PENSION AND OTHER POSTRETIREMENT BENEFITS - AMOUNT RECOGNIZED IN ACCUMULATED OTHER COMPREHENSIVE INCOME (Details) - CAD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Canada \| Pension
Amount included in AOCI
Net actuarial loss (gain)	$ 542	$ 445
Prior service credit	0	0
Total amount recognized in AOCI	542	445
Canada \| OPEB
Amount included in AOCI
Net actuarial loss (gain)	15	(7)
Prior service credit	(1)	(1)
Total amount recognized in AOCI	14	(8)
United States \| Pension
Amount included in AOCI
Net actuarial loss (gain)	233	134
Prior service credit	(1)	(2)
Total amount recognized in AOCI	232	132
United States \| OPEB
Amount included in AOCI
Net actuarial loss (gain)	(7)	(23)
Prior service credit	(44)	(13)
Total amount recognized in AOCI	$ (51)	$ (36)

Amount, before tax, of accumulated other comprehensive (income) loss for defined benefit plan, that has not been recognized in net periodic benefit cost (credit).

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Amount, before tax, of accumulated other comprehensive income (loss) for gain (loss) of defined benefit plan, that has not been recognized in net periodic benefit (cost) credit.

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Amount, before tax, of accumulated other comprehensive (income) loss for cost (credit) of benefit change attributable to participants' prior service from plan amendment or plan initiation of defined benefit plan, that has not been recognized in net periodic benefit cost (credit).

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