Financial Risk Management Activities - Additional Information (Detail) - West Texas Intermediate Crude Oil Price Hedging Contracts	12 Months Ended
	Dec. 31, 2018 bbl / d $ / bbl	Dec. 31, 2017 bbl / d $ / bbl
2018
Derivative Instruments Gain Loss [Line Items]
Monthly floor price		50
Monthly ceiling price		65
Notional amount of outstanding average barrels of oil per day | bbl / d		115,000
2018 | Designated as Cash Flow Hedges
Derivative Instruments Gain Loss [Line Items]
Monthly floor price		50
Monthly ceiling price		65
2019
Derivative Instruments Gain Loss [Line Items]
Monthly floor price	60
Monthly ceiling price	0
2019 | Designated as Cash Flow Hedges
Derivative Instruments Gain Loss [Line Items]
Monthly floor price	60
Notional amount of outstanding average barrels of oil per day | bbl / d	95,000