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Financial Instruments - Summary of Changes in Inputs to Option Pricing Model, Resulted in Unrealized Gains (Losses) Impacting Earnings Before Income Tax (Detail)
$ in Millions
 12 Months Ended
Dec. 31, 2020
CAD ($)
Dec. 31, 2019
CAD ($)
WCS Forward Prices [Member]    
Disclosure Of Sensitivity Of Fair Value Measurement To Changes In Unobservable Inputs Liabilities [Line Items]    
Sensitivity Price Range 5.00 5.00
Increase in volatility $ (41) $ (129)
Decrease in volatility $ 32 $ 80
WTI Option Volatility [Member]    
Disclosure Of Sensitivity Of Fair Value Measurement To Changes In Unobservable Inputs Liabilities [Line Items]    
Sensitivity Range 5.00% 5.00%
Increase in volatility $ (18) $ (45)
Decrease in volatility $ 17 $ 42
Foreign Exchange Rate Option Volatility [Member]    
Disclosure Of Sensitivity Of Fair Value Measurement To Changes In Unobservable Inputs Liabilities [Line Items]    
Sensitivity Range 5.00% 5.00%
Increase in volatility $ 7 $ 10
Decrease in volatility $ (10) $ (19)