RISK MANAGEMENT AND FINANCIAL INSTRUMENTS - Fair Value Derivatives (Details) - Fair Value Hedges - CAD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Foreign Currency Fair Value Hedge Derivative [Line Items]
Unrealized gain/(loss) on derivative	$ (60)	$ 0	$ 25	$ 0
Unrealized gain/(loss) on hedged item	59	0	(6)	0
Realized loss on derivative	$ 0	$ 0	$ (12)	$ 0