PENSION AND OTHER POSTRETIREMENT BENEFITS (Details) - CAD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Service cost	$ 58	$ 51	$ 100	$ 102
Interest cost	54	50	87	101
Expected return on plan assets	(113)	(84)	(180)	(168)
Amortization of actuarial loss and prior service costs	10	8	19	15
Net periodic benefit costs	9	$ 25	26	$ 50
Severance costs	$ 236		$ 236