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PENSION AND OTHER POSTRETIREMENT BENEFITS (Details) - CAD ($)
$ in Millions
 3 Months Ended
Mar. 31, 2021
Mar. 31, 2020
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]    
Service cost $ 48 $ 42
Interest cost 32 33
Expected return on plan assets (84) (67)
Amortization of actuarial loss and prior service costs 14 9
Net periodic benefit costs $ 10 $ 17