REVENUES - Narrative (Details) - CAD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Revenue from Contract with Customer [Abstract]
Mark-to-market gains (losses) from hedging program	$ 131	$ 531	$ 261	$ (575)
Revenues from lease contracts	143	$ 157	302	$ 315
Revenue recognized	80		225
Increase (decrease) in contract with customers, liability	76		145
Remaining performance obligation	55,200		55,200
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2021-07-01
Revenue from Contract with Customer [Abstract]
Remaining performance obligation	$ 3,300		$ 3,300
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]: 2022-01-01
Revenue from Contract with Customer [Abstract]
Remaining performance obligation	$ 5,600		$ 5,600
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Remaining performance obligation, period	1 year		1 year