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EMPLOYEE POST-RETIREMENT BENEFITS - Net Change in Level III Fair Value (Details) - CAD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Net change in the Level III fair value category    
Plan assets at fair value – beginning of year $ 3,697  
Plan assets at fair value – end of year 4,099 $ 3,697
Significant Unobservable Inputs (Level III)    
Net change in the Level III fair value category    
Plan assets at fair value – beginning of year 362 216
Purchases and sales 35 127
Realized and unrealized losses (18) 19
Plan assets at fair value – end of year $ 379 $ 362