Supplemental Oil and Gas Disclosures (Unaudited) - Additional Information (Detail) bbl in Thousands, Mcf in Thousands		12 Months Ended
		Dec. 31, 2017 well bbl Mcf	Dec. 31, 2016 well $ / MMBTU $ / Boe bbl Mcf	Dec. 31, 2015 Boe bbl Mcf	Dec. 31, 2015 Boe well bbl Mcf	Dec. 31, 2015 Boe $ / MMBTU bbl Mcf	Dec. 31, 2015 Boe bbl Mcf	Dec. 31, 2015 Boe bbl Mcf	Dec. 31, 2015 Boe $ / Boe bbl Mcf	Dec. 31, 2014 Boe bbl Mcf
Supplemental Oil And Gas Reserve Information [Line Items]
New well addition | well		0	0		36
Wells which company has royalty interest in		0	473	534
Wells which company has working interest in | well		0	32		400
Crude Oil and NGL
Supplemental Oil And Gas Reserve Information [Line Items]
Average spot price | $ / Boe			42.75						50.28
Natural Gas in Mmbtu
Supplemental Oil And Gas Reserve Information [Line Items]
Average spot price | $ / MMBTU			2.48			2.59
Oil
Supplemental Oil And Gas Reserve Information [Line Items]
Proved developed and undeveloped reserves, net	[1]	0	446	5,179	5,179	5,179	5,179	5,179	5,179
Oil | Consolidated Entities
Supplemental Oil And Gas Reserve Information [Line Items]
Proved developed and undeveloped reserves, net	[1]	0	446	5,179	5,179	5,179	5,179	5,179	5,179	7,672
Proved developed and undeveloped reserves, sales of minerals in place	[1]	446	4,460				704
Proved developed and undeveloped reserves, revisions of previous estimates	[1]	0	11				855
Oil Less Natural Gas Liquids
Supplemental Oil And Gas Reserve Information [Line Items]
Proved developed and undeveloped reserves, net	[1]		446
Oil Less Natural Gas Liquids | Consolidated Entities
Supplemental Oil And Gas Reserve Information [Line Items]
Proved developed and undeveloped reserves, sales of minerals in place	[1]		4,155
Natural Gas Liquids | Consolidated Entities
Supplemental Oil And Gas Reserve Information [Line Items]
Proved developed and undeveloped reserves, sales of minerals in place			305
Natural Gas
Supplemental Oil And Gas Reserve Information [Line Items]
Proved developed and undeveloped reserves, net | Mcf		0	5,035	9,220	9,220	9,220	9,220	9,220	9,220
Natural Gas | Consolidated Entities
Supplemental Oil And Gas Reserve Information [Line Items]
Proved developed and undeveloped reserves, net | Mcf		0	3,836	7,957	7,957	7,957	7,957	7,957	7,957	12,649
Proved developed and undeveloped reserves, sales of minerals in place | Mcf		3,836	3,756					1,223
Proved developed and undeveloped reserves, revisions of previous estimates		0	(631)				1,995	1,675
BOE
Supplemental Oil And Gas Reserve Information [Line Items]
Proved undeveloped reserve (Energy) | Boe				0	0	0	0	0	0	2,703,000

[1] Includes natural gas liquids (NGLs).