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Description of Business and Segmented Disclosures - Schedule of Capital Expenditures (Detail) - CAD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Disclosure Of Reportable Segments [Line Items]      
Capital Investment $ 841 $ 1,176 $ 1,363
Total Capital Expenditures 859 1,189 1,704
Oil Sands [Member]      
Disclosure Of Reportable Segments [Line Items]      
Capital Investment 427 656 870
Acquisition Capital 6 2 319
Conventional [Member]      
Disclosure Of Reportable Segments [Line Items]      
Capital Investment 78 103 228
Acquisition Capital 12 7 22
Refining and Marketing [Member]      
Disclosure Of Reportable Segments [Line Items]      
Capital Investment 276 280 208
Acquisition Capital   4  
Corporate and Eliminations [Member]      
Disclosure Of Reportable Segments [Line Items]      
Capital Investment $ 60 $ 137 $ 57