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PENSION AND OTHER POSTRETIREMENT BENEFITS - Plan Assets (Details) - CAD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2023
Dec. 31, 2022
Change in plan assets    
Fair value of plan assets at beginning of year $ 32  
Fair value of plan assets at end of year $ 61 $ 32
Canada | Equity securities    
Pension and Other Postretirement Benefit Disclosures    
Allocation of plan assets (as a percent) 46.00%  
Plan asset allocations 41.40% 38.20%
Canada | Fixed income securities    
Pension and Other Postretirement Benefit Disclosures    
Allocation of plan assets (as a percent) 23.20%  
Plan asset allocations 29.60% 31.70%
Canada | Alternatives    
Pension and Other Postretirement Benefit Disclosures    
Allocation of plan assets (as a percent) 30.80%  
Plan asset allocations 29.00% 30.10%
Canada | Pension    
Change in plan assets    
Fair value of plan assets at beginning of year $ 4,234 $ 4,536
Fair value of plan assets at end of year 4,528 4,234
Canada | Pension | Level 1    
Change in plan assets    
Fair value of plan assets at beginning of year 473  
Fair value of plan assets at end of year 227 473
Canada | Pension | Level 2    
Change in plan assets    
Fair value of plan assets at beginning of year 2,470  
Fair value of plan assets at end of year 3,011 2,470
Canada | Pension | Level 3    
Change in plan assets    
Fair value of plan assets at beginning of year 1,291 1,064
Unrealized and realized gains/(losses) (41) 155
Purchases and settlements, net 40 72
Fair value of plan assets at end of year 1,290 1,291
Canada | Pension | Alternatives    
Change in plan assets    
Fair value of plan assets at beginning of year 1,291  
Fair value of plan assets at end of year 1,290 1,291
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 1,291  
Fair value of plan assets at end of year 1,290 1,291
Canada | Pension | Cash and Cash Equivalents    
Change in plan assets    
Fair value of plan assets at beginning of year 272  
Fair value of plan assets at end of year 227 272
Canada | Pension | Cash and Cash Equivalents | Level 1    
Change in plan assets    
Fair value of plan assets at beginning of year 272  
Fair value of plan assets at end of year 227 272
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 1,263  
Fair value of plan assets at end of year 1,871 1,263
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 1,263  
Fair value of plan assets at end of year 1,871 1,263
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, Global    
Change in plan assets    
Fair value of plan assets at beginning of year 355  
Fair value of plan assets at end of year 3 355
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 355  
Fair value of plan assets at end of year 3 355
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 636  
Fair value of plan assets at end of year 446 636
Canada | Pension | Government | Level 1    
Change in plan assets    
Fair value of plan assets at beginning of year 201  
Fair value of plan assets at end of year 0 201
Canada | Pension | Government | Level 2    
Change in plan assets    
Fair value of plan assets at beginning of year 435  
Fair value of plan assets at end of year 446 435
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 433  
Fair value of plan assets at end of year 667 433
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 433  
Fair value of plan assets at end of year 667 433
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 (16)  
Fair value of plan assets at end of year 24 (16)
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 (16)  
Fair value of plan assets at end of year 24 (16)
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
US | Equity securities    
Pension and Other Postretirement Benefit Disclosures    
Allocation of plan assets (as a percent) 45.00%  
Plan asset allocations 39.50% 38.30%
US | Fixed income securities    
Pension and Other Postretirement Benefit Disclosures    
Allocation of plan assets (as a percent) 20.00%  
Plan asset allocations 19.40% 20.50%
US | Alternatives    
Pension and Other Postretirement Benefit Disclosures    
Allocation of plan assets (as a percent) 35.00%  
Plan asset allocations 41.10% 41.20%
US | Pension    
Change in plan assets    
Fair value of plan assets at beginning of year $ 1,080 $ 1,160
Fair value of plan assets at end of year 1,052 1,080
US | Pension | Level 1    
Change in plan assets    
Fair value of plan assets at beginning of year 13  
Fair value of plan assets at end of year 8 13
US | Pension | Level 2    
Change in plan assets    
Fair value of plan assets at beginning of year 622  
Fair value of plan assets at end of year 611 622
US | Pension | Level 3    
Change in plan assets    
Fair value of plan assets at beginning of year 445 337
Unrealized and realized gains/(losses) (12) 78
Purchases and settlements, net 0 30
Fair value of plan assets at end of year 433 445
US | Pension | Alternatives    
Change in plan assets    
Fair value of plan assets at beginning of year 445  
Fair value of plan assets at end of year 433 445
US | 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
US | 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
US | Pension | Alternatives | Level 3    
Change in plan assets    
Fair value of plan assets at beginning of year 445  
Fair value of plan assets at end of year 433 445
US | Pension | Cash and Cash Equivalents    
Change in plan assets    
Fair value of plan assets at beginning of year 13  
Fair value of plan assets at end of year 8 13
US | Pension | Cash and Cash Equivalents | Level 1    
Change in plan assets    
