PENSION AND OTHER POSTRETIREMENT BENEFITS - Plan Assets (Details) - CAD ($) $ in Millions	12 Months Ended
	Dec. 31, 2022	Dec. 31, 2021
Canada | Equity securities
Pension and Other Postretirement Benefit Disclosures
Allocation of plan assets (as a percent)	43.80%
Plan asset allocations	38.20%	46.70%
Canada | Fixed income securities
Pension and Other Postretirement Benefit Disclosures
Allocation of plan assets (as a percent)	28.40%
Plan asset allocations	31.70%	29.80%
Canada | Alternatives
Pension and Other Postretirement Benefit Disclosures
Allocation of plan assets (as a percent)	27.80%
Plan asset allocations	30.10%	23.50%
Canada | Pension
Change in plan assets
Fair value of plan assets at beginning of year	$ 4,536	$ 4,077
Fair value of plan assets at end of year	4,234	4,536
Canada | Pension | Level 1
Change in plan assets
Fair value of plan assets at beginning of year	637
Fair value of plan assets at end of year	473	637
Canada | Pension | Level 2
Change in plan assets
Fair value of plan assets at beginning of year	2,835
Fair value of plan assets at end of year	2,470	2,835
Canada | Pension | Level 3
Change in plan assets
Fair value of plan assets at beginning of year	1,064	912
Unrealized and realized gains	155	77
Purchases and settlements, net	72	75
Fair value of plan assets at end of year	1,291	1,064
Canada | Pension | Alternatives
Change in plan assets
Fair value of plan assets at beginning of year	1,064
Fair value of plan assets at end of year	1,291	1,064
Canada | Pension | Alternatives | Level 1
Change in plan assets
Fair value of plan assets at beginning of year	0
Fair value of plan assets at end of year	0	0
Canada | Pension | Alternatives | Level 2
Change in plan assets
Fair value of plan assets at beginning of year	0
Fair value of plan assets at end of year	0	0
Canada | Pension | Alternatives | Level 3
Change in plan assets
Fair value of plan assets at beginning of year	1,064
Fair value of plan assets at end of year	1,291	1,064
Canada | Pension | Cash and Cash Equivalents
Change in plan assets
Fair value of plan assets at beginning of year	180
Fair value of plan assets at end of year	272	180
Canada | Pension | Cash and Cash Equivalents | Level 1
Change in plan assets
Fair value of plan assets at beginning of year	180
Fair value of plan assets at end of year	272	180
Canada | Pension | Cash and Cash Equivalents | Level 2
Change in plan assets
Fair value of plan assets at beginning of year	0
Fair value of plan assets at end of year	0	0
Canada | Pension | Cash and Cash Equivalents | Level 3
Change in plan assets
Fair value of plan assets at beginning of year	0
Fair value of plan assets at end of year	0	0
Canada | Pension | Equity Securities, Global
Change in plan assets
Fair value of plan assets at beginning of year	1,693
Fair value of plan assets at end of year	1,263	1,693
Canada | Pension | Equity Securities, Global | Level 1
Change in plan assets
Fair value of plan assets at beginning of year	0
Fair value of plan assets at end of year	0	0
Canada | Pension | Equity Securities, Global | Level 2
Change in plan assets
Fair value of plan assets at beginning of year	1,693
Fair value of plan assets at end of year	1,263	1,693
Canada | Pension | Equity Securities, Global | Level 3
Change in plan assets
Fair value of plan assets at beginning of year	0
Fair value of plan assets at end of year	0	0
Canada | Pension | Equity Securities, US
Change in plan assets
Fair value of plan assets at beginning of year	1
Fair value of plan assets at end of year		1
Canada | Pension | Equity Securities, US | Level 1
Change in plan assets
Fair value of plan assets at beginning of year	1
Fair value of plan assets at end of year		1
Canada | Pension | Equity Securities, US | Level 2
Change in plan assets
Fair value of plan assets at beginning of year	0
Fair value of plan assets at end of year		0
Canada | Pension | Equity Securities, US | Level 3
Change in plan assets
Fair value of plan assets at beginning of year	0
Fair value of plan assets at end of year		0
Canada | Pension | Equity Securities, Global
Change in plan assets
Fair value of plan assets at beginning of year	426
Fair value of plan assets at end of year	355	426
Canada | Pension | Equity Securities, Global | Level 1
Change in plan assets
Fair value of plan assets at beginning of year	198
Fair value of plan assets at end of year	0	198
Canada | Pension | Equity Securities, Global | Level 2
Change in plan assets
Fair value of plan assets at beginning of year	228
Fair value of plan assets at end of year	355	228
Canada | Pension | Equity Securities, Global | Level 3
Change in plan assets
Fair value of plan assets at beginning of year	0
