Supplemental Oil and Gas Disclosures (Unaudited) - Changes in Standardized Measure of Discounted Future Net Cash Flow (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Schedule Of Changes In Standardized Measure Of Discounted Future Net Cash Flows [Line Items]
Beginning Balance	$ 165,616	$ 136,853	$ 107,956
Changes resulting from:
Net change in sales prices and production costs	(137,648)	507	23,837
Net change in future development costs	92	1,308	(2,897)
Sales of oil and gas, net of production costs	(32,160)	(63,979)	(57,360)
Net change due to extensions and discoveries	11,747	58,228	54,539
Increase Due to Purchases of Minerals in Place	0	2,778	1,160
Proved Developed and Undeveloped Reserves, Sales of Minerals in Place	(15,855)	(5,804)
Net change due to revisions of quantity estimates	(15,431)	14,960	8,679
Previously estimated development costs incurred	(15,096)	(15,497)	(4,124)
Accretion of discount	22,886	18,277	13,768
Standardized Measure of Discounted Future Net Cash Flow of Proved Oil and Gas Reserves, Other	3,808	4,313	(749)
Net change in income taxes	49,347	(17,322)	(16,204)
Aggregate change for the year	(98,118)	28,763	28,897
Ending Balance	67,498	165,616	136,853
Consolidated Entities [Member]
Schedule Of Changes In Standardized Measure Of Discounted Future Net Cash Flows [Line Items]
Beginning Balance	163,841	135,553	106,543
Changes resulting from:
Net change in sales prices and production costs	(136,536)	(1,064)	23,422
Net change in future development costs	92	1,308	(2,897)
Sales of oil and gas, net of production costs	(31,732)	(63,192)	(56,559)
Net change due to extensions and discoveries	11,747	58,228	54,539
Increase Due to Purchases of Minerals in Place	0	2,778	1,160
Proved Developed and Undeveloped Reserves, Sales of Minerals in Place	(15,855)	(5,804)
Net change due to revisions of quantity estimates	(15,164)	15,303	8,673
Previously estimated development costs incurred	(15,096)	(15,497)	(4,124)
Accretion of discount	22,600	18,067	13,540
Standardized Measure of Discounted Future Net Cash Flow of Proved Oil and Gas Reserves, Other	4,018	4,198	(718)
Net change in income taxes	48,689	(17,031)	(16,274)
Aggregate change for the year	(97,045)	28,288	29,010
Ending Balance	66,796	163,841	135,553
Equity Method Investee [Member]
Schedule Of Changes In Standardized Measure Of Discounted Future Net Cash Flows [Line Items]
Beginning Balance	1,775	1,300	1,413
Changes resulting from:
Net change in sales prices and production costs	(1,112)	1,571	415
Net change in future development costs	0	0	0
Sales of oil and gas, net of production costs	(428)	(787)	(801)
Net change due to extensions and discoveries	0	0	0
Increase Due to Purchases of Minerals in Place	0	0	0
Proved Developed and Undeveloped Reserves, Sales of Minerals in Place		0
Net change due to revisions of quantity estimates	(267)	(343)	6
Previously estimated development costs incurred	0	0	0
Accretion of discount	286	210	228
Standardized Measure of Discounted Future Net Cash Flow of Proved Oil and Gas Reserves, Other	(210)	115	(31)
Net change in income taxes	658	(291)	70
Aggregate change for the year	(1,073)	475	(113)
Ending Balance	$ 702	$ 1,775	$ 1,300