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employee future benefits - Assumptions (Details) - Pension plans - CAD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
employee future benefits    
Discount rate used to determine net benefit costs 3.10% 3.90%
Discount rate used to determine defined benefit obligations 2.50% 3.10%
Discount rate used to determine current service cost in subsequent fiscal year 2.70% 3.20%
Rate of future increases in compensation used to determine net benefit costs 2.90% 2.80%
Rate of future increases in compensation used to defined benefit obligations 2.90% 2.90%
Increase in life expectancy in actuarial assumption 1 year 1 year
Decrease in actuarial assumption (as a per cent) 0.25% 0.25%
Life expectancy    
employee future benefits    
Effect of increase in actuarial assumption on obligation $ 349 $ 297
Effect of increase in actuarial assumption on expense 11 9
Discount rate    
employee future benefits    
Effect of decrease in actuarial assumption on obligation 386 342
Effect of decrease in actuarial assumption on expense 15 13
Rate of future increases in compensation    
employee future benefits    
Effect of decrease in actuarial assumption on obligation (36) (32)
Effect of decrease in actuarial assumption on expense $ (3) $ (4)