Supplemental Oil and Gas Disclosures (Unaudited) - Changes in Standardized Measure of Discounted Future Net Cash Flow (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Schedule Of Changes In Standardized Measure Of Discounted Future Net Cash Flows [Line Items]
Beginning Balance	$ 10,985	$ 67,498	$ 165,616
Changes resulting from:
Net change in sales prices and production costs	0	(3,645)	(137,648)
Net change in future development costs	0	0	92
Sales of oil and gas, net of production costs	0	(5,871)	(32,160)
Net change due to extensions and discoveries	0	410	11,747
Net change due to acquisition of reserves	0	0	0
Net change due to divestitures of reserves	(10,985)	(63,535)	(15,855)
Net change due to revisions of quantity estimates	0	1,367	(15,431)
Previously estimated development costs incurred	0	0	15,096
Accretion of discount	0	3,105	22,886
Net change in timing and other	0	(208)	3,808
Net change in income taxes	0	11,864	49,347
Aggregate change for the year	(10,985)	(56,513)	(98,118)
Ending Balance	0	10,985	67,498
Consolidated Entities
Schedule Of Changes In Standardized Measure Of Discounted Future Net Cash Flows [Line Items]
Beginning Balance	10,313	66,796	163,841
Changes resulting from:
Net change in sales prices and production costs	0	(3,585)	(136,536)
Net change in future development costs	0	0	92
Sales of oil and gas, net of production costs	0	(5,663)	(31,732)
Net change due to extensions and discoveries	0	410	11,747
Net change due to acquisition of reserves	0	0	0
Net change due to divestitures of reserves	(10,313)	(63,535)	(15,855)
Net change due to revisions of quantity estimates	0	1,304	(15,164)
Previously estimated development costs incurred	0	0	15,096
Accretion of discount	0	2,992	22,600
Net change in timing and other	0	(128)	4,018
Net change in income taxes	0	11,722	48,689
Aggregate change for the year	(10,313)	(56,483)	(97,045)
Ending Balance	0	10,313	66,796
Equity Method Investee
Schedule Of Changes In Standardized Measure Of Discounted Future Net Cash Flows [Line Items]
Beginning Balance	672	702	1,775
Changes resulting from:
Net change in sales prices and production costs	0	(60)	(1,112)
Net change in future development costs	0	0	0
Sales of oil and gas, net of production costs	0	(208)	(428)
Net change due to extensions and discoveries	0	0	0
Net change due to acquisition of reserves	0	0	0
Net change due to divestitures of reserves	(672)	0	0
Net change due to revisions of quantity estimates	0	63	(267)
Previously estimated development costs incurred	0	0	0
Accretion of discount	0	113	286
Net change in timing and other	0	(80)	(210)
Net change in income taxes	0	142	658
Aggregate change for the year	(672)	(30)	(1,073)
Ending Balance	$ 0	$ 672	$ 702