PENSION AND OTHER POSTRETIREMENT BENEFITS (Details) - CAD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Service cost	$ 48	$ 58	$ 96	$ 100
Interest cost	32	54	64	87
Expected return on plan assets	(84)	(113)	(168)	(180)
Amortization of actuarial loss and prior service costs	14	10	28	19
Net periodic benefit costs	10	9	20	26
Severance costs	$ 0	$ 236	$ 0	$ 236