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Financial and derivative instruments - Additional Information (Detail) - CAD ($)
$ in Millions
Mar. 31, 2021
Dec. 31, 2020
Financial Instruments And Derivatives [Line Items]    
Long-term Debt, Fair Value $ 4,447 $ 4,447
Master netting arrangements [Member]    
Financial Instruments And Derivatives [Line Items]    
Derivative collateral receivable $ 14 $ 5