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STOCK COMPENSATION PLANS - Nonemployee Directors (Details) - USD ($) | 12 Months Ended
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Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |
Share-based Payment Arrangement, Nonvested Award, Cost Not yet Recognized, Amount | $ 14,000,000 |  |
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 |  |
Share-based Payment Arrangement, Nonemployee [Member] | Director [Member] | Directors Plan [Member] | Restricted Stock Units (RSUs) [Member] |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |
Granted shares | 253,809 | 86,477 | 92,718
Share-based Payment Arrangement, Nonvested Award, Cost Not yet Recognized, Amount | $ 100,000 |  |
Share-based Payment Arrangement, Nonemployee [Member] | Director [Member] | Deferred Compensation, Share-based Payments [Member] | Directors Plan [Member] |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |
Granted shares | 54,794 | 23,659 | 17,170

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