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Share-based payments - Narrative (Details) - CAD ($)
$ in Millions
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Disclosure of terms and conditions of share-based payment arrangement [line items]    
Number of shares authorized (in shares) 4,360,087  
ESP    
Disclosure of terms and conditions of share-based payment arrangement [line items]    
Award vesting period 2 years  
RSUs/PSUs    
Disclosure of terms and conditions of share-based payment arrangement [line items]    
Award vesting period 3 years  
DSP    
Disclosure of terms and conditions of share-based payment arrangement [line items]    
Liabilities from share-based payment transactions $ 19 $ 22
Stock Options    
Disclosure of terms and conditions of share-based payment arrangement [line items]    
Award vesting period 3 years  
Number of shares authorized (in shares) 4,193,370  
Top of range | ESP    
Disclosure of terms and conditions of share-based payment arrangement [line items]    
Maximum employee contribution rate 12.00%  
Maximum employer contribution rate 2.00%  
Top of range | Stock Options    
Disclosure of terms and conditions of share-based payment arrangement [line items]    
Award exercise period 10 years  
Bottom of range | DSU    
Disclosure of terms and conditions of share-based payment arrangement [line items]    
Percent of employee compensation paid in DSUs after minimum share ownership requirement 50.00%  
Bottom of range | Stock Options    
Disclosure of terms and conditions of share-based payment arrangement [line items]    
Award exercise period 7 years