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Income taxes - Narrative (Details) - CAD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2023
Dec. 31, 2022
Dec. 31, 2021
Disclosure of temporary difference, unused tax losses and unused tax credits [line items]      
Applicable statutory tax rate 26.80% 26.80%  
Deferred tax asset $ (4,773) $ (4,869) $ (4,574)
Non-capital tax loss carryforwards      
Disclosure of temporary difference, unused tax losses and unused tax credits [line items]      
Unused tax loss carryforward 156 251  
Deferred tax asset 36 60 $ 63
Unused tax losses for which no deferred tax asset recognised 13 20  
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 143 231  
Capital loss carryforwards      
Disclosure of temporary difference, unused tax losses and unused tax credits [line items]      
Unused tax loss carryforward $ 55 $ 67