PENSION AND OTHER POSTRETIREMENT BENEFITS (Details) - CAD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Service cost	$ 42	$ 51
Interest cost	33	51
Expected return on plan assets	(67)	(84)
Amortization of actuarial loss and prior service costs	9	7
Net periodic benefit costs	$ 17	$ 25