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, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Discounted Future Net Cash Flows Relating to Proved Oil and Gas Reserves [Line Items]
Standardized measure of discounted future net cash flows	$ 0	$ 10,985	$ 67,498	$ 165,616
Consolidated Entities
Discounted Future Net Cash Flows Relating to Proved Oil and Gas Reserves [Line Items]
Future cash inflows	0	24,304	216,588
Future Net Cash Flows Relating to Proved Oil and Gas Reserves, Production Costs	0	(2,988)	(93,623)
Future income tax expenses	0	(3,926)	(22,218)
Future net cash flows	0	17,390	100,747
10% annual discount for estimated timing of cash flows	0	(7,077)	(33,951)
Standardized measure of discounted future net cash flows	0	10,313	66,796	163,841
Equity Method Investee
Discounted Future Net Cash Flows Relating to Proved Oil and Gas Reserves [Line Items]
Future cash inflows	0	2,010	2,283
Future Net Cash Flows Relating to Proved Oil and Gas Reserves, Production Costs	0	(216)	(245)
Future income tax expenses	0	(537)	(774)
Future net cash flows	0	1,257	1,264
10% annual discount for estimated timing of cash flows	0	(585)	(562)
Standardized measure of discounted future net cash flows	$ 0	$ 672	$ 702	$ 1,775