Supplemental Oil and Gas Disclosures (Unaudited) - Summary of Estimated Quantities of Proved Developed Oil and Natural Gas Reserves (Detail) bbl in Thousands, Mcf in Thousands		12 Months Ended
		Dec. 31, 2017 bbl Mcf	Dec. 31, 2016 bbl Mcf	Dec. 31, 2015 bbl Mcf	Dec. 31, 2015 bbl Mcf
Oil
Proved Developed and Undeveloped Reserves [Roll Forward]
Beginning Balance	[1]	446	5,179
Proved developed reserves	[1]	0	446	5,179	5,179
Proved undeveloped reserves	[1]	0	0	0	0
Ending Balance	[1]	0	446	5,179
Natural Gas
Proved Developed and Undeveloped Reserves [Roll Forward]
Beginning Balance | Mcf		5,035	9,220
Proved developed reserves | Mcf		0	5,035	9,220	9,220
Proved undeveloped reserves | Mcf		0	0	0	0
Ending Balance | Mcf		0	5,035		9,220
Consolidated Entities | Oil
Proved Developed and Undeveloped Reserves [Roll Forward]
Beginning Balance	[1]	446	5,179	7,672
Revisions of previous estimates	[1]	0	(11)	(855)
Extensions and discoveries	[1]	0	29	224
Acquisitions	[1]	0	0	0
Sales	[1]	(446)	(4,460)	(704)
Production	[1]	0	(291)	(1,158)
Ending Balance	[1]	0	446	5,179
Consolidated Entities | Natural Gas
Proved Developed and Undeveloped Reserves [Roll Forward]
Beginning Balance | Mcf		3,836	7,957		12,649
Revisions of previous estimates		0	631	(1,995)	(1,675)
Extensions and discoveries | Mcf		0	0		173
Acquisitions | Mcf		0	0		0
Sales | Mcf		(3,836)	(3,756)		(1,223)
Production | Mcf		0	(996)		(1,967)
Ending Balance | Mcf		0	3,836		7,957
Equity Method Investee | Oil
Proved Developed and Undeveloped Reserves [Roll Forward]
Beginning Balance	[1]	0	0	0
Revisions of previous estimates	[1]		0	0
Sales	[1]	0
Production	[1]		0	0
Ending Balance	[1]	0	0	0
Equity Method Investee | Natural Gas
Proved Developed and Undeveloped Reserves [Roll Forward]
Beginning Balance | Mcf		1,199	1,263		1,751
Revisions of previous estimates | Mcf			(79)		320
Sales | Mcf		(1,199)
Production | Mcf			(143)		(168)
Ending Balance | Mcf		0	1,199		1,263

[1] Includes natural gas liquids (NGLs).