Annual report pursuant to Section 13 and 15(d)

STOCK COMPENSATION PLANS - Narrative (Details)

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STOCK COMPENSATION PLANS - Narrative (Details)
12 Months Ended
Mar. 13, 2020
shares
Dec. 31, 2021
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Dec. 31, 2020
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Dec. 31, 2019
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2021 Equity Plan        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Number of additional shares authorized (in shares)   26,000,000    
Number of shares authorized (in shares)   28,000,000    
A&R 2015 Equity Plan        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Number of shares authorized (in shares)   2,500,000    
AK Steel        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Exchange ratio 0.400      
Stock options granted (in shares) 2,000,000 0 2,010,277 0
AK Steel | A&R 2015 Equity Plan        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Number of additional shares authorized (in shares) 5,700,000      
Long-term Performance Plan | AK Steel        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Granted (in shares) 1,000,000      
Performance Shares        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Granted (in shares)   652,888 960,637 572,104
Performance Shares | AK Steel        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Granted (in shares) 500,000 0 1,550,216 0
Restricted Stock | AK Steel        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Granted (in shares) 200,000      
Restricted Stock Units (RSUs)        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Granted (in shares)   678,420 960,637 572,104
Restricted Stock Units (RSUs) | AK Steel        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Granted (in shares) 300,000 0 200,291 0