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CAPITAL STOCK - Narrative (Details) $ / shares in Units, $ in Millions |  |  |  |  |  | 3 Months Ended | 9 Months Ended |  |  |  |  |
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Jul. 27, 2021 USD ($) shares | Mar. 11, 2021 USD ($) | Dec. 09, 2020 shares | Apr. 24, 2020 USD ($) | Mar. 13, 2020 USD ($) shares | Sep. 30, 2021 USD ($) $ / shares shares | Sep. 30, 2021 USD ($) $ / shares shares | Sep. 30, 2020 USD ($) | Jun. 30, 2021 shares | Apr. 29, 2021 shares | Mar. 31, 2021 shares | Feb. 11, 2021 $ / shares shares | Dec. 31, 2020 shares
Class of Stock [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Common shares, issued (in shares) |  |  |  |  |  | 506,832,537 | 506,832,537 |  |  |  |  | 20,000,000 | 506,832,537
Debt Extinguished | $ |  |  |  | $ 736 |  | $ 2 | $ 1,280 | $ 1,488 |  |  |  |  |
Common shares, par value (in dollars per share) | $ / shares |  |  |  |  |  | $ 0.125 | $ 0.125 |  |  |  |  |  |
Common shares, authorized (in shares) |  |  |  |  |  | 1,200,000,000 | 1,200,000,000 |  |  | 1,200,000,000 | 600,000,000 |  | 600,000,000
Series B Participating Redeemable Preferred Stock, issued (in shares) |  |  |  |  |  |  |  |  | 583,273 |  |  |  | 583,273
Series B Participating Redeemable Preferred Stock, outstanding (in shares) |  |  |  |  |  |  |  |  | 583,273 |  |  |  | 583,273
9.875% 2025 Senior Secured Notes |  |  |  |  |  |  |  |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Extinguished | $ |  | $ 322 |  |  |  | $ 0 | $ 347 |  |  |  |  |  |
Arcelor Mittal |  |  |  |  |  |  |  |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Common shares sold (in shares) |  |  |  |  |  |  |  |  |  |  |  | 40,000,000 |
Cleveland-Cliffs Inc. | 9.875% 2025 Senior Secured Notes |  |  |  |  |  |  |  |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Stated interest rate |  |  |  | 9.875% |  | 9.875% | 9.875% |  |  |  |  |  | 9.875%
AK Steel |  |  |  |  |  |  |  |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Exchange ratio |  |  |  |  | 0.400 |  |  |  |  |  |  |  |
Fair value of Cliffs common shares issued for AK Steel outstanding common stock | $ |  |  |  |  | $ 618 |  |  |  |  |  |  |  |
Common Stock |  |  |  |  |  |  |  |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Share price (in dollars per share) | $ / shares |  |  |  |  |  |  |  |  |  |  |  | $ 16.12 |
Common Stock | ArcelorMittal USA |  |  |  |  |  |  |  |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Stock issued during period, shares, acquisitions (in shares) |  |  | 78,186,671 |  |  |  |  |  |  |  |  |  |
Common Stock | AK Steel |  |  |  |  |  |  |  |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Stock issued during period, shares, acquisitions (in shares) |  |  |  |  | 127,000,000 |  |  |  |  |  |  |  |
Exchange ratio |  |  |  |  | 0.400 |  |  |  |  |  |  |  |
Series B Preferred Stock |  |  |  |  |  |  |  |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Preferred stock authorized (in shares) |  |  |  |  |  | 4,000,000 | 4,000,000 |  |  |  |  |  |
Preferred stock outstanding (in shares) |  |  |  |  |  | 0 | 0 |  |  |  |  |  |
Preferred stock issued (in shares) |  |  |  |  |  | 0 | 0 |  |  |  |  |  |
Stock redeemed (in shares) | 583,273 |  |  |  |  |  |  |  |  |  |  |  |
Preferred stock redemption value | $ | $ 1,300 |  |  |  |  |  |  |  |  |  |  |  |
Series B Preferred Stock | ArcelorMittal USA |  |  |  |  |  |  |  |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Stock issued during period, shares, acquisitions (in shares) |  |  | 583,273 |  |  |  |  |  |  |  |  |  |
Series A Preferred Stock |  |  |  |  |  |  |  |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Preferred stock authorized (in shares) |  |  |  |  |  | 3,000,000 | 3,000,000 |  |  |  |  |  |
Preferred stock outstanding (in shares) |  |  |  |  |  | 0 | 0 |  |  |  |  |  |
Preferred stock issued (in shares) |  |  |  |  |  | 0 | 0 |  |  |  |  |  |

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