PENSION AND OTHER POSTRETIREMENT BENEFITS - Amount Recognized in Accumulated Other Comprehensive Income (Details) - CAD ($) $ in Millions	Dec. 31, 2022	Dec. 31, 2021
Canada \| OPEB
Amount included in AOCI
Net actuarial (gain)/loss	$ (101)	$ (35)
Prior service (credit)/cost	(1)	(1)
Total amount recognized in AOCI	(102)	(36)
Canada \| Pension
Amount included in AOCI
Net actuarial (gain)/loss	(64)	226
Prior service (credit)/cost	0	0
Total amount recognized in AOCI	(64)	226
United States \| OPEB
Amount included in AOCI
Net actuarial (gain)/loss	(102)	(104)
Prior service (credit)/cost	(30)	(37)
Total amount recognized in AOCI	(132)	(141)
United States \| Pension
Amount included in AOCI
Net actuarial (gain)/loss	40	92
Prior service (credit)/cost	1	(1)
Total amount recognized in AOCI	$ 41	$ 91

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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