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employee future benefits - Assumptions (Details) - Pension plans - CAD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
employee future benefits    
Discount rate used to determine net benefit costs 3.40% 3.80%
Discount rate used to determine defined benefit obligations 3.90% 3.40%
Discount rate used to determine current service cost in subsequent fiscal year 4.00% 3.50%
Rate of future increases in compensation used to determine net benefit costs 2.70% 2.51%
Rate of future increases in compensation used to defined benefit obligations 2.80% 2.70%
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 change in actuarial assumption in obligation $ 242 $ 270
Effect of change in actuarial assumption in expense 11 10
Discount rate    
employee future benefits    
Effect of change in actuarial assumption in obligation 292 337
Effect of change in actuarial assumption in expense 16 16
Rate of future increases in compensation    
employee future benefits    
Effect of change in actuarial assumption in obligation (27) (34)
Effect of change in actuarial assumption in expense $ (3) $ (3)