Derivative Instruments - Oil and Natural-Gas Production/Processing Derivative Activities Table (Detail) - Not Designated as Hedging Instrument [Member]	9 Months Ended
Sep. 30, 2018 MMBTU / d MBbls / d $ / bbl $ / MMBTU
Two Way Collars Oil 2018 [Member]
Derivative [Line Items]
Oil or NGL derivative nonmonetary notional amount per day \| MBbls / d	108
Two Way Collars Oil 2018 [Member] \| Call Option [Member] \| Short [Member]
Average price per MMBtu or barrel
Average ceiling price \| $ / bbl	60.48
Two Way Collars Oil 2018 [Member] \| Put Option [Member] \| Long [Member]
Average price per MMBtu or barrel
Average floor price \| $ / bbl	50.00
Three Way Collars Oil 2019 [Member]
Derivative [Line Items]
Oil or NGL derivative nonmonetary notional amount per day \| MBbls / d	87
Three Way Collars Oil 2019 [Member] \| Call Option [Member] \| Short [Member]
Average price per MMBtu or barrel
Average ceiling price \| $ / bbl	72.98
Three Way Collars Oil 2019 [Member] \| Put Option [Member] \| Short [Member]
Average price per MMBtu or barrel
Average floor price	46.72
Three Way Collars Oil 2019 [Member] \| Put Option [Member] \| Long [Member]
Average price per MMBtu or barrel
Average floor price \| $ / bbl	56.72
Forward Contracts Oil 2018 [Member]
Derivative [Line Items]
Oil or NGL derivative nonmonetary notional amount per day \| MBbls / d	84
Average price per MMBtu or barrel
Average fixed price	61.45
Three Way Collars Natural Gas 2018 [Member]
Derivative [Line Items]
Natural-gas derivative nonmonetary notional amount per day \| MMBTU / d	250
Three Way Collars Natural Gas 2018 [Member] \| Call Option [Member] \| Short [Member]
Average price per MMBtu or barrel
Average ceiling price	3.54
Three Way Collars Natural Gas 2018 [Member] \| Put Option [Member] \| Short [Member]
Average price per MMBtu or barrel
Average floor price	2.00
Three Way Collars Natural Gas 2018 [Member] \| Put Option [Member] \| Long [Member]
Average price per MMBtu or barrel
Average floor price	2.75
Forward Contracts Natural Gas 2018 [Member]
Derivative [Line Items]
Natural-gas derivative nonmonetary notional amount per day \| MMBTU / d	280
Average price per MMBtu or barrel
Average fixed price	3.02

Average price per MMBtu or barrel [Abstract]

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- Definition

Nominal flow rate used to calculate payments on a derivative instrument.

+ References

No definition available.

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- Definition

The average cap rate on a group of price risk derivatives such as caps or collars. A payment or receipt is triggered if the market rate exceeds the cap rate on the contract.

+ References

No definition available.

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- Definition

The average floor rate on a group of price risk derivatives such as floors or collars. A payment or receipt is triggered if the market rate falls below the floor rate on the contract.

+ References

No definition available.

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- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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- Definition

Nominal flow rate used to calculate payments on a derivative instrument.

+ References

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- Definition

Average fixed price related to the group of price risk swap derivatives.

+ References

No definition available.

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