,553
Equity Method Investee
Discounted Future Net Cash Flows Relating to Proved Oil and Gas Reserves [Line Items]
Future cash inflows	2,010	2,283	6,186
Future Net Cash Flows Relating to Proved Oil and Gas Reserves, Production Costs	(216)	(245)	(664)
Future income tax expenses	(537)	(774)	(2,098)
Future net cash flows	1,257	1,264	3,424
10% annual discount for estimated timing of cash flows	(585)	(562)	(1,649)
Standardized measure of discounted future net cash flows	$ 672	$ 702	$ 1,775	$ 1,300