Supplemental Oil and Gas Disclosures (Unaudited) - Estimates of Future Cash Flows from Proved Developed Oil and Natural Gas Reserves (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2010
Discounted Future Net Cash Flows Relating to Proved Oil and Gas Reserves [Line Items]
Standardized measure of discounted future net cash flows	$ 165,616	$ 136,853	$ 107,956	$ 56,206
Discounted Future Net Cash Flows Relating to Proved Oil and Gas Reserves, Entity's Share of Equity Method Investees' Standardized Discounted Future Net Cash Flows	165,616	136,853	107,956
Consolidated Entities [Member]
Discounted Future Net Cash Flows Relating to Proved Oil and Gas Reserves [Line Items]
Future Net Cash Flows Relating to Proved Oil and Gas Reserves, Cash Inflows	665,657	544,098	322,098
Future Net Cash Flows Relating to Proved Oil and Gas Reserves, Production Costs	(271,735)	(231,801)	(104,441)
Future income tax expenses	(106,002)	(77,361)	(50,350)
Future net cash flows	287,920	234,936	167,307
10% annual discount for estimated timing of cash flows	(124,079)	(99,383)	(60,764)
Standardized measure of discounted future net cash flows	163,841	135,553	106,543	52,698
Equity Method Investee [Member]
Discounted Future Net Cash Flows Relating to Proved Oil and Gas Reserves [Line Items]
Future Net Cash Flows Relating to Proved Oil and Gas Reserves, Cash Inflows	6,186	4,765	5,125
Future Net Cash Flows Relating to Proved Oil and Gas Reserves, Production Costs	(664)	(512)	(551)
Future income tax expenses	(2,098)	(1,616)	(1,738)
Future net cash flows	3,424	2,637	2,836
10% annual discount for estimated timing of cash flows	(1,649)	(1,337)	(1,423)
Standardized measure of discounted future net cash flows	$ 1,775	$ 1,300	$ 1,413	$ 3,508