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ACQUISITIONS - AK Steel Consideration (Details) $ / shares in Units, $ in Millions | Mar. 13, 2020 USD ($) $ / shares shares | Sep. 30, 2021 shares | Jun. 30, 2021 shares | Mar. 31, 2021 shares | Dec. 31, 2020 shares | Sep. 30, 2020 shares | Jun. 30, 2020 shares | Mar. 31, 2020 shares | Dec. 31, 2019 shares
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Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |
Common shares, outstanding (in shares) | shares |  | 500,055,941 |  |  | 477,517,372 |  |  |  |
AK Steel |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |
Fair value of debt included in consideration | $ 914 |  |  |  |  |  |  |  |
Fair value of Cliffs common shares issued for AK Steel outstanding common stock | 618 |  |  |  |  |  |  |  |
Other | 3 |  |  |  |  |  |  |  |
Consideration transferred | $ 1,535 |  |  |  |  |  |  |  |
Common shares, outstanding (in shares) | shares | 317,000,000 |  |  |  |  |  |  |  |
Exchange ratio | 0.400 |  |  |  |  |  |  |  |
Shares of Cliffs common shares issued to AK Steel stockholders (in shares) | shares | 127,000,000 |  |  |  |  |  |  |  |
Price per share of Cliffs common shares (in dollars per share) | $ / shares | $ 4.87 |  |  |  |  |  |  |  |
ABL Facility | AK Steel |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |
Fair value of debt included in consideration | $ 590 |  |  |  |  |  |  |  |
7.500% Senior Secured Notes due July 2023 | AK Steel |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |
Fair value of debt included in consideration | 324 |  |  |  |  |  |  |  |
Common Stock |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |
Common shares, outstanding (in shares) | shares |  | 500,000,000 | 500,000,000 | 499,000,000 | 478,000,000 | 399,000,000 | 399,000,000 | 399,000,000 | 271,000,000
Common Stock | AK Steel |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |
Fair value of Cliffs common shares issued for AK Steel outstanding common stock | $ 618 |  |  |  |  |  |  |  |
Common Stock | AK Steel |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |
Exchange ratio | 0.400 |  |  |  |  |  |  |  |

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