RISK MANAGEMENT AND FINANCIAL INSTRUMENTS - Fair Value Derivatives (Details) - CAD ($) $ in Millions	3 Months Ended	6 Months Ended
Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Foreign Currency Fair Value Hedge Derivative [Line Items]
Realized gain/(loss) on derivative	$ 131	$ 531	$ 261	$ (575)
Fair Value Hedges
Foreign Currency Fair Value Hedge Derivative [Line Items]
Unrealized gain/(loss) on derivative	(32)	(133)	(35)	85
Unrealized gain/(loss) on hedged item	32	138	28	(65)
Fair Value Hedges \| Interest rate contracts
Foreign Currency Fair Value Hedge Derivative [Line Items]
Realized gain/(loss) on derivative	0	0	(39)	(12)
Realized gain on hedged item	$ 0	$ 0	$ 45	$ 0

Amount of gain (loss) from the increase (decrease) in fair value of derivative and nonderivative instruments designated as fair value hedging instruments recognized in the income statement.

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Amount of the increase (decrease) in fair value of the hedged item in a fair value hedge recognized in the income statement.

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Amount of increase (decrease) in the fair value of derivatives recognized in the income statement.

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Gain (loss) incurred in hedging activities.

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