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SHAREHOLDERS' EQUITY - Narrative (Details) - USD ($) $ / shares in Units, $ in Millions | 3 Months Ended | 6 Months Ended |  |  |  |
--- | --- | --- | --- | --- | --- | ---
Mar. 31, 2020 | Jun. 30, 2019 | Mar. 31, 2019 | Jun. 30, 2020 | Jun. 30, 2019 | Mar. 13, 2020 | Dec. 31, 2019 | Apr. 24, 2019 | Nov. 26, 2018
Class of Stock [Line Items] |  |  |  |  |  |  |  |  |
Common shares, par value |  |  |  | $ 0.125 |  |  | $ 0.125 |  |
Fair value of Cliffs common shares issued for AK Steel outstanding common stock |  |  |  |  |  | $ 617.6 |  |  |
Stock Repurchase Program, Authorized Amount |  |  |  |  |  |  |  | $ 100.0 | $ 200.0
Stock Repurchased During Period, Shares |  |  |  |  | 24,400,000 |  |  |  |
Payments for Repurchase of common shares |  |  |  | $ 0.0 | $ 252.9 |  |  |  |
Preferred Class A [Member] |  |  |  |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |  |  |  |
Preferred stock, shares authorized (in shares) |  |  |  | 3,000,000 |  |  |  |  |
Preferred Stock, Shares Issued |  |  |  | 0 |  |  |  |  |
Preferred Stock, Shares Outstanding |  |  |  | 0 |  |  |  |  |
Preferred Class B [Member] |  |  |  |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |  |  |  |
Preferred stock, shares authorized (in shares) |  |  |  | 4,000,000 |  |  |  |  |
Preferred Stock, Shares Issued |  |  |  | 0 |  |  |  |  |
Preferred Stock, Shares Outstanding |  |  |  | 0 |  |  |  |  |
Common Stock [Member] |  |  |  |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |  |  |  |
Stock Issued During Period, Shares, Acquisitions | 126,800,000 |  |  | 126,800,000 |  |  |  |  |
Stock Repurchased During Period, Shares |  | 12,900,000 | 11,500,000 |  |  |  |  |  |

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