Quarterly report pursuant to Section 13 or 15(d)

CAPITAL STOCK - Narrative (Details)

v3.21.1
CAPITAL STOCK - Narrative (Details)
$ / shares in Units, $ in Millions
3 Months Ended
Mar. 11, 2021
USD ($)
Dec. 09, 2020
shares
Mar. 13, 2020
USD ($)
shares
Mar. 31, 2021
USD ($)
$ / shares
shares
Mar. 31, 2020
USD ($)
Feb. 11, 2021
$ / shares
shares
Dec. 31, 2020
shares
Class of Stock [Line Items]              
Common shares, issued (in shares)       506,832,537   20,000,000 506,832,537
Debt Extinguished | $       $ 857 $ 740    
Common shares, par value (in dollars per share) | $ / shares       $ 0.125      
Series B Participating Redeemable Preferred Stock, issued (in shares)       583,273     583,273
Series B Participating Redeemable Preferred Stock, outstanding (in shares)       583,273     583,273
9.875% 2025 Senior Secured Notes              
Class of Stock [Line Items]              
Debt Extinguished | $ $ 322     $ 322      
Arcelor Mittal              
Class of Stock [Line Items]              
Common shares sold (in shares)           40,000,000  
AK Steel              
Class of Stock [Line Items]              
Exchange ratio     0.400        
Fair value of Cliffs common shares issued for AK Steel outstanding common stock | $     $ 618        
Common Stock              
Class of Stock [Line Items]              
Share price (in dollars per share) | $ / shares           $ 16.12  
Common Stock | ArcelorMittal USA              
Class of Stock [Line Items]              
Stock issued during period, shares, acquisitions (in shares)   78,186,671          
Common Stock | AK Steel              
Class of Stock [Line Items]              
Stock issued during period, shares, acquisitions (in shares)     127,000,000        
Exchange ratio       0.400      
Series B Preferred Stock              
Class of Stock [Line Items]              
Preferred stock authorized (in shares)       4,000,000      
Series B Participating Redeemable Preferred Stock, issued (in shares)       583,273      
Series B Participating Redeemable Preferred Stock, outstanding (in shares)       583,273      
Series B Preferred Stock | ArcelorMittal USA              
Class of Stock [Line Items]              
Stock issued during period, shares, acquisitions (in shares)   583,273   583,273      
Series A Preferred Stock              
Class of Stock [Line Items]              
Preferred stock authorized (in shares)       3,000,000      
Preferred stock outstanding (in shares)       3,000,000      
Preferred stock issued (in shares)       3,000,000