PENSION AND OTHER POSTRETIREMENT BENEFITS (Details) - CAD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Service cost	$ 50	$ 46	$ 152	$ 162
Interest cost	51	39	152	126
Expected return on plan assets	(84)	(72)	(252)	(234)
Amortization of actuarial loss	8	6	24	21
Plan curtailments	0	0	0	2
Amortization of prior service costs	(1)	0	(2)	(1)
Net periodic benefit costs	$ 24	$ 19	$ 74	$ 76