PENSION AND OTHER POSTRETIREMENT BENEFITS (Details) - CAD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Service cost	$ 51	$ 65
Interest cost	51	45
Expected return on plan assets	(84)	(82)
Amortization of actuarial loss	8	7
Amortization of prior service costs	(1)	(1)
Net periodic benefit costs	$ 25	$ 34