Fair value of plan assets at end of year	0	0
Canada | Pension | Government
Change in plan assets
Fair value of plan assets at beginning of year	717
Fair value of plan assets at end of year	636	717
Canada | Pension | Government | Level 1
Change in plan assets
Fair value of plan assets at beginning of year	258
Fair value of plan assets at end of year	201	258
Canada | Pension | Government | Level 2
Change in plan assets
Fair value of plan assets at beginning of year	459
Fair value of plan assets at end of year	435	459
Canada | Pension | Government | Level 3
Change in plan assets
Fair value of plan assets at beginning of year	0
Fair value of plan assets at end of year	0	0
Canada | Pension | Corporate
Change in plan assets
Fair value of plan assets at beginning of year	453
Fair value of plan assets at end of year	433	453
Canada | Pension | Corporate | Level 1
Change in plan assets
Fair value of plan assets at beginning of year	0
Fair value of plan assets at end of year	0	0
Canada | Pension | Corporate | Level 2
Change in plan assets
Fair value of plan assets at beginning of year	453
Fair value of plan assets at end of year	433	453
Canada | Pension | Corporate | Level 3
Change in plan assets
Fair value of plan assets at beginning of year	0
Fair value of plan assets at end of year	0	0
Canada | Pension | Foreign exchange contracts
Change in plan assets
Fair value of plan assets at beginning of year	2
Fair value of plan assets at end of year	(16)	2
Canada | Pension | Foreign exchange contracts | Level 1
Change in plan assets
Fair value of plan assets at beginning of year	0
Fair value of plan assets at end of year	0	0
Canada | Pension | Foreign exchange contracts | Level 2
Change in plan assets
Fair value of plan assets at beginning of year	2
Fair value of plan assets at end of year	(16)	2
Canada | Pension | Foreign exchange contracts | Level 3
Change in plan assets
Fair value of plan assets at beginning of year	0
Fair value of plan assets at end of year	0	0
Canada | OPEB
Change in plan assets
Fair value of plan assets at beginning of year	0	0
Fair value of plan assets at end of year	$ 0	$ 0
United States | Equity securities
Pension and Other Postretirement Benefit Disclosures
Allocation of plan assets (as a percent)	45.00%
Plan asset allocations	38.30%	52.50%
United States | Fixed income securities
Pension and Other Postretirement Benefit Disclosures
Allocation of plan assets (as a percent)	20.00%
Plan asset allocations	20.50%	18.40%
United States | Alternatives
Pension and Other Postretirement Benefit Disclosures
Allocation of plan assets (as a percent)	35.00%
Plan asset allocations	41.20%	29.10%
United States | Pension
Change in plan assets
Fair value of plan assets at beginning of year	$ 1,160	$ 1,062
Fair value of plan assets at end of year	1,080	1,160
United States | Pension | Level 1
Change in plan assets
Fair value of plan assets at beginning of year	10
Fair value of plan assets at end of year	13	10
United States | Pension | Level 2
Change in plan assets
Fair value of plan assets at beginning of year	813
Fair value of plan assets at end of year	622	813
United States | Pension | Level 3
Change in plan assets
Fair value of plan assets at beginning of year	337	289
Unrealized and realized gains	78	38
Purchases and settlements, net	30	10
Fair value of plan assets at end of year	445	337
United States | Pension | Alternatives
Change in plan assets
Fair value of plan assets at beginning of year	337
Fair value of plan assets at end of year	445	337
United States | Pension | Alternatives | Level 1
Change in plan assets
Fair value of plan assets at beginning of year	0
Fair value of plan assets at end of year	0	0
United States | Pension | Alternatives | Level 2
Change in plan assets
Fair value of plan assets at beginning of year	0
Fair value of plan assets at end of year	0	0
United States | Pension | Alternatives | Level 3
Change in plan assets
Fair value of plan assets at beginning of year	337
Fair value of plan assets at end of year	445	337
United States | Pension | Cash and Cash Equivalents
Change in plan assets
Fair value of plan assets at beginning of year	10
Fair value of plan assets at end of year	13	10
United States | Pension | Cash and Cash Equivalents | Level 1
Change in plan assets
Fair value of plan assets at beginning of year	10
Fair value of plan assets at end of year	13	10
United States | Pension | Cash and Cash Equivalents | Level 2
Change in plan assets
Fair value of plan assets at beginning of year	0
Fair value of plan assets at end of year	0	0
United States | Pension | Cash and Cash Equivalents | Level 3
Change in plan assets
Fair value of plan assets at beginning of year	0
