PENSION AND OTHER POSTRETIREMENT BENEFITS - PLAN ASSETS (Details) - CAD ($) $ in Millions	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020
Canada | Equity securities
Pension and Other Postretirement Benefit Disclosures
Allocation of plan assets (as a percent)	43.80%
Plan asset allocations	46.70%	47.20%
Canada | Fixed income securities
Pension and Other Postretirement Benefit Disclosures
Allocation of plan assets (as a percent)	28.90%
Plan asset allocations	29.80%	29.60%
Canada | Alternatives
Pension and Other Postretirement Benefit Disclosures
Allocation of plan assets (as a percent)	27.30%
Plan asset allocations	23.50%	23.20%
Canada | Pension
Change in plan assets
Fair value of plan assets at beginning of year	$ 4,077	$ 3,827
Fair value of plan assets at end of year	4,536	4,077
Canada | Pension | Level 1
Change in plan assets
Fair value of plan assets at beginning of year	600
Fair value of plan assets at end of year	637	600
Canada | Pension | Level 2
Change in plan assets
Fair value of plan assets at beginning of year	2,565
Fair value of plan assets at end of year	2,835	2,565
Canada | Pension | Level 3
Change in plan assets
Fair value of plan assets at beginning of year	912	852
Unrealized and realized gains/(losses)	77	(27)
Purchases and settlements, net	75	87
Fair value of plan assets at end of year	1,064	912
Canada | Pension | Cash and Cash Equivalents
Change in plan assets
Fair value of plan assets at beginning of year	213
Fair value of plan assets at end of year	180	213
Canada | Pension | Cash and Cash Equivalents | Level 1
Change in plan assets
Fair value of plan assets at beginning of year	213
Fair value of plan assets at end of year	180	213
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, Non-US
Change in plan assets
Fair value of plan assets at beginning of year	366
Fair value of plan assets at end of year	426	366
Canada | Pension | Equity Securities, Non-US | Level 1
Change in plan assets
Fair value of plan assets at beginning of year	178
Fair value of plan assets at end of year	198	178
Canada | Pension | Equity Securities, Non-US | Level 2
Change in plan assets
Fair value of plan assets at beginning of year	188
Fair value of plan assets at end of year	228	188
Canada | Pension | Equity Securities, Non-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
Canada | Pension | Equity Securities, US
Change in plan assets
Fair value of plan assets at beginning of year	2
Fair value of plan assets at end of year	1	2
Canada | Pension | Equity Securities, US | Level 1
Change in plan assets
Fair value of plan assets at beginning of year	2
Fair value of plan assets at end of year	1	2
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	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	0
Canada | Pension | Equity Securities, Global
Change in plan assets
Fair value of plan assets at beginning of year	1,556
Fair value of plan assets at end of year	1,693	1,556
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	1,556
Fair value of plan assets at end of year	1,693	1,556
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	585
Fair value of plan assets at end of year	717	585
Canada | Pension | Government | Level 1
Change in plan assets
Fair value of plan assets at beginning of year	207
Fair value of plan assets at end of year	258	207
Canada | Pension | Government | Level 2
Change in plan assets
Fair value of plan assets at beginning of year	378
Fair value of plan assets at end of year	459	378
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	410
Fair value of plan assets at end of year	453	410
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	410
Fair value of plan assets at end of year	453	410
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 | Infrastructure and real estate
Change in plan assets
Fair value of plan assets at beginning of year	912
Fair value of plan assets at end of year	1,064	912
Canada | Pension | Infrastructure and real estate | 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 | Infrastructure and real estate | 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 | Infrastructure and real estate | Level 3
Change in plan assets
Fair value of plan assets at beginning of year	912
Fair value of plan assets at end of year	1,064	912
Canada | Pension | Foreign exchange contracts
Change in plan assets
Fair value of plan assets at beginning of year	33
Fair value of plan assets at end of year	2	33
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	33
Fair value of plan assets at end of year	2	33
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	52.50%	55.60%
United States | Fixed income securities
Pension and Other Postretirement Benefit Disclosures
Allocation of plan assets (as a percent)	20.10%
Plan asset allocations	18.40%	17.20%
United States | Alternatives
Pension and Other Postretirement Benefit Disclosures
Allocation of plan assets (as a percent)	34.90%
Plan asset allocations	29.10%	27.20%
United States | Pension
Change in plan assets
Fair value of plan assets at beginning of year	$ 1,062	$ 1,104
Fair value of plan assets at end of year	1,160	1,062
United States | Pension | Level 1
Change in plan assets
Fair value of plan assets at beginning of year	5
Fair value of plan assets at end of year	10	5
United States | Pension | Level 2
Change in plan assets
Fair value of plan assets at beginning of year	768
Fair value of plan assets at end of year	813	768
United States | Pension | Level 3
Change in plan assets
