PENSION AND OTHER POSTRETIREMENT BENEFITS - AMOUNT RECOGNIZED IN ACCUMULATED OTHER COMPREHENSIVE INCOME (Details) - CAD ($) $ in Millions	Dec. 31, 2021	Dec. 31, 2020
Canada \| OPEB
Amount included in AOCI
Net actuarial loss	$ (35)	$ 15
Prior service credit	(1)	(1)
Total amount recognized in AOCI	(36)	14
Canada \| Pension
Amount included in AOCI
Net actuarial loss	226	542
Prior service credit	0	0
Total amount recognized in AOCI	226	542
United States \| OPEB
Amount included in AOCI
Net actuarial loss	(104)	(7)
Prior service credit	(37)	(44)
Total amount recognized in AOCI	(141)	(51)
United States \| Pension
Amount included in AOCI
Net actuarial loss	92	233
Prior service credit	(1)	(1)
Total amount recognized in AOCI	$ 91	$ 232

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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