Fair value of plan assets at end of year	0	0
United States | Pension | Equity Securities, Global
Change in plan assets
Fair value of plan assets at beginning of year	609
Fair value of plan assets at end of year	414	609
United States | Pension | Equity Securities, Global | Level 1
Change in plan assets
Fair value of plan assets at beginning of year	0
Fair value of plan assets at end of year	0	0
United States | Pension | Equity Securities, Global | Level 2
Change in plan assets
Fair value of plan assets at beginning of year	609
Fair value of plan assets at end of year	414	609
United States | Pension | Equity Securities, Global | Level 3
Change in plan assets
Fair value of plan assets at beginning of year	0
Fair value of plan assets at end of year	0	0
United States | Pension | Equity Securities, US
Change in plan assets
Fair value of plan assets at beginning of year	0
Fair value of plan assets at end of year		0
United States | Pension | Equity Securities, US | Level 1
Change in plan assets
Fair value of plan assets at beginning of year	0
Fair value of plan assets at end of year		0
United States | Pension | Equity Securities, US | Level 2
Change in plan assets
Fair value of plan assets at beginning of year	0
Fair value of plan assets at end of year		0
United States | Pension | Equity Securities, US | Level 3
Change in plan assets
Fair value of plan assets at beginning of year	0
Fair value of plan assets at end of year		0
United States | Pension | Equity Securities, Global
Change in plan assets
Fair value of plan assets at beginning of year	0
Fair value of plan assets at end of year	0	0
United States | Pension | Equity Securities, Global | Level 1
Change in plan assets
Fair value of plan assets at beginning of year	0
Fair value of plan assets at end of year	0	0
United States | Pension | Equity Securities, Global | Level 2
Change in plan assets
Fair value of plan assets at beginning of year	0
Fair value of plan assets at end of year	0	0
United States | Pension | Equity Securities, Global | Level 3
Change in plan assets
Fair value of plan assets at beginning of year	0
Fair value of plan assets at end of year	0	0
United States | Pension | Government
Change in plan assets
Fair value of plan assets at beginning of year	86
Fair value of plan assets at end of year	87	86
United States | Pension | Government | Level 1
Change in plan assets
Fair value of plan assets at beginning of year	0
Fair value of plan assets at end of year	0	0
United States | Pension | Government | Level 2
Change in plan assets
Fair value of plan assets at beginning of year	86
Fair value of plan assets at end of year	87	86
United States | Pension | Government | Level 3
Change in plan assets
Fair value of plan assets at beginning of year	0
Fair value of plan assets at end of year	0	0
United States | Pension | Corporate
Change in plan assets
Fair value of plan assets at beginning of year	118
Fair value of plan assets at end of year	121	118
United States | Pension | Corporate | Level 1
Change in plan assets
Fair value of plan assets at beginning of year	0
Fair value of plan assets at end of year	0	0
United States | Pension | Corporate | Level 2
Change in plan assets
Fair value of plan assets at beginning of year	118
Fair value of plan assets at end of year	121	118
United States | Pension | Corporate | Level 3
Change in plan assets
Fair value of plan assets at beginning of year	0
Fair value of plan assets at end of year	0	0
United States | Pension | Foreign exchange contracts
Change in plan assets
Fair value of plan assets at beginning of year	0
Fair value of plan assets at end of year	0	0
United States | Pension | Foreign exchange contracts | Level 1
Change in plan assets
Fair value of plan assets at beginning of year	0
Fair value of plan assets at end of year	0	0
United States | Pension | Foreign exchange contracts | Level 2
Change in plan assets
Fair value of plan assets at beginning of year	0
Fair value of plan assets at end of year	0	0
United States | Pension | Foreign exchange contracts | Level 3
Change in plan assets
Fair value of plan assets at beginning of year	0
Fair value of plan assets at end of year	0	0
United States | OPEB
Change in plan assets
Fair value of plan assets at beginning of year	201	188
Fair value of plan assets at end of year	185	201
United States | OPEB | Fair Value
Change in plan assets
Fair value of plan assets at beginning of year	201
Fair value of plan assets at end of year	185	201
United States | OPEB | Level 1 | Fair Value
Change in plan assets
Fair value of plan assets at beginning of year	51
Fair value of plan assets at end of year	48	51
United States | OPEB | Level 2 | Fair Value
Change in plan assets
