Supplemental Oil and Gas Disclosures (Unaudited) - Information About Results of Operations of Oil and Natural Gas Interests (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Consolidated entities
Results of operations	$ (193)	$ (5,777)	$ (170,567)
Consolidated Entities
Consolidated entities
Revenues	1,399	10,111	51,553
Production costs	(209)	(4,392)	(19,820)
Exploration costs	(34)	(124)	(11,864)
Depreciation, depletion, amortization	0	(2,157)	(28,774)
Results of Operations, Impairment of Oil and Gas Properties	(224)	(612)	(164,831)
Oil and gas administrative expenses	(1,197)	(8,700)	(11,700)
Results of Operations, Accretion of Asset Retirement Obligations	0	(56)	(144)
Income tax expenses	(7)	(20)	14,717
Results of operations	(272)	(5,950)	(170,863)
Equity Method Investee
Consolidated entities
Royalty revenues	100	284	428
Production costs	(19)	(76)	(102)
Oil and gas administrative expenses	(2)	(35)	(51)
Income tax expenses	0	0	21
Results of operations	$ 79	$ 173	$ 296