PENSION AND OTHER POSTRETIREMENT BENEFITS (Details) - CAD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Service cost	$ 50	$ 50	$ 150	$ 152
Interest cost	44	51	131	152
Expected return on plan assets	(90)	(84)	(270)	(252)
Amortization of actuarial loss and prior service costs	9	7	28	22
Net periodic benefit costs	13	$ 24	39	$ 74
Severance costs	$ 236		$ 236