Financial Risk Management Activities - Summary of Gross Notional Amounts of Outstanding West Texas Intermediate (WTI) Commodity Contracts (Detail) - WTI - 2019	9 Months Ended	12 Months Ended
	Sep. 30, 2019 MMBbls $ / bbl	Dec. 31, 2018 MMBbls $ / bbl
Derivative [Line Items]
Calendar year program	2019	2019
Contract type	Puts	Puts
Effective date	Oct. 01, 2019	Jan. 01, 2019
End date	Dec. 31, 2019	Dec. 31, 2019
Crude oil volumes (millions of barrels) | MMBbls	8.7	34.7
Floor price per barrel – WTI | $ / bbl	60	60