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POST-EMPLOYMENT BENEFITS - Narrative (Details) - CAD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Disclosure of defined benefit plans [line items]    
Defined contribution plan expense $ 12 $ 8
Entity's own securities included in plan assets 10 $ 5
Estimate of contributions expected to be paid to plan for next annual reporting period $ 145  
Weighted average duration of defined benefit obligation 17 years 18 years
Return gain (loss) on plan assets $ 355 $ (44)
Cumulative loss in equity, employee benefit plans 503 384
Domestic defined benefit plans    
Disclosure of defined benefit plans [line items]    
Reduction in past service cost 21 43
Return gain (loss) on plan assets 277 (114)
Accrued benefit obligation | Domestic defined benefit plans    
Disclosure of defined benefit plans [line items]    
Reduction in past service cost $ 21 $ 43