Annual report pursuant to Section 13 and 15(d)

STOCK COMPENSATION PLANS - Narrative (Details)

v3.20.4
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