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POST-EMPLOYMENT BENEFITS - Narrative (Details) - CAD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Disclosure of defined benefit plans [line items]    
Defined contribution plan expense $ 15 $ 12
Entity's own securities included in plan assets 10 $ 10
Estimate of contributions expected to be paid to plan for next annual reporting period $ 169  
Weighted average duration of defined benefit obligation 18 years 17 years
Return gain (loss) on plan assets $ 240 $ 355
Cumulative loss in equity, employee benefit plans 592 503
Domestic defined benefit plans    
Disclosure of defined benefit plans [line items]    
Reduction in past service cost 0 21
Return gain (loss) on plan assets 163 277
Accrued benefit obligation | Domestic defined benefit plans    
Disclosure of defined benefit plans [line items]    
Reduction in past service cost $ 0 $ 21