REVENUES - Narrative (Details) - CAD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Revenue from Contract with Customer [Abstract]
Mark-to-market gains (losses) from hedging program	$ 531	$ 126	$ (575)	$ 384
Revenues from lease contracts	157	$ 151	315	$ 315
Revenue recognized	21		107
Increase (decrease) in contract with customers, liability	104		180
Remaining performance obligation	63,700		63,700
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2020-07-01
Revenue from Contract with Customer [Abstract]
Remaining performance obligation	$ 3,500		$ 3,500
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Remaining performance obligation, period	6 months		6 months
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2021-01-01
Revenue from Contract with Customer [Abstract]
Remaining performance obligation	$ 6,100		$ 6,100
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Remaining performance obligation, period	1 year		1 year