Financial Risk Management Activities - Gross Notional Amounts of Outstanding Financial Risk Management Derivative Contracts (Detail) - WTI	3 Months Ended	12 Months Ended
	Mar. 31, 2018 MMBbls $ / bbl	Dec. 31, 2017 MMBbls $ / bbl
Derivative [Line Items]
Instrument type	Puts	Collars
Effective date	Apr. 01, 2018	Jan. 01, 2018
End date	Dec. 31, 2018	Dec. 31, 2018
Crude oil volumes (millions of barrels) | MMBbls	31.6	42.0
Ceiling price	0	65
Floor price	50	50