Financial Risk Management Activities - Gross Notional Amounts of Outstanding Financial Risk Management Derivative Contracts (Detail) - WTI	9 Months Ended	12 Months Ended
	Sep. 30, 2018 MMBbls $ / bbl	Dec. 31, 2017 MMBbls $ / bbl
2018
Derivative [Line Items]
Calendar year program	2018	2018
Instrument type	Puts	Collars
Effective date	Oct. 01, 2018	Jan. 01, 2018
End date	Dec. 31, 2018	Dec. 31, 2018
Crude oil volumes (millions of barrels) | MMBbls	10.6	42.0
Ceiling price	0	65
Floor price	50	50
2019
Derivative [Line Items]
Calendar year program	2019
Instrument type	Puts
Effective date	Jan. 01, 2019
End date	Dec. 31, 2019
Crude oil volumes (millions of barrels) | MMBbls	32.9
Ceiling price	0
Floor price	60