INCOME TAXES - Components of tax expense (Details) - CAD ($) $ in Millions	12 Months Ended
Dec. 31, 2020	Dec. 31, 2019
Income Taxes [Abstract]
Total current tax expense	$ 712	$ 269
Deferred tax (recovery) expense:
(Reversal) origination of temporary differences	(129)	466
Revaluation of deferred tax balances due to legislative changes	(3)	(23)
Total deferred tax (recovery) expense	(132)	443
Total income tax expense	$ 580	$ 712

The amount of current tax expense (income) and adjustments for the current tax of prior periods. [Refer: Current tax expense (income); Adjustments for current tax of prior periods]

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The amount of deferred tax expense or income relating to the creation or reversal of temporary differences. [Refer: Temporary differences [member]; Deferred tax expense (income)]

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The amount of deferred tax expense or income relating to tax rate changes or the imposition of new taxes. [Refer: Deferred tax expense (income)]

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The aggregate amount included in the determination of profit (loss) for the period in respect of current tax and deferred tax. [Refer: Current tax expense (income); Deferred tax expense (income)]

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Income Taxes [Abstract]

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