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employee future benefits - Pensions funded status (Details) - CAD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Funded Status - Planned surplus (deficit)    
Interest expense $ (16) $ (1)
Pension plans    
Funded Status - Planned surplus (deficit)    
Balance at beginning of year (425)  
Balance at end of year (913) (425)
Pension plans | Present value of the defined benefit obligations    
Funded Status - Planned surplus (deficit)    
Balance at beginning of year 9,684 8,723
Current service cost 111 91
Past service cost 3  
Interest expense 297 335
Actuarial loss (gain) arising from changes in demographic assumptions 67 20
Actuarial loss (gain) arising from changes in financial assumptions 836 984
Benefits paid (477) (469)
Balance at end of year 10,521 9,684
Pension plans | Plan assets    
Funded Status - Planned surplus (deficit)    
Balance at beginning of year 9,380 9,043
Interest expense 285 344
Return on plan assets - Actual return on plan assets (less than) greater than discount rate 480 408
Contributions - Employer contributions 51 41
Contributions - Employees' contributions 19 19
Benefits paid (477) (469)
Administrative fees (7) (6)
Balance at end of year 9,731 9,380
Pension plans | Plan assets net of Effect of asset ceiling limit    
Funded Status - Planned surplus (deficit)    
Balance at beginning of year 9,259  
Balance at end of year 9,608 9,259
Pension plans | Effect of asset ceiling limit    
Funded Status - Planned surplus (deficit)    
Balance at beginning of year (121) (263)
Change (2) 142
Balance at end of year $ (123) $ (121)