Fair value of plan assets at beginning of year	289	276
Unrealized and realized gains/(losses)	38	7
Purchases and settlements, net	10	6
Fair value of plan assets at end of year	337	289
United States | Pension | Cash and Cash Equivalents
Change in plan assets
Fair value of plan assets at beginning of year	5
Fair value of plan assets at end of year	10	5
United States | Pension | Cash and Cash Equivalents | Level 1
Change in plan assets
Fair value of plan assets at beginning of year	5
Fair value of plan assets at end of year	10	5
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, Non-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	0
United States | Pension | Equity Securities, Non-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 | Pension | Equity Securities, Non-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	0
United States | Pension | Equity Securities, Non-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 | 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	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	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	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	0
United States | Pension | Equity Securities, Global
Change in plan assets
Fair value of plan assets at beginning of year	590
Fair value of plan assets at end of year	609	590
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	590
Fair value of plan assets at end of year	609	590
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	75
Fair value of plan assets at end of year	86	75
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	75
Fair value of plan assets at end of year	86	75
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	103
Fair value of plan assets at end of year	118	103
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	103
Fair value of plan assets at end of year	118	103
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 | Infrastructure and real estate
Change in plan assets
Fair value of plan assets at beginning of year	289
Fair value of plan assets at end of year	337	289
United States | Pension | Infrastructure and real estate | 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 | Infrastructure and real estate | 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 | Infrastructure and real estate | Level 3
Change in plan assets
Fair value of plan assets at beginning of year	289
Fair value of plan assets at end of year	337	289
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	188	188
Fair value of plan assets at end of year	201	188
United States | OPEB | Fair Value
Change in plan assets
Fair value of plan assets at beginning of year	188
Fair value of plan assets at end of year	201	188
United States | OPEB | Level 1 | Fair Value
Change in plan assets
Fair value of plan assets at beginning of year	38
Fair value of plan assets at end of year	51	38
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	128	128
United States | OPEB | Level 3
Change in plan assets
Fair value of plan assets at beginning of year	22	18
Unrealized and realized gains/(losses)	2	1
Purchases and settlements, net	(2)	3
Fair value of plan assets at end of year	22	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	22	22
United States | OPEB | Cash and Cash Equivalents | Fair Value
Change in plan assets
Fair value of plan assets at end of year	4
United States | OPEB | Cash and Cash Equivalents | Level 1 | Fair Value
Change in plan assets
Fair value of plan assets at end of year	4
United States | OPEB | Cash and Cash Equivalents | Level 2 | Fair Value
Change in plan assets
Fair value of plan assets at end of year	0
United States | OPEB | Cash and Cash Equivalents | Level 3 | Fair Value
Change in plan assets
Fair value of plan assets at end of year	0
United States | OPEB | Equity Securities, Non-US
Change in plan assets
Fair value of plan assets at beginning of year	79
Fair value of plan assets at end of year	75	79
United States | OPEB | Equity Securities, Non-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, Non-US | Level 2
Change in plan assets
Fair value of plan assets at beginning of year	79
Fair value of plan assets at end of year	75	79
United States | OPEB | Equity Securities, Non-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, US
Change in plan assets
Fair value of plan assets at beginning of year	35
Fair value of plan assets at end of year	39	35
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	35
Fair value of plan assets at end of year	39	35
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 | Government | Fair Value
Change in plan assets
Fair value of plan assets at beginning of year	44
Fair value of plan assets at end of year	53	44
United States | OPEB | Government | Level 1 | Fair Value
Change in plan assets
Fair value of plan assets at beginning of year	38
Fair value of plan assets at end of year	47	38
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	6	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 end of year	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
United States | OPEB | Infrastructure and real estate | 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	22	22
United States | OPEB | Infrastructure and real estate | 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 | Infrastructure and real estate | 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 | Infrastructure and real estate | 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	$ 22	$ 22