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Business Segments (Parenthetical) (Detail) - CAD ($)
$ in Millions
12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Jan. 01, 2019
Segment Reporting Information [Line Items]        
Impairment charge, before tax   $ 46    
Plant and equipment under construction $ 2,149 1,553 $ 1,047  
Operating Lease, Right-of-Use Asset 260     $ 298
Upstream | Alberta        
Segment Reporting Information [Line Items]        
Noncash impact 662      
Accounts payable and accrued liabilities        
Segment Reporting Information [Line Items]        
Non-cash adjustments to accrued liabilities and all other items 570      
United States Exports        
Segment Reporting Information [Line Items]        
Operating revenues $ 7,190 $ 6,661 4,392  
Horn River Development        
Segment Reporting Information [Line Items]        
Impairment charge, before tax     396  
Mackenzie Gas Project        
Segment Reporting Information [Line Items]        
Impairment charge, before tax     $ 379