RETIREMENT AND POSTRETIREMENT BENEFITS - NET BENEFITS COSTS RECOGNIZED (Details) - CAD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Service cost	$ 51	$ 62	$ 116	$ 116
Interest cost	42	47	87	79
Expected return on plan assets	(80)	(73)	(162)	(124)
Amortization of actuarial loss	8	8	15	17
Plan curtailments	2	0	2	0
Amortization of prior service costs	0	0	(1)	0
Net periodic benefit costs	$ 23	$ 44	$ 57	$ 88