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Risk Management - Net Fair Value of Risk Management Positions (Detail)
$ in Millions
 12 Months Ended
Dec. 31, 2020
CAD ($)
$ / bbl
MMBbls
Dec. 31, 2019
CAD ($)
Dec. 31, 2018
CAD ($)
Disclosure of Derivative Financial Instruments [Line Items]      
Fair Value Asset (Liability) $ (53) $ 3 $ 160
Fair value derivative financial instruments [Member]      
Disclosure of Derivative Financial Instruments [Line Items]      
Fair Value Asset (Liability) $ (53)    
Fair value derivative financial instruments [Member] | WTI Fixed - Sell [Member]      
Disclosure of Derivative Financial Instruments [Line Items]      
Notional Volumes | MMBbls 19.6    
Terms January 2021 - June 2022    
Weighted Average Price | $ / bbl 43.99    
Fair Value Asset (Liability) $ (113)    
Fair value derivative financial instruments [Member] | WTI Fixed - Buy [Member]      
Disclosure of Derivative Financial Instruments [Line Items]      
Notional Volumes | MMBbls 11.7    
Terms February 2021 - June 2022    
Weighted Average Price | $ / bbl 44.55    
Fair Value Asset (Liability) $ 59    
Fair value derivative financial instruments [Member] | Other Financial Positions [Member]      
Disclosure of Derivative Financial Instruments [Line Items]      
Fair Value Asset (Liability) $ 1