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Description of Business and Segmented Disclosures - Schedule of Geographical Information (Detail) - CAD ($)
$ in Millions
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Disclosure Of Geographical Areas [Line Items]      
Revenue $ 13,227 $ 20,180 $ 20,843
Non-Current Assets 29,758 32,429  
Canada [Member]      
Disclosure Of Geographical Areas [Line Items]      
Revenue 8,399 11,798 11,694
Non-Current Assets 26,168 28,336  
United States [Member]      
Disclosure Of Geographical Areas [Line Items]      
Revenue 4,828 8,382 $ 9,149
Non-Current Assets $ 3,590 $ 4,093