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Debt - Finance Lease Liabilities - Additional Information (Detail) - CAD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Disclosure of detailed information about borrowings [line items]    
Finance lease liabilities $ 338 $ 313
Fort Hills Energy Limited Partnership [member]    
Disclosure of detailed information about borrowings [line items]    
Finance lease liabilities $ 207 $ 229
Lease term 25 years 25 years
Lease renewal option periods Four additional five-year periods Four additional five-year periods
Property plant and equipment under finance leases [member]    
Disclosure of detailed information about borrowings [line items]    
Carrying amount of assets under finance leases $ 613 $ 406