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STOCK COMPENSATION PLANS - Narrative (Details) |  | 12 Months Ended
--- | --- | ---
Mar. 13, 2020 | Dec. 31, 2020 shares | Dec. 31, 2019 shares | Dec. 31, 2018 shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |
Granted shares |  | 2,010,277 | 0 | 0
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Authorized |  | 4,900,000 |  |
AK Steel [Member] |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award, Number of Additional Shares Authorized |  | 5,700,000 |  |
Business Acquisition, Shares Exchange Ratio | 0.400 |  |  |
Granted shares |  | 2,000,000.0 |  |
Performance Shares [Member] |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |
Granted shares |  | 2,510,853 | 572,104 | 675,599
Performance Shares [Member] | AK Steel [Member] |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |
Granted shares |  | 500,000 |  |
Restricted Stock Units (RSUs) [Member] |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |
Granted shares |  | 1,160,928 | 572,104 | 685,599
Restricted Stock Units (RSUs) [Member] | AK Steel [Member] |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |
Granted shares |  | 300,000 |  |
Long-term Performance Plan | AK Steel [Member] |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |
Granted shares |  | 1,000,000.0 |  |
Restricted Stock [Member] | AK Steel [Member] |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |
Granted shares |  | 200,000 |  |

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