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CAPITAL STOCK - Narrative (Details) $ / shares in Units, $ in Millions |  | 3 Months Ended | 9 Months Ended |  |  |
--- | --- | --- | --- | --- | --- | ---
Nov. 01, 2024 USD ($) shares | Jun. 30, 2024 shares | Mar. 31, 2024 shares | Sep. 30, 2025 USD ($) shares | Sep. 30, 2024 USD ($) shares | Nov. 01, 2024 $ / shares | Nov. 01, 2024 $ / shares | Apr. 22, 2024 USD ($) | Feb. 10, 2022 USD ($)
Class of Stock [Line Items] |  |  |  |  |  |  |  |  |
Authorized amount of stock repurchase | $ |  |  |  |  |  |  |  | $ 1,500 | $ 1,000
Repurchase of common shares | $ |  |  |  | $ 0 | $ 733 |  |  |  |
Remaining authorized amount of stock repurchase | $ |  |  |  | $ 1,400 |  |  |  |  |
Stelco Holdings Inc |  |  |  |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |  |  |  |
Price per share (in dollars per share) | (per share) |  |  |  |  |  | $ 60.00 | $ 13.27 |  |
Business Acquisition, Shares Exchange Ratio | 0.454 |  |  |  |  |  |  |  |
Stock Issued During Period, Shares, Acquisitions | 25,900,000 |  |  |  |  |  |  |  |
Stock Issued During Period, Value, Acquisitions | $ | $ 343 |  |  |  |  |  |  |  |
Common Stock |  |  |  |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |  |  |  |
Common stock repurchases (shares) |  | 7,500,000 | 30,400,000 |  | 37,900,000 |  |  |  |
Common Shares in Treasury |  |  |  |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |  |  |  |
Repurchase of common shares | $ |  |  |  |  | $ 733 |  |  |  |
Series A Preferred Stock |  |  |  |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |  |  |  |
Preferred stock authorized (in shares) |  |  |  | 3,000,000 |  |  |  |  |
Preferred stock outstanding (in shares) |  |  |  | 0 |  |  |  |  |
Preferred stock issued (in shares) |  |  |  | 0 |  |  |  |  |
Series B Preferred Stock |  |  |  |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |  |  |  |
Preferred stock authorized (in shares) |  |  |  | 4,000,000 |  |  |  |  |
Preferred stock outstanding (in shares) |  |  |  | 0 |  |  |  |  |
Preferred stock issued (in shares) |  |  |  | 0 |  |  |  |  |

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