Annual report pursuant to Section 13 and 15(d)

SUBSEQUENT EVENTS SUBSEQUENT EVENTS (Details)

v3.20.4
SUBSEQUENT EVENTS SUBSEQUENT EVENTS (Details) - USD ($)
$ / shares in Units, $ in Millions
3 Months Ended 12 Months Ended
Mar. 12, 2021
Mar. 11, 2021
Dec. 09, 2020
Apr. 24, 2020
Mar. 27, 2020
Mar. 13, 2020
Mar. 31, 2021
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Feb. 17, 2021
Feb. 11, 2021
Jun. 19, 2020
Apr. 17, 2020
Mar. 16, 2020
Subsequent Event [Line Items]                              
Common shares, issued (in shares)               506,832,537 301,886,794            
Debt Instrument, Par Value               $ 5,595              
Debt Extinguished       $ 736       1,513 $ 724 $ 227          
Subsequent Event [Member]                              
Subsequent Event [Line Items]                              
Common shares, issued (in shares)                       20,000,000      
Common Stock, Shares, Option to Purchase Additional Shares                       9,000,000      
Arcelor Mittal [Member] | Subsequent Event [Member]                              
Subsequent Event [Line Items]                              
Common Stock, Shares, Sold                       40,000,000      
AK Steel [Member]                              
Subsequent Event [Line Items]                              
Debt Instrument, Par Value               132              
9.875% 2025 Senior Secured Notes | Subsequent Event [Member]                              
Subsequent Event [Line Items]                              
Debt Extinguished   $ 334         $ 334                
9.875% 2025 Senior Secured Notes | Cleveland-Cliffs Inc. [Member]                              
Subsequent Event [Line Items]                              
Debt Instrument, Par Value       $ 555       $ 955           $ 400  
9.875% 2025 Senior Secured Notes | Cleveland-Cliffs Inc. [Member] | Subsequent Event [Member]                              
Subsequent Event [Line Items]                              
Debt Instrument, Redemption Price, Percentage   109.875%                          
Debt Extinguished   $ 322                          
9.875% 2025 Senior Secured Notes | Debt Instrument, Redemption, Period One, Upon Equity Issuance | Cleveland-Cliffs Inc. [Member]                              
Subsequent Event [Line Items]                              
Debt Instrument, Redemption Price, Percentage               109.875%              
5.875% 2027 Senior Notes                              
Subsequent Event [Line Items]                              
Debt Extinguished               $ 195              
5.875% 2027 Senior Notes | Cleveland-Cliffs Inc. [Member]                              
Subsequent Event [Line Items]                              
Debt Instrument, Par Value               $ 556 750            
5.875% 2027 Senior Notes | Debt Instrument, Redemption, Period One, Upon Equity Issuance | Cleveland-Cliffs Inc. [Member]                              
Subsequent Event [Line Items]                              
Debt Instrument, Redemption Price, Percentage               105.875%              
6.75% 2026 Senior Secured Notes | Cleveland-Cliffs Inc. [Member]                              
Subsequent Event [Line Items]                              
Debt Instrument, Par Value           $ 725   $ 845         $ 120    
6.75% 2026 Senior Secured Notes | Debt Instrument, Redemption, Period One, Upon Equity Issuance | Cleveland-Cliffs Inc. [Member]                              
Subsequent Event [Line Items]                              
Debt Instrument, Redemption Price, Percentage               106.75%              
4.625% 2029 Senior Notes | Cleveland-Cliffs Inc. [Member] | Subsequent Event [Member]                              
Subsequent Event [Line Items]                              
Debt Instrument, Par Value                     $ 500        
4.875% 2031 Senior Notes | Cleveland-Cliffs Inc. [Member] | Subsequent Event [Member]                              
Subsequent Event [Line Items]                              
Debt Instrument, Par Value                     $ 500        
4.875% 2024 Senior Secured Notes                              
Subsequent Event [Line Items]                              
Debt Extinguished               $ 6              
4.875% 2024 Senior Secured Notes | Cleveland-Cliffs Inc. [Member]                              
Subsequent Event [Line Items]                              
Debt Instrument, Par Value               395 $ 400            
4.875% 2024 Senior Secured Notes | Cleveland-Cliffs Inc. [Member] | Subsequent Event [Member]                              
Subsequent Event [Line Items]                              
Debt Instrument, Redemption Price, Percentage 102.438%                            
Debt Extinguished $ 395                            
6.375% 2025 Senior Notes                              
Subsequent Event [Line Items]                              
Debt Extinguished               168              
6.375% 2025 Senior Notes | Cleveland-Cliffs Inc. [Member]                              
Subsequent Event [Line Items]                              
Debt Instrument, Par Value               64             $ 232
6.375% 2025 Senior Notes | Cleveland-Cliffs Inc. [Member] | Subsequent Event [Member]                              
Subsequent Event [Line Items]                              
Debt Instrument, Redemption Price, Percentage 103.188%                            
Debt Extinguished $ 64                            
7.625% 2021 AK Senior Notes                              
Subsequent Event [Line Items]                              
Debt Extinguished         $ 9 364   373              
7.625% 2021 AK Senior Notes | AK Steel [Member]                              
Subsequent Event [Line Items]                              
Debt Instrument, Par Value               34              
7.625% 2021 AK Senior Notes | AK Steel [Member] | Subsequent Event [Member]                              
Subsequent Event [Line Items]                              
Debt Instrument, Redemption Price, Percentage 100.00%                            
Debt Extinguished $ 34                            
7.50% 2023 AK Senior Notes [Member]                              
Subsequent Event [Line Items]                              
Debt Extinguished         $ 56 $ 311   367              
7.50% 2023 AK Senior Notes [Member] | AK Steel [Member]                              
Subsequent Event [Line Items]                              
Debt Instrument, Par Value               13              
7.50% 2023 AK Senior Notes [Member] | AK Steel [Member] | Subsequent Event [Member]                              
Subsequent Event [Line Items]                              
Debt Instrument, Redemption Price, Percentage 101.875%                            
Debt Extinguished $ 13                            
6.375% 2025 AK Senior Notes                              
Subsequent Event [Line Items]                              
Debt Extinguished               9              
6.375% 2025 AK Senior Notes | AK Steel [Member]                              
Subsequent Event [Line Items]                              
Debt Instrument, Par Value               $ 29              
6.375% 2025 AK Senior Notes | AK Steel [Member] | Subsequent Event [Member]                              
Subsequent Event [Line Items]                              
Debt Instrument, Redemption Price, Percentage 103.188%                            
Debt Extinguished $ 29                            
Common Stock [Member] | Subsequent Event [Member]                              
Subsequent Event [Line Items]                              
Common stock, shares issued, price per share                       $ 16.12      
ArcelorMittal USA [Member] | Common Stock [Member]                              
Subsequent Event [Line Items]                              
Stock Issued During Period, Shares, Acquisitions     78,186,671         78,000,000