RETIREMENT AND POSTRETIREMENT BENEFITS - NET BENEFITS COSTS RECOGNIZED (Details) - CAD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Service cost	$ 65	$ 54
Interest cost	45	32
Expected return on plan assets	(82)	(51)
Amortization of prior service costs	(1)	0
Amortization of actuarial loss	7	9
Net periodic benefit costs	$ 34	$ 44