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Non-Controlling Interests - Summary of Information About Subsidiaries with Non-controlling Interests and Non-controlling Interest Balances (Parenthetical) (Detail) - CAD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Disclosure of subsidiaries [line items]    
Total fair value $ 175  
Inversiones Mineras S A [member]    
Disclosure of subsidiaries [line items]    
Proportion of ownership interest in associate 13.50%  
Retained earnings [member]    
Disclosure of subsidiaries [line items]    
Purchase of non-controlling interests $ (159) $ (63)
Non-controlling interests [member]    
Disclosure of subsidiaries [line items]    
Purchase of non-controlling interests $ (16)