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Financial Risk Management Activities - Gross Notional Amounts of Outstanding Financial Risk Management Derivative Contracts (Detail) - WTI
12 Months Ended
Dec. 31, 2018
MMBbls
$ / bbl
Dec. 31, 2017
MMBbls
$ / bbl
2019    
Derivative [Line Items]    
Calendar year program 2019  
Instrument type Puts  
Crude oil volumes (millions of barrels) | MMBbls 34.7  
Ceiling price 0  
Floor price 60  
2018    
Derivative [Line Items]    
Calendar year program   2018
Instrument type Collars  
Crude oil volumes (millions of barrels) | MMBbls   42.0
Ceiling price   65
Floor price   50