Quarterly report pursuant to Section 13 or 15(d)

DEBT AND CREDIT FACILITIES (Narrative) (Details)

v3.20.1
DEBT AND CREDIT FACILITIES (Narrative) (Details) - USD ($)
$ in Millions
3 Months Ended
Apr. 24, 2020
Mar. 27, 2020
Mar. 13, 2020
Mar. 31, 2020
Mar. 31, 2019
Apr. 17, 2020
Dec. 31, 2019
Debt Instrument [Line Items]              
Long-term Debt, Gross       $ 4,570.5      
Extinguishment of Debt, Amount       739.9 $ 10.0    
6.75% 2026 Senior Notes [Member]              
Debt Instrument [Line Items]              
Long-term Debt, Gross       $ 725.0      
Debt issuance, discount rate       98.783%      
Debt Instrument, Interest Rate, Stated Percentage       6.75%      
Repurchase price if triggering event occurs       101.00%      
Debt Issuance Costs, Gross       $ 20.5      
6.375% 2025 Senior Notes [Member]              
Debt Instrument [Line Items]              
Long-term Debt, Gross       $ 231.8      
Debt Instrument, Interest Rate, Stated Percentage       6.375%      
Repurchase price if triggering event occurs       101.00%      
Debt Issuance Costs, Gross       $ 0.9      
7.00% 2027 Senior Notes [Member]              
Debt Instrument [Line Items]              
Long-term Debt, Gross       $ 335.4      
Debt Instrument, Interest Rate, Stated Percentage       7.00%      
Debt Issuance Costs, Gross       $ 1.3      
7.625% 2021 AK Senior Notes [Member]              
Debt Instrument [Line Items]              
Debt Instrument, Redemption Price, Percentage       100.00%      
Extinguishment of Debt, Amount   $ 8.5 $ 364.2 $ 372.7      
7.50% 2023 AK Senior Notes [Member]              
Debt Instrument [Line Items]              
Extinguishment of Debt, Amount   $ 56.5 $ 310.7 367.2      
Credit Facility              
Debt Instrument [Line Items]              
Long-term Debt, Gross       2,000.0      
Sublimit for letters of credit       555.0      
Sublimit for swingline loans       125.0      
FILO Tranche Commitments       $ 150.0      
Fixed Charge Coverage Ratio       1.0      
Cleveland-Cliffs Inc. [Member] | 6.375% 2025 Senior Notes [Member]              
Debt Instrument [Line Items]              
Long-term Debt, Gross       $ 231.8      
Cleveland-Cliffs Inc. [Member] | 7.00% 2027 Senior Notes [Member]              
Debt Instrument [Line Items]              
Long-term Debt, Gross       $ 335.4      
Repurchase price if triggering event occurs       101.00%      
Cleveland-Cliffs Inc. [Member] | Credit Facility              
Debt Instrument [Line Items]              
Long-term Debt, Gross       $ 2,000.0     $ 450.0
AK Steel Holding Corporation [Member]              
Debt Instrument [Line Items]              
Unsecured Long-term Debt, Noncurrent       141.0      
AK Steel Holding Corporation [Member] | Unsecured Industrial Revenue Bonds [Member]              
Debt Instrument [Line Items]              
Long-term Debt, Gross       73.3      
AK Steel Holding Corporation [Member] | Industrial Revenue Bonds [Member]              
Debt Instrument [Line Items]              
Long-term Debt, Gross       $ 99.3      
Debt, Weighted Average Interest Rate       6.80%      
AK Steel Holding Corporation [Member] | Secured Industrial Revenue Bonds [Member]              
Debt Instrument [Line Items]              
Long-term Debt, Gross