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Exploration and Evaluation Assets, Net - Summary of Exploration and Valuation Assets, Net (Detail) - CAD ($)
$ in Millions
 3 Months Ended 9 Months Ended 12 Months Ended
Sep. 30, 2021
Sep. 30, 2020
Sep. 30, 2021
Sep. 30, 2020
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Disclosure Of Exploration And Evaluation Assets [Line Items]              
Beginning Balance     $ 623        
Expense arising from exploration for and evaluation of mineral resources $ (5) $ (25) (15) $ (32) $ (91) $ (82) $ (2,123)
Ending Balance 655   655   623    
E&E Assets [Member]              
Disclosure Of Exploration And Evaluation Assets [Line Items]              
Beginning Balance     623        
Additions     37        
Expense arising from exploration for and evaluation of mineral resources     (12)        
Change in Decommissioning Liabilities     8        
Exchange Rate Movements and Other     (1)        
Ending Balance $ 655   $ 655   $ 623