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Post-employee benefit plans - Disclosure of Contributions to Post-Employment Benefit Plans (Details) - CAD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Disclosure of defined benefit plans [line items]    
Contributions, defined contribution plan $ (114) $ (110)
Estimated of contributions expected to be paid to plan for next annual reporting period, defined contribution plan 120  
DB pension    
Disclosure of defined benefit plans [line items]    
Estimate of contributions expected to be paid to plan for next annual reporting period, defined benefit plan 180  
OPEBs    
Disclosure of defined benefit plans [line items]    
Estimate of contributions expected to be paid to plan for next annual reporting period, defined benefit plan 70  
Plan assets    
Disclosure of defined benefit plans [line items]    
Contributions, defined benefit plan (244) (252)
Plan assets | DB pension    
Disclosure of defined benefit plans [line items]    
Contributions, defined benefit plan (183) (180)
Plan assets | OPEBs    
Disclosure of defined benefit plans [line items]    
Contributions, defined benefit plan $ (61) $ (72)