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Income taxes - Narrative (Details) - CAD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Jan. 01, 2019
Income Taxes [Abstract]      
Applicable statutory tax rate 26.90% 27.00%  
Disclosure of temporary difference, unused tax losses and unused tax credits [line items]      
Deferred tax asset $ (3,704) $ (3,463) $ (3,044)
Non-capital tax loss carryforwards      
Disclosure of temporary difference, unused tax losses and unused tax credits [line items]      
Unused tax loss carryforward 357 215  
Deferred tax asset 69 31 $ 129
Unused tax losses for which no deferred tax asset recognised 94 93  
Unused tax loss carryforward 357 215  
Non-capital Loss Carryforwards With Deferred Tax Asset      
Disclosure of temporary difference, unused tax losses and unused tax credits [line items]      
Unused tax loss carryforward 263 122  
Unused tax loss carryforward 263 122  
Capital loss carryforwards      
Disclosure of temporary difference, unused tax losses and unused tax credits [line items]      
Unused tax loss carryforward 64 734  
Unused tax loss carryforward $ 64 $ 734