Quarterly report pursuant to Section 13 or 15(d)

Commodity Derivative Positions (Detail)

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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