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Financial Instruments - Additional information (Detail)
$ in Millions
 12 Months Ended
Dec. 31, 2020
CAD ($)
$ / bbl
Dec. 31, 2019
CAD ($)
Dec. 31, 2018
CAD ($)
Disclosure of detailed information about financial instruments [Line Items]      
Long-term debt, carrying value $ 7,441 $ 6,699  
Cash collateral for risk management contracts price change $ 59    
Discounted Credit adjusted risk free rate 2.00%    
Estimated fair value of contingent payment $ 63 143 $ 132
WCS Forward Prices [Member]      
Disclosure of detailed information about financial instruments [Line Items]      
Average forward price for Western Canadian Select Crude Oil for the remaining term | $ / bbl 42.93    
WTI Option Volatility [Member]      
Disclosure of detailed information about financial instruments [Line Items]      
Average implied volatility contingent payment percentage 35.60%    
U.S. to Canadian Dollar Foreign Exchange Average Rate Volatility Contingent Payment [Member]      
Disclosure of detailed information about financial instruments [Line Items]      
Average implied volatility contingent payment percentage 6.80%    
Level 2 of Fair Value Hierarchy [Member]      
Disclosure of detailed information about financial instruments [Line Items]      
Debt, fair value $ 8,608 $ 7,610