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Supplementary Information - Standardized Measure of Discounted Future Net Cash Flows and Changes therein Relating to Proved Oil and Gas Reserves (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Sales of oil and gas produced, net of production costs	$ (36,003)	$ (10,762)
Net changes in prices and production costs	12,432	(6,895)
Extensions, discoveries and improved recovery	76,694	27,706
Revisions of previous quantity estimates	19,808	(5,214)
Net change in development costs	(5,199)	(26,953)
Reserves sold	(21)
Reserves purchased	1,372
Accretion of discount	5,224	5,880
Net change in income taxes	(13,001)	(2,600)
Changes in production rates (timing) and other	(6,628)	12,273
Net change	54,677	(6,565)
Standardized measure of discounted future net cash flow:
Beginning of year	52,237	58,802
End of year	$ 106,914	$ 52,237

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Amount of decrease in standardized measure of discounted future net cash flow as a result of sales of minerals in place.

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-Publisher FASB
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-SubTopic 235
-Section 50
-Paragraph 35
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Amount of increase (decrease) in standardized measure of discounted future net cash flow as a result of change in the estimated future income tax on future pretax net cash flows related to the entity's proved oil and gas reserves.

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Amount of increase (decrease) in standardized measure of discounted future net cash flow as a result of revisions of estimated quantity of oil and gas in proved reserves.

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