Quarterly report [Sections 13 or 15(d)]

DEBT AND CREDIT FACILITIES - Schedule Of Long-Term Debt (Details)

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DEBT AND CREDIT FACILITIES - Schedule Of Long-Term Debt (Details) - USD ($)
$ in Millions
Sep. 30, 2025
Sep. 08, 2025
Feb. 06, 2025
Dec. 31, 2024
Debt Instrument [Line Items]        
Long-term Debt, Gross $ 8,135     $ 7,148
Unamortized discounts and issuance costs (96)     (83)
Long-term Debt 8,039     7,065
ABL Facility        
Debt Instrument [Line Items]        
Long-term Line of Credit $ 847      
Cleveland-Cliffs Inc. | 7.000% 2027 Senior Notes        
Debt Instrument [Line Items]        
Stated interest rate 7.00%      
Annual Effective Interest Rate 9.24%      
Long-term Debt, Gross $ 73     73
Cleveland-Cliffs Inc. | 5.875% 2027 Senior Notes        
Debt Instrument [Line Items]        
Stated interest rate 5.875%      
Annual Effective Interest Rate 6.49%      
Long-term Debt, Gross $ 556     556
Cleveland-Cliffs Inc. | 4.625% 2029 Senior Notes        
Debt Instrument [Line Items]        
Stated interest rate 4.625%      
Annual Effective Interest Rate 4.625%      
Long-term Debt, Gross $ 368     368
Cleveland-Cliffs Inc. | 6.875% 2029 Senior Notes [Member]        
Debt Instrument [Line Items]        
Stated interest rate 6.875%      
Annual Effective Interest Rate 6.875%      
Long-term Debt, Gross $ 900     900
Cleveland-Cliffs Inc. | 6.750% 2030 Senior Notes        
Debt Instrument [Line Items]        
Stated interest rate 6.75%      
Annual Effective Interest Rate 6.75%      
Long-term Debt, Gross $ 750     750
Cleveland-Cliffs Inc. | 4.875% 2031 Senior Notes        
Debt Instrument [Line Items]        
Stated interest rate 4.875%      
Annual Effective Interest Rate 4.875%      
Long-term Debt, Gross $ 325     325
Cleveland-Cliffs Inc. | 7.500% 2031 Senior Notes [Member]        
Debt Instrument [Line Items]        
Stated interest rate 7.50%   7.50%  
Annual Effective Interest Rate 7.50%      
Long-term Debt, Gross $ 850   $ 850 0
Cleveland-Cliffs Inc. | 7.000% 2032 Senior Notes        
Debt Instrument [Line Items]        
Stated interest rate 7.00%      
Annual Effective Interest Rate 7.054%      
Long-term Debt, Gross $ 1,425     1,425
Cleveland-Cliffs Inc. | 7.375% 2033 Senior Notes [Member]        
Debt Instrument [Line Items]        
Stated interest rate 7.375%      
Annual Effective Interest Rate 7.375%      
Long-term Debt, Gross $ 900     900
Cleveland-Cliffs Inc. | 7.625% 2034 Senior Notes [Member]        
Debt Instrument [Line Items]        
Stated interest rate 7.625% 7.625%    
Annual Effective Interest Rate 7.625%      
Long-term Debt, Gross $ 850 $ 850   0
Cleveland-Cliffs Inc. | 6.250% 2040 Senior Notes        
Debt Instrument [Line Items]        
Stated interest rate 6.25%      
Annual Effective Interest Rate 6.34%      
Long-term Debt, Gross $ 235     235
Cleveland-Cliffs Inc. | ABL Facility        
Debt Instrument [Line Items]        
Annual Effective Interest Rate 5.87%      
Long-term Line of Credit $ 847     1,560
AK Steel | 7.000% 2027 AK Senior Notes        
Debt Instrument [Line Items]        
Stated interest rate 7.00%      
Annual Effective Interest Rate 9.24%      
Long-term Debt, Gross $ 56     $ 56