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Financial Instruments - Narrative (Detail)
$ in Millions
 9 Months Ended
Sep. 30, 2021
CAD ($)
$ / bbl
Sep. 30, 2020
Dec. 31, 2020
CAD ($)
Disclosure of detailed information about financial instruments [Line Items]      
Long-Term Debt $ 12,986   $ 7,441
Discounted credit adjusted risk free rate 2.30%    
Estimated fair value of contingent payment $ 392   63
WCS Forward Prices      
Disclosure of detailed information about financial instruments [Line Items]      
Average forward price for Western Canadian Select Crude Oil for the remaining term (in CAD per barrel) | $ / bbl 77.66    
WTI Option Implied Volatility      
Disclosure of detailed information about financial instruments [Line Items]      
Average volatility contingent payment percentage 35.80%    
U.S. to Canadian Dollar Foreign Exchange Average Rate Volatility Contingent Payment [Member]      
Disclosure of detailed information about financial instruments [Line Items]      
Average volatility contingent payment percentage   6.80%  
Level 2 of fair value hierarchy      
Disclosure of detailed information about financial instruments [Line Items]      
Debt, fair value $ 14,427   $ 8,608