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ACQUISITIONS - Consideration (Details) $ / shares in Units, $ in Millions |  |  | 12 Months Ended |
--- | --- | --- | --- | ---
Dec. 09, 2020 USD ($) $ / shares shares | Mar. 13, 2020 USD ($) $ / shares shares | Dec. 31, 2020 shares | Dec. 31, 2019 shares
Business Acquisition [Line Items] |  |  |  |
Cash consideration (subject to customary working capital adjustments) | $ 631 |  |  |
Common shares, outstanding (in shares) | shares |  |  | 477,517,372 | 270,084,005
Series B Preferred Stock |  |  |  |
Business Acquisition [Line Items] |  |  |  |
Fair value of Cliffs issued stock | 738 |  |  |
ArcelorMittal USA [Member] |  |  |  |
Business Acquisition [Line Items] |  |  |  |
Fair value of settlement of a pre-existing relationship | 237 |  |  |
Cash consideration (subject to customary working capital adjustments) | 631 |  |  |
Total purchase consideration | 2,596 |  |  |
Payments to Acquire Businesses, Pursuant to Transaction Agreement | 505 |  |  |
Payments to Acquire Businesses, Purchase of JV Interest | 182 |  |  |
Payments to Acquire Businesses, Cash Consideration Receivable | (56) |  |  |
Business Combination, Separately Recognized Transactions, Accounts Receivable | 97 |  |  |
Business Combination, Separately Recognized Transactions, Assets Recognized | 140 |  |  |
ArcelorMittal USA [Member] | Common Stock [Member] |  |  |  |
Business Acquisition [Line Items] |  |  |  |
Fair value of Cliffs issued stock | $ 990 |  |  |
Number of shares issued (in shares) | shares | 78,186,671 |  | 78,000,000 |
Share price (in dollars per share) | $ / shares | $ 12.66 |  |  |
ArcelorMittal USA [Member] | Series B Preferred Stock |  |  |  |
Business Acquisition [Line Items] |  |  |  |
Number of shares issued (in shares) | shares | 583,273 |  | 583,273 |
Share price (in dollars per share) | $ / shares | $ 1,266 |  |  |
AK Steel [Member] |  |  |  |
Business Acquisition [Line Items] |  |  |  |
Total purchase consideration |  | $ 1,535 |  |
Share price (in dollars per share) | $ / shares |  | $ 4.87 |  |
Business Combination, Consideration Transferred, Liabilities Incurred |  | $ 914 |  |
Common shares, outstanding (in shares) | shares |  | 317,000,000 |  |
Business Acquisition, Shares Exchange Ratio |  | 0.400 |  |
Business Acquisition, Equity Interest Issued or Issuable, Number of Shares | shares |  | 127,000,000 |  |
AK Steel [Member] | Common Stock [Member] |  |  |  |
Business Acquisition [Line Items] |  |  |  |
Number of shares issued (in shares) | shares |  |  | 127,000,000 |
Business Acquisition, Shares Exchange Ratio |  |  | 0.400 |
Business Acquisition, Equity Interest Issued or Issuable, Value Assigned |  | $ 618 |  |
Business Combination, Consideration Transferred, Other |  | 3 |  |
AK Steel [Member] | ABL Facility [Member] |  |  |  |
Business Acquisition [Line Items] |  |  |  |
Business Combination, Consideration Transferred, Liabilities Incurred |  | 590 |  |
AK Steel [Member] | 7.50% 2023 AK Senior Notes [Member] |  |  |  |
Business Acquisition [Line Items] |  |  |  |
Business Combination, Consideration Transferred, Liabilities Incurred |  | $ 324 |  |

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