REVENUE - Narrative (Details) $ in Millions	3 Months Ended	9 Months Ended
	Sep. 30, 2018 CAD ($)	Sep. 30, 2018 CAD ($)
Revenue from Contract with Customer [Abstract]
Revenue recognized	$ 19	$ 143
Increase (decrease) in contract with customers, liability	147	345
Remaining performance obligation	64,700	64,700
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2018-10-01
Revenue from Contract with Customer [Abstract]
Remaining performance obligation	1,700	$ 1,700
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Remaining performance obligation, period		1 year
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2019-10-01
Revenue from Contract with Customer [Abstract]
Remaining performance obligation	$ 5,800	$ 5,800