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Income Taxes - Narrative (Details) - CAD ($)
$ in Millions
 6 Months Ended 12 Months Ended 18 Months Ended 24 Months Ended
Jan. 01, 2022
Dec. 31, 2019
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Jun. 30, 2019
Jan. 01, 2022
Disclosure of temporary difference, unused tax losses and unused tax credits [line items]              
Provincial corporate income tax rate 8.00% 11.00%   12.00% 11.00%    
Provincial corporate income tax rate, annual rate reduction             1.00%
Increase in deferred income tax liability due to change in provincial corporate income tax rate     $ 1,618 $ 10      
Unused tax losses              
Disclosure of temporary difference, unused tax losses and unused tax credits [line items]              
Deductible temporary differences for which no deferred tax asset is recognised   $ 1,000 1,000        
Unused tax credits              
Disclosure of temporary difference, unused tax losses and unused tax credits [line items]              
Deductible temporary differences for which no deferred tax asset is recognised   $ 750 $ 750        
Changes in tax rate              
Disclosure of temporary difference, unused tax losses and unused tax credits [line items]              
Provincial corporate income tax rate           12.00%