RETIREMENT AND POSTRETIREMENT BENEFITS - NET BENEFITS COSTS RECOGNIZED (Details) - CAD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Service cost	$ 46	$ 65	$ 162	$ 181
Interest cost	39	46	126	125
Expected return on plan assets	(72)	(71)	(234)	(195)
Amortization of actuarial loss	6	11	21	28
Plan curtailments	0	0	2	0
Amortization of prior service costs	0	(1)	(1)	(1)
Net periodic benefit costs	$ 19	$ 50	$ 76	$ 138