Quarterly report pursuant to Section 13 or 15(d)

CAPITAL STOCK - Narrative (Details)

v3.21.2
CAPITAL STOCK - Narrative (Details)
$ / shares in Units, $ in Millions
3 Months Ended 9 Months Ended
Jul. 27, 2021
USD ($)
shares
Mar. 11, 2021
USD ($)
Dec. 09, 2020
shares
Apr. 24, 2020
USD ($)
Mar. 13, 2020
USD ($)
shares
Sep. 30, 2021
USD ($)
$ / shares
shares
Sep. 30, 2021
USD ($)
$ / shares
shares
Sep. 30, 2020
USD ($)
Jun. 30, 2021
shares
Apr. 29, 2021
shares
Mar. 31, 2021
shares
Feb. 11, 2021
$ / shares
shares
Dec. 31, 2020
shares
Class of Stock [Line Items]                          
Common shares, issued (in shares)           506,832,537 506,832,537         20,000,000 506,832,537
Debt Extinguished | $       $ 736   $ 2 $ 1,280 $ 1,488          
Common shares, par value (in dollars per share) | $ / shares           $ 0.125 $ 0.125            
Common shares, authorized (in shares)           1,200,000,000 1,200,000,000     1,200,000,000 600,000,000   600,000,000
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       $ 0 $ 347            
Arcelor Mittal                          
Class of Stock [Line Items]                          
Common shares sold (in shares)                       40,000,000  
Cleveland-Cliffs Inc. | 9.875% 2025 Senior Secured Notes                          
Class of Stock [Line Items]                          
Stated interest rate       9.875%   9.875% 9.875%           9.875%
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 4,000,000            
Preferred stock outstanding (in shares)           0 0            
Preferred stock issued (in shares)           0 0            
Stock redeemed (in shares) 583,273                        
Preferred stock redemption value | $ $ 1,300                        
Series B Preferred Stock | ArcelorMittal USA                          
Class of Stock [Line Items]                          
Stock issued during period, shares, acquisitions (in shares)     583,273                    
Series A Preferred Stock                          
Class of Stock [Line Items]                          
Preferred stock authorized (in shares)           3,000,000 3,000,000            
Preferred stock outstanding (in shares)           0 0            
Preferred stock issued (in shares)           0 0