Fair value of plan assets at beginning of year 13  
Fair value of plan assets at end of year 8 13
US | 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
US | 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
US | Pension | Equity Securities, Global    
Change in plan assets    
Fair value of plan assets at beginning of year 414  
Fair value of plan assets at end of year 416 414
US | 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
US | Pension | Equity Securities, Global | Level 2    
Change in plan assets    
Fair value of plan assets at beginning of year 414  
Fair value of plan assets at end of year 416 414
US | 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
US | 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
US | 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
US | 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
US | 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
US | Pension | Government    
Change in plan assets    
Fair value of plan assets at beginning of year 87  
Fair value of plan assets at end of year 46 87
US | 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
US | Pension | Government | Level 2    
Change in plan assets    
Fair value of plan assets at beginning of year 87  
Fair value of plan assets at end of year 46 87
US | 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
US | Pension | Corporate    
Change in plan assets    
Fair value of plan assets at beginning of year 121  
Fair value of plan assets at end of year 149 121
US | 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
US | Pension | Corporate | Level 2    
Change in plan assets    
Fair value of plan assets at beginning of year 121  
Fair value of plan assets at end of year 149 121
US | 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
US | 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
US | 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
US | 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
US | 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
US | OPEB    
Change in plan assets    
Fair value of plan assets at beginning of year 185 201
Fair value of plan assets at end of year 187 185
US | OPEB | Fair Value    
Change in plan assets    
Fair value of plan assets at beginning of year 185  
Fair value of plan assets at end of year 187 185
US | OPEB | Level 1 | Fair Value    
Change in plan assets    
Fair value of plan assets at beginning of year 48  
Fair value of plan assets at end of year 45 48
US | OPEB | Level 2 | Fair Value    
Change in plan assets    
Fair value of plan assets at beginning of year 109  
Fair value of plan assets at end of year 113 109
US | OPEB | Level 3    
Change in plan assets    
Fair value of plan assets at beginning of year 28 22
Unrealized and realized gains/(losses) 1 4
Purchases and settlements, net 0 2
Fair value of plan assets at end of year 29 28
US | OPEB | Level 3 | Fair Value    
Change in plan assets    
Fair value of plan assets at beginning of year 28  
Fair value of plan assets at end of year 29 28
US | OPEB | Alternatives | Fair Value    
Change in plan assets    
Fair value of plan assets at beginning of year 28  
Fair value of plan assets at end of year 29 28
US | 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
US | 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
US | OPEB | Alternatives | Level 3 | Fair Value    
Change in plan assets    
Fair value of plan assets at beginning of year 28  
Fair value of plan assets at end of year 29 28
US | OPEB | Cash and Cash Equivalents | Fair Value    
Change in plan assets    
Fair value of plan assets at beginning of year 2  
Fair value of plan assets at end of year 3 2
US | OPEB | Cash and Cash Equivalents | Level 1 | Fair Value    
Change in plan assets    
Fair value of plan assets at beginning of year 2  
Fair value of plan assets at end of year 3 2
US | 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
US | 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
US | OPEB | Equity Securities, US    
Change in plan assets    
Fair value of plan assets at beginning of year 34  
Fair value of plan assets at end of year 36 34
US | 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
US | OPEB | Equity Securities, US | Level 2    
Change in plan assets    
Fair value of plan assets at beginning of year 34  
Fair value of plan assets at end of year 36 34
US | 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
US | OPEB | Equity Securities, Global    
Change in plan assets    
Fair value of plan assets at beginning of year 62  
Fair value of plan assets at end of year 62 62
US | 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
US | OPEB | Equity Securities, Global | Level 2    
Change in plan assets    
Fair value of plan assets at beginning of year 62  
Fair value of plan assets at end of year 62 62
US | 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
US | OPEB | Government | 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 45 51
US | OPEB | Government | Level 1 | Fair Value    
Change in plan assets    
Fair value of plan assets at beginning of year 46  
Fair value of plan assets at end of year 42 46
US | OPEB | Government | Level 2 | Fair Value    
Change in plan assets    
Fair value of plan assets at beginning of year 5  
Fair value of plan assets at end of year 3 5
US | 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
US | 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 12 8
US | 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
US | 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 12 8
US | 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