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BASIS OF PRESENTATION AND SIGNIFICANT ACCOUNTING POLICIES - Narrative (Details) $ / shares in Units, $ in Millions |  | 12 Months Ended |  |
--- | --- | --- | --- | ---
Nov. 01, 2024 $ / shares shares | Dec. 31, 2024 USD ($) employee segment | Nov. 01, 2024 $ / shares | Dec. 31, 2023 USD ($)
Basis of Presentation and Significant Accounting Policies [Line Items] |  |  |  |
Number of employees | employee |  | 30,000 |  |
Number of operating segments |  | 4 |  |
Number of reportable segments |  | 1 |  |
Stelco Holdings Inc |  |  |  |
Basis of Presentation and Significant Accounting Policies [Line Items] |  |  |  |
Share price (in dollars per share) | (per share) | $ 60.00 |  | $ 13.27 |
Common stock per share | shares | 25,862,021 |  |  |
Business Acquisition, Shares Exchange Ratio | 0.454 |  |  |
Stelco Holdings Inc | Stelco Holdings Inc |  |  |  |
Basis of Presentation and Significant Accounting Policies [Line Items] |  |  |  |
Share price (in dollars per share) | $ / shares | $ 60.00 |  |  |
Other non-current assets |  |  |  |
Basis of Presentation and Significant Accounting Policies [Line Items] |  |  |  |
Investment in affiliates | $ |  | $ 131 |  | $ 123

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