PENSION AND OTHER POSTRETIREMENT BENEFITS (Details) - CAD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Service cost	$ 48	$ 50	$ 144	$ 150
Interest cost	32	44	96	131
Expected return on plan assets	(84)	(90)	(252)	(270)
Amortization of actuarial loss and prior service costs	14	9	42	28
Net periodic benefit costs	10	13	30	39
Severance costs	$ 0	$ 0	$ 0	$ 236