Supplemental Oil and Gas Disclosures (Unaudited) - Changes in Standardized Measure of Discounted Future Net Cash Flow (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2010
Schedule Of Changes In Standardized Measure Of Discounted Future Net Cash Flows [Line Items]
Beginning Balance	$ 136,853	$ 107,956		$ 56,206
Changes resulting from:
Net change in sales prices and production costs	507	23,837	(8,206)
Net change in future development costs	1,308	(2,897)	(1,834)
Sales of oil and gas, net of production costs	(63,979)	(57,360)	(32,364)
Net change due to extensions and discoveries	58,228	54,539	5,596
Increase Due to Purchases of Minerals in Place	2,778	1,160	86,013
Proved Developed and Undeveloped Reserves, Sales of Minerals in Place	(5,804)
Net change due to revisions of quantity estimates	14,960	8,679	(2,236)
Previously estimated development costs incurred	15,497	4,124	1,007
Accretion of discount	18,277	13,768	7,778
Standardized Measure of Discounted Future Net Cash Flow of Proved Oil and Gas Reserves, Other	4,313	(749)
Net change in income taxes	(17,322)	(16,204)	(4,004)
Aggregate change for the year	28,763	28,897	51,750
Ending Balance	165,616	136,853	107,956	56,206
Consolidated Entities [Member]
Schedule Of Changes In Standardized Measure Of Discounted Future Net Cash Flows [Line Items]
Beginning Balance	135,553	106,543		52,698
Changes resulting from:
Net change in sales prices and production costs	(1,064)	23,422	(5,709)
Net change in future development costs	1,308	(2,897)	(1,834)
Sales of oil and gas, net of production costs	(63,192)	(56,559)	(31,732)
Net change due to extensions and discoveries	58,228	54,539	5,596
Increase Due to Purchases of Minerals in Place	2,778	1,160	86,013
Proved Developed and Undeveloped Reserves, Sales of Minerals in Place	(5,804)
Net change due to revisions of quantity estimates	15,303	8,673	(2,254)
Previously estimated development costs incurred	15,497	4,124	1,007
Accretion of discount	18,067	13,540	7,377
Standardized Measure of Discounted Future Net Cash Flow of Proved Oil and Gas Reserves, Other	4,198	(718)
Net change in income taxes	(17,031)	(16,274)	(4,619)
Aggregate change for the year	28,288	29,010	53,845
Ending Balance	163,841	135,553	106,543	52,698
Equity Method Investee [Member]
Schedule Of Changes In Standardized Measure Of Discounted Future Net Cash Flows [Line Items]
Beginning Balance	1,300	1,413		3,508
Changes resulting from:
Net change in sales prices and production costs	1,571	415	(2,497)
Net change in future development costs	0	0	0
Sales of oil and gas, net of production costs	(787)	(801)	(632)
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	(343)	6	18
Previously estimated development costs incurred	0	0	0
Accretion of discount	210	228	401
Standardized Measure of Discounted Future Net Cash Flow of Proved Oil and Gas Reserves, Other	115	(31)
Net change in income taxes	(291)	70	615
Aggregate change for the year	475	(113)	(2,095)
Ending Balance	$ 1,775	$ 1,300	$ 1,413	$ 3,508