REVENUES - Narrative (Details) $ in Millions	3 Months Ended	9 Months Ended
	Sep. 30, 2019 CAD ($)	Sep. 30, 2019 CAD ($)
Revenue from Contract with Customer [Abstract]
Revenue recognized	$ 19	$ 149
Increase (decrease) in contract with customers, liability	171	314
Remaining performance obligation	64,700	64,700
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2019-10-01
Revenue from Contract with Customer [Abstract]
Remaining performance obligation	$ 1,800	$ 1,800
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Remaining performance obligation, period	3 months	3 months
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2020-01-01
Revenue from Contract with Customer [Abstract]
Remaining performance obligation	$ 6,000	$ 6,000
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Remaining performance obligation, period	1 year	1 year