REVENUES - Narrative (Details) - CAD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Revenue from Contract with Customer [Abstract]
Mark-to-market gains (losses) from hedging program	$ (225)	$ 276	$ 36	$ (298)
Revenues from lease contracts	140	$ 144	442	$ 459
Revenue recognized	44		269
Increase (decrease) in contract with customers, liability	154		299
Remaining performance obligation	55,600		55,600
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2021-10-01
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	3 months		3 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