Annual report pursuant to Section 13 and 15(d)

ACQUISITIONS - Consideration (Details)

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ACQUISITIONS - Consideration (Details)
$ / shares in Units, $ in Millions
12 Months Ended
Nov. 18, 2021
USD ($)
Dec. 09, 2020
USD ($)
$ / shares
shares
Mar. 13, 2020
USD ($)
$ / shares
shares
Dec. 31, 2021
shares
Dec. 31, 2020
shares
Business Acquisition [Line Items]          
Cash consideration   $ 639      
Common shares, outstanding (in shares) | shares       500,158,955 477,517,372
Series B Preferred Stock          
Business Acquisition [Line Items]          
Fair value of Cliffs issued stock   738      
Ferrous Processing and Trading          
Business Acquisition [Line Items]          
Fair value of settlement of a pre-existing relationship $ (20)        
Cash consideration 775        
Total purchase consideration $ 755        
ArcelorMittal USA          
Business Acquisition [Line Items]          
Fair value of settlement of a pre-existing relationship   (237)      
Cash consideration   639      
Total purchase consideration   2,604      
Cash consideration pursuant to the AM USA Transaction Agreement   505      
Cash consideration for purchase of the remaining JV partner's interest of Kote and Tek   182      
Total cash consideration receivable   (48)      
Accounts receivable   97      
Freestanding derivative asset from customer supply agreement   140      
ArcelorMittal USA | Common Stock          
Business Acquisition [Line Items]          
Fair value of Cliffs issued stock   $ 990      
Number of shares issued (in shares) | shares   78,186,671      
Share price (in dollars per share) | $ / shares   $ 12.66      
ArcelorMittal USA | Series B Preferred Stock          
Business Acquisition [Line Items]          
Number of shares issued (in shares) | shares   583,273   583,273  
Share price (in dollars per share) | $ / shares   $ 1,266      
AK Steel          
Business Acquisition [Line Items]          
Total purchase consideration     $ 1,535    
Share price (in dollars per share) | $ / shares     $ 4.87    
Fair value of AK Steel debt     $ 914    
Common shares, outstanding (in shares) | shares     317,000,000    
Exchange ratio     0.400    
Shares of Cliffs common shares issued to AK Steel stockholders (in shares) | shares     127,000,000    
AK Steel | Common Stock          
Business Acquisition [Line Items]          
Number of shares issued (in shares) | shares       127,000,000  
Other consideration transferred     $ 3    
Exchange ratio       0.400  
Fair value of Cliffs common shares issued for outstanding AK Steel common stock     618    
AK Steel | ABL Facility          
Business Acquisition [Line Items]          
Fair value of AK Steel debt     590    
AK Steel | 7.500% 2023 AK Senior Notes          
Business Acquisition [Line Items]          
Fair value of AK Steel debt     $ 324