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Income Taxes - Schedule of Amounts of Tax Pools Available, Including Tax Losses (Detail) - CAD ($)
$ in Millions
Dec. 31, 2020
Dec. 31, 2019
Disclosure of temporary difference, unused tax losses and unused tax credits [Line Items]    
Amounts of tax pools available, including tax losses $ 9,657 $ 8,761
Canada [Member]    
Disclosure of temporary difference, unused tax losses and unused tax credits [Line Items]    
Amounts of tax pools available, including tax losses 6,540 6,113
United States [Member]    
Disclosure of temporary difference, unused tax losses and unused tax credits [Line Items]    
Amounts of tax pools available, including tax losses $ 3,117 $ 2,648