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POST-EMPLOYMENT BENEFITS - Benefit Plan Costs (Details) - Domestic defined benefit plans - CAD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Disclosure of net defined benefit liability (asset) [line items]    
Current service cost $ 146 $ 121
Past service recovery 0 (21)
Net interest cost 10 8
Net pension expense 156 108
Administrative expense 4 4
Total pension cost recognized in net income 160 112
Plan assets    
Disclosure of net defined benefit liability (asset) [line items]    
Net interest cost (81) (81)
Administrative expense 4 3
Accrued benefit obligation    
Disclosure of net defined benefit liability (asset) [line items]    
Current service cost 146 121
Past service recovery 0 (21)
Net interest cost $ 91 $ 89