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Exploration and Evaluation Assets - Summary of Exploration and Valuation Assets (Detail) - CAD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Disclosure Of Exploration And Evaluation Assets [Line Items]      
Beginning Balance $ 787    
Exploration Expense (91) $ (82) $ (2,123)
Ending Balance 623 787  
E&E Assets [Member]      
Disclosure Of Exploration And Evaluation Assets [Line Items]      
Beginning Balance 787 785  
Additions 48 73  
Transfers to PP&E (Note 18) (1) [1] (47)    
Exploration Expense (91) (82)  
Depletion (18)    
Change in Decommissioning Liabilities 5 9  
Exchange Rate Movements and Other   2  
Divestitures (Note 8) (61)    
Ending Balance $ 623 $ 787 $ 785
[1] (1)Includes the $41 million reclassification of the GORR retained in the sale of the Marten Hills assets (see Note 8).