Commodity Derivative Positions (Detail) $ in Thousands	9 Months Ended
Sep. 30, 2018 USD ($) bbl $ / bbl
Settlement Agreement [Member]
Derivative Instruments Related to Oil and Gas Production [Line Items]
Fair Value Asset
Fair Value Liability	$ 6,327
Crude Oil \| Fourth Quarter 2018 [Member]
Derivative Instruments Related to Oil and Gas Production [Line Items]
Derivative, Type of Instrument	Swaps-WTI
Notional Volume, per day \| bbl	10,455
Derivative, Swap Type, Average Fixed Price \| $ / bbl	57.05
Fair Value Asset	$ 0
Fair Value Liability	$ 15,125
Crude Oil \| First Quarter 2019 [Member]
Derivative Instruments Related to Oil and Gas Production [Line Items]
Derivative, Type of Instrument	Swaps-WTI
Notional Volume, per day \| bbl	6,446
Derivative, Swap Type, Average Fixed Price \| $ / bbl	54.46
Fair Value Asset	$ 0
Fair Value Liability	$ 9,948
Crude Oil \| Second Quarter 2019 [Member]
Derivative Instruments Related to Oil and Gas Production [Line Items]
Derivative, Type of Instrument	Swaps-WTI
Notional Volume, per day \| bbl	6,421
Derivative, Swap Type, Average Fixed Price \| $ / bbl	54.48
Fair Value Asset	$ 0
Fair Value Liability	$ 9,413
Crude Oil \| Third Quarter 2019 [Member]
Derivative Instruments Related to Oil and Gas Production [Line Items]
Derivative, Type of Instrument	Swaps-WTI
Notional Volume, per day \| bbl	6,397
Derivative, Swap Type, Average Fixed Price \| $ / bbl	54.50
Fair Value Asset	$ 0
Fair Value Liability	$ 8,722
Crude Oil \| Fourth Quarter 2019 [Member]
Derivative Instruments Related to Oil and Gas Production [Line Items]
Derivative, Type of Instrument	Swaps-WTI
Notional Volume, per day \| bbl	6,398
Derivative, Swap Type, Average Fixed Price \| $ / bbl	54.50
Fair Value Asset	$ 0
Fair Value Liability	$ 7,925
Crude Oil \| First Quarter 2020 [Member]
Derivative Instruments Related to Oil and Gas Production [Line Items]
Derivative, Type of Instrument	Swaps-WTI
Notional Volume, per day \| bbl	6,000
Derivative, Swap Type, Average Fixed Price \| $ / bbl	54.09
Fair Value Asset	$ 0
Fair Value Liability	$ 6,786
Crude Oil \| Second Quarter 2020 [Member]
Derivative Instruments Related to Oil and Gas Production [Line Items]
Derivative, Type of Instrument	Swaps-WTI
Notional Volume, per day \| bbl	6,000
Derivative, Swap Type, Average Fixed Price \| $ / bbl	54.09
Fair Value Asset	$ 0
Fair Value Liability	$ 6,142
Crude Oil \| Third Quarter 2020 [Member]
Derivative Instruments Related to Oil and Gas Production [Line Items]
Derivative, Type of Instrument	Swaps-WTI
Notional Volume, per day \| bbl	6,000
Derivative, Swap Type, Average Fixed Price \| $ / bbl	54.09
Fair Value Asset	$ 0
Fair Value Liability	$ 5,593
Crude Oil \| Fourth Quarter 2020 [Member]
Derivative Instruments Related to Oil and Gas Production [Line Items]
Derivative, Type of Instrument	Swaps-WTI
Notional Volume, per day \| bbl	6,000
Derivative, Swap Type, Average Fixed Price \| $ / bbl	54.09
Fair Value Asset	$ 0
Fair Value Liability	$ 5,068
Louisiana Light Sweet [Member] \| Fourth Quarter 2018 [Member]
Derivative Instruments Related to Oil and Gas Production [Line Items]
Derivative, Type of Instrument	Swaps-LLS
Notional Volume, per day \| bbl	6,000
Derivative, Swap Type, Average Fixed Price \| $ / bbl	65.27
Fair Value Asset	$ 0
Fair Value Liability	$ 8,128
Louisiana Light Sweet [Member] \| First Quarter 2019 [Member]
Derivative Instruments Related to Oil and Gas Production [Line Items]
Derivative, Type of Instrument	Swaps-LLS
Notional Volume, per day \| bbl	5,000
Derivative, Swap Type, Average Fixed Price \| $ / bbl	59.17
Fair Value Asset	$ 0
Fair Value Liability	$ 8,386
Louisiana Light Sweet [Member] \| Second Quarter 2019 [Member]
Derivative Instruments Related to Oil and Gas Production [Line Items]
Derivative, Type of Instrument	Swaps-LLS
Notional Volume, per day \| bbl	5,000
Derivative, Swap Type, Average Fixed Price \| $ / bbl	59.17
Fair Value Asset	$ 0
Fair Value Liability	$ 7,717
Louisiana Light Sweet [Member] \| Third Quarter 2019 [Member]
Derivative Instruments Related to Oil and Gas Production [Line Items]
Derivative, Type of Instrument	Swaps-LLS
Notional Volume, per day \| bbl	5,000
Derivative, Swap Type, Average Fixed Price \| $ / bbl	59.17
Fair Value Asset	$ 0
Fair Value Liability	$ 6,874
Louisiana Light Sweet [Member] \| Fourth Quarter 2019 [Member]
Derivative Instruments Related to Oil and Gas Production [Line Items]
Derivative, Type of Instrument	Swaps-LLS
Notional Volume, per day \| bbl	5,000
Derivative, Swap Type, Average Fixed Price \| $ / bbl	59.17
Fair Value Asset	$ 0
Fair Value Liability	$ 6,057

Aggregate notional amount of derivative expressed in nonmonetary units. For example, the number of barrels specified in a fuel oil forward purchase contract.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Regulation S-X (SX)
-Number 210
-Article 12
-Section 13
-Sentence Column B

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Fair value as of the balance sheet date of derivative instrument not designated as hedging instrument and classified as an asset.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4B
-Subparagraph (c)(2)
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Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 1B
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-Name Accounting Standards Codification
-Topic 235
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-Section S99
-Paragraph 1
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-URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e23780-122690

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