PENSION AND OTHER POSTRETIREMENT BENEFITS - Amount Recognized in Accumulated Other Comprehensive Income (Details) - CAD ($) $ in Millions	Dec. 31, 2023	Dec. 31, 2022
Canada \| Pension
Amount included in AOCI
Net actuarial (gain)/loss	$ 51	$ (64)
Prior service cost	0	0
Total amount recognized in AOCI	51	(64)
Canada \| OPEB
Amount included in AOCI
Net actuarial (gain)/loss	(82)	(101)
Prior service cost	(1)	(1)
Total amount recognized in AOCI	(83)	(102)
US \| Pension
Amount included in AOCI
Net actuarial (gain)/loss	74	40
Prior service cost	1	1
Total amount recognized in AOCI	75	41
US \| OPEB
Amount included in AOCI
Net actuarial (gain)/loss	(96)	(102)
Prior service cost	(22)	(30)
Total amount recognized in AOCI	$ (118)	$ (132)

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

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Name Accounting Standards Codification
-Topic 220
-SubTopic 10
-Section 45
-Paragraph 14A
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147482790/220-10-45-14A

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Name Accounting Standards Codification
-Section 45
-Paragraph 10A
-Subparagraph (i-k)
-SubTopic 10
-Topic 220
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147482790/220-10-45-10A

Reference 3: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 715
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 1
-Subparagraph (j)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147480506/715-20-50-1

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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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