PENSION AND OTHER POSTRETIREMENT BENEFITS (Details) - CAD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Service cost	$ 51	$ 51	$ 102	$ 116
Interest cost	50	42	101	87
Expected return on plan assets	(84)	(80)	(168)	(162)
Amortization of actuarial loss	8	8	16	15
Plan curtailments	0	2	0	2
Amortization of prior service costs	0	0	(1)	(1)
Net periodic benefit costs	$ 25	$ 23	$ 50	$ 57