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CAPITAL STOCK - Narrative (Details) $ / shares in Units, $ in Millions |  |  | 12 Months Ended |  |
--- | --- | --- | --- | --- | ---
Dec. 09, 2020 shares | Mar. 13, 2020 USD ($) | Dec. 31, 2020 USD ($) shareMultiple $ / shares shares | Dec. 31, 2019 USD ($) shares | Dec. 31, 2018 USD ($) | Apr. 24, 2019 USD ($) | Nov. 26, 2018 USD ($)
Class of Stock [Line Items] |  |  |  |  |  |  |
Common shares, par value | $ / shares |  |  | $ 0.125 |  |  |  |
Temporary Equity, Shares Issued |  |  | 583,273 |  |  |  |
Temporary Equity, Shares Outstanding |  |  | 583,273 |  |  |  |
Stock Repurchase Program, Authorized Amount | $ |  |  |  |  |  | $ 100 | $ 200
Repurchase of common shares | $ |  |  | $ 0 | $ 253 | $ 48 |  |
Preferred Stock, Share Multiple | shareMultiple |  |  | 100 |  |  |  |
AK Steel [Member] |  |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |  |
Business Acquisition, Shares Exchange Ratio |  | 0.400 |  |  |  |  |
Common Stock [Member] |  |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |  |
Common shares, par value | $ / shares |  |  | $ 0.125 |  |  |  |
Stock Repurchased During Period, Shares |  |  |  | 24,000,000 |  |  |
Common Stock [Member] | AK Steel [Member] |  |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |  |
Business Acquisition, Shares Exchange Ratio |  |  | 0.400 |  |  |  |
Stock Issued During Period, Shares, Acquisitions |  |  | 127,000,000 |  |  |  |
Business Acquisition, Equity Interest Issued or Issuable, Value Assigned | $ |  | $ 618 |  |  |  |  |
Common Stock [Member] | ArcelorMittal USA [Member] |  |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |  |
Stock Issued During Period, Shares, Acquisitions | 78,186,671 |  | 78,000,000 |  |  |  |
Series A Preferred Stock |  |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |  |
Preferred Stock, Shares Authorized |  |  | 3,000,000 |  |  |  |
Preferred Stock, Shares Issued |  |  | 0 |  |  |  |
Preferred Stock, Shares Outstanding |  |  | 0 |  |  |  |
Series B Preferred Stock |  |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |  |
Preferred Stock, Shares Authorized |  |  | 4,000,000 |  |  |  |
Temporary Equity, Shares Issued |  |  | 583,273 |  |  |  |
Temporary Equity, Shares Outstanding |  |  | 583,273 |  |  |  |
Series B Preferred Stock | ArcelorMittal USA [Member] |  |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |  |
Stock Issued During Period, Shares, Acquisitions | 583,273 |  | 583,273 |  |  |  |
Series B Redeemable Preferred Stock |  |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |  |
Preferred Stock, Dividend Rate, Percentage, Daily Compounding Rate |  |  | 10.00% |  |  |  |
Preferred Stock, Dividend Rate, Percentage |  |  | 2.00% |  |  |  |

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