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SUBSEQUENT EVENTS (Details) | Nov. 01, 2024 USD ($) shares | Jul. 12, 2024 USD ($) | Nov. 01, 2024 $ / shares | Oct. 22, 2024 USD ($) | Sep. 30, 2024 USD ($) | Sep. 13, 2024 USD ($) | Dec. 31, 2023 USD ($)
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Subsequent Events [Abstract] |  |  |  |  |  |  |
Long-term Debt, Gross |  |  |  |  | $ 3,835,000,000 |  | $ 3,192,000,000
Subsequent Event [Line Items] |  |  |  |  |  |  |
Long-term Debt, Gross |  |  |  |  | 3,835,000,000 |  | $ 3,192,000,000
Stelco Holdings Inc |  |  |  |  |  |  |
Subsequent Event [Line Items] |  |  |  |  |  |  |
Expected enterprise value of acquisition |  | $ 2,500,000,000 |  |  |  |  |
Subsequent Event | Stelco Holdings Inc |  |  |  |  |  |  |
Subsequent Event [Line Items] |  |  |  |  |  |  |
Shares of Cliffs common shares issued to Stelco Holdings stockholders (in shares) | shares | 0.454 |  |  |  |  |  |
Expected enterprise value of acquisition | $ 3,200,000,000 |  |  |  |  |  |
Subsequent Event | Stelco Holdings Inc | Stelco Holdings Inc |  |  |  |  |  |  |
Subsequent Event [Line Items] |  |  |  |  |  |  |
Price per share (in dollars per share) | $ / shares |  |  | $ 60.00 |  |  |  |
7.375% 2033 Senior Guaranteed Note | Subsequent Event |  |  |  |  |  |  |
Subsequent Events [Abstract] |  |  |  |  |  |  |
Long-term Debt, Gross |  |  |  | $ 900,000,000 |  |  |
Subsequent Event [Line Items] |  |  |  |  |  |  |
Long-term Debt, Gross |  |  |  | $ 900,000,000 |  |  |
Stated interest rate |  |  |  | 7.375% |  |  |
6.875% 2029 Senior Guaranteed Note | Subsequent Event |  |  |  |  |  |  |
Subsequent Events [Abstract] |  |  |  |  |  |  |
Long-term Debt, Gross |  |  |  | $ 900,000,000 |  |  |
Subsequent Event [Line Items] |  |  |  |  |  |  |
Long-term Debt, Gross |  |  |  | $ 900,000,000 |  |  |
Stated interest rate |  |  |  | 6.875% |  |  |
Bridge Loan | Line of Credit |  |  |  |  |  |  |
Subsequent Event [Line Items] |  |  |  |  |  |  |
Maximum borrowing capacity |  |  |  |  | $ 3,300,000,000 | $ 2,400,000,000 |
Bridge Loan | Subsequent Event | Line of Credit |  |  |  |  |  |  |
Subsequent Event [Line Items] |  |  |  |  |  |  |
Maximum borrowing capacity |  |  |  | $ 1,800,000,000 |  |  |

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