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Description of Business and Segmented Disclosures - Additional Information (Detail)
$ in Millions
 12 Months Ended
Dec. 31, 2020
CAD ($)
Refinery
Customer
Dec. 31, 2019
CAD ($)
Customer
Dec. 31, 2018
CAD ($)
Customer
Disclosure Of Operating Segments [Line Items]      
Number of major customers | Customer 3 2 3
Gross Sales $ 13,591 $ 21,353 $ 21,389
Customer One [Member]      
Disclosure Of Operating Segments [Line Items]      
Gross Sales 4,323 6,922 7,840
Customer Two [Member]      
Disclosure Of Operating Segments [Line Items]      
Gross Sales 1,834 $ 2,316 2,285
Customer Three [Member]      
Disclosure Of Operating Segments [Line Items]      
Gross Sales $ 1,472   $ 2,263
Bottom of Range [Member]      
Disclosure Of Operating Segments [Line Items]      
Percentage of entity's revenue from gross sales 10.00% 10.00% 10.00%
Canada [Member]      
Disclosure Of Operating Segments [Line Items]      
Sales of crude oil, natural gas and NGLs $ 2,639 $ 4,002 $ 2,500
Refining and Marketing [Member]      
Disclosure Of Operating Segments [Line Items]      
Number of refineries | Refinery 2    
Gross Sales $ 6,051 $ 10,513 $ 11,183