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ACQUISITION OF AK STEEL - Transaction Consideration (Details) $ / shares in Units, $ in Millions | Mar. 13, 2020 USD ($) $ / shares shares | Jun. 30, 2020 shares | Mar. 31, 2020 shares | Dec. 31, 2019 shares | Jun. 30, 2019 shares | Mar. 31, 2019 shares | Dec. 31, 2018 shares
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Business Acquisition [Line Items] |  |  |  |  |  |  |
Fair value of Cliffs common shares issued for AK Steel outstanding common stock | $ 617.6 |  |  |  |  |  |
Number of shares of AK Steel common stock issued and outstanding | shares |  | 399,159,988 |  | 270,084,005 |  |  |
AK Steel Holding Corporation [Member] |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |
Fair value of Cliffs common shares issued for AK Steel outstanding common stock | 617.6 |  |  |  |  |  |
Fair value of debt included in consideration | 913.6 |  |  |  |  |  |
Business Combination, Consideration Transferred | $ 1,535.1 |  |  |  |  |  |
Number of shares of AK Steel common stock issued and outstanding | shares | 316,900,000 |  |  |  |  |  |
Business Acquisition, Shares Exchange Ratio | 0.400 |  |  |  |  |  |
Shares of Cliffs common shares issued to AK Steel stockholders | shares | 126,800,000 |  |  |  |  |  |
Price per share of Cliffs common shares | $ / shares | $ 4.87 |  |  |  |  |  |
Credit Facility | AK Steel Holding Corporation [Member] |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |
Fair value of debt included in consideration | $ 590.0 |  |  |  |  |  |
7.50% 2023 AK Senior Notes [Member] | AK Steel Holding Corporation [Member] |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |
Fair value of debt included in consideration | 323.6 |  |  |  |  |  |
Share-based Payment Arrangement [Member] | AK Steel Holding Corporation [Member] |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |
Fair value of Cliffs common shares issued for AK Steel outstanding common stock | 3.9 |  |  |  |  |  |
Common Stock [Member] |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |
Number of shares of AK Steel common stock issued and outstanding | shares |  | 399,200,000 | 398,600,000 | 270,100,000 | 270,000,000.0 | 282,800,000 | 292,600,000
Common Stock [Member] | AK Steel Holding Corporation [Member] |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |
Fair value of Cliffs common shares issued for AK Steel outstanding common stock | $ 617.6 |  |  |  |  |  |

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