Fair value of plan assets at beginning of year	128
Fair value of plan assets at end of year	109	128
United States | OPEB | Level 3
Change in plan assets
Fair value of plan assets at beginning of year	22	22
Unrealized and realized gains	4	2
Purchases and settlements, net	2	(2)
Fair value of plan assets at end of year	28	22
United States | OPEB | Level 3 | Fair Value
Change in plan assets
Fair value of plan assets at beginning of year	22
Fair value of plan assets at end of year	28	22
United States | OPEB | Alternatives | Fair Value
Change in plan assets
Fair value of plan assets at beginning of year	22
Fair value of plan assets at end of year	28	22
United States | OPEB | Alternatives | Level 1 | Fair Value
Change in plan assets
Fair value of plan assets at beginning of year	0
Fair value of plan assets at end of year	0	0
United States | OPEB | Alternatives | Level 2 | Fair Value
Change in plan assets
Fair value of plan assets at beginning of year	0
Fair value of plan assets at end of year	0	0
United States | OPEB | Alternatives | Level 3 | Fair Value
Change in plan assets
Fair value of plan assets at beginning of year	22
Fair value of plan assets at end of year	28	22
United States | OPEB | Cash and Cash Equivalents | Fair Value
Change in plan assets
Fair value of plan assets at beginning of year	4
Fair value of plan assets at end of year	2	4
United States | OPEB | Cash and Cash Equivalents | Level 1 | Fair Value
Change in plan assets
Fair value of plan assets at beginning of year	4
Fair value of plan assets at end of year	2	4
United States | OPEB | Cash and Cash Equivalents | Level 2 | Fair Value
Change in plan assets
Fair value of plan assets at beginning of year	0
Fair value of plan assets at end of year	0	0
United States | OPEB | Cash and Cash Equivalents | Level 3 | Fair Value
Change in plan assets
Fair value of plan assets at beginning of year	0
Fair value of plan assets at end of year	0	0
United States | OPEB | Equity Securities, US
Change in plan assets
Fair value of plan assets at beginning of year	39
Fair value of plan assets at end of year	34	39
United States | OPEB | Equity Securities, US | Level 1
Change in plan assets
Fair value of plan assets at beginning of year	0
Fair value of plan assets at end of year	0	0
United States | OPEB | Equity Securities, US | Level 2
Change in plan assets
Fair value of plan assets at beginning of year	39
Fair value of plan assets at end of year	34	39
United States | OPEB | Equity Securities, US | Level 3
Change in plan assets
Fair value of plan assets at beginning of year	0
Fair value of plan assets at end of year	0	0
United States | OPEB | Equity Securities, Global
Change in plan assets
Fair value of plan assets at beginning of year	75
Fair value of plan assets at end of year	62	75
United States | OPEB | Equity Securities, Global | Level 1
Change in plan assets
Fair value of plan assets at beginning of year	0
Fair value of plan assets at end of year	0	0
United States | OPEB | Equity Securities, Global | Level 2
Change in plan assets
Fair value of plan assets at beginning of year	75
Fair value of plan assets at end of year	62	75
United States | OPEB | Equity Securities, Global | Level 3
Change in plan assets
Fair value of plan assets at beginning of year	0
Fair value of plan assets at end of year	0	0
United States | OPEB | Government | Fair Value
Change in plan assets
Fair value of plan assets at beginning of year	53
Fair value of plan assets at end of year	51	53
United States | OPEB | Government | Level 1 | Fair Value
Change in plan assets
Fair value of plan assets at beginning of year	47
Fair value of plan assets at end of year	46	47
United States | OPEB | Government | Level 2 | Fair Value
Change in plan assets
Fair value of plan assets at beginning of year	6
Fair value of plan assets at end of year	5	6
United States | OPEB | Government | Level 3 | Fair Value
Change in plan assets
Fair value of plan assets at beginning of year	0
Fair value of plan assets at end of year	0	0
United States | OPEB | Corporate | Fair Value
Change in plan assets
Fair value of plan assets at beginning of year	8
Fair value of plan assets at end of year	8	8
United States | OPEB | Corporate | Level 1 | Fair Value
Change in plan assets
Fair value of plan assets at beginning of year	0
Fair value of plan assets at end of year	0	0
United States | OPEB | Corporate | Level 2 | Fair Value
Change in plan assets
Fair value of plan assets at beginning of year	8
Fair value of plan assets at end of year	8	8
United States | OPEB | Corporate | Level 3 | Fair Value
Change in plan assets
Fair value of plan assets at beginning of year	0
Fair value of plan assets at end of year	$ 0	$ 0