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PENSION AND OTHER POSTRETIREMENT BENEFITS - ASSUMED HEALTH CARE COST TREND RATES (Details) - CAD ($)
$ in Millions
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Canada    
MEDICAL COST TRENDS    
Health care cost trend rate assumed for next year 5.60% 5.50%
Rate to which the cost trend is assumed to decline (the ultimate trend rate) 4.40% 4.40%
Year that the rate reaches the ultimate trend rate 2034 2034
Effect of 1% change in assumed medical care trend rate    
Increase in service and interest cost due to 1% increase in the assumed medical care trend rate $ 1  
Decrease in service and interest cost due to 1% decrease in the assumed medical care trend rate (1)  
Increase in benefit obligation due to 1% increase in the assumed medical care trend rate 20  
Decrease in benefit obligation due to 1% decrease in the assumed medical care trend rate $ (16)  
United States    
MEDICAL COST TRENDS    
Health care cost trend rate assumed for next year 7.40% 7.40%
Rate to which the cost trend is assumed to decline (the ultimate trend rate) 4.50% 4.50%
Year that the rate reaches the ultimate trend rate 2037 2037
Effect of 1% change in assumed medical care trend rate    
Increase in service and interest cost due to 1% increase in the assumed medical care trend rate $ 1  
Decrease in service and interest cost due to 1% decrease in the assumed medical care trend rate (1)  
Increase in benefit obligation due to 1% increase in the assumed medical care trend rate 18  
Decrease in benefit obligation due to 1% decrease in the assumed medical care trend rate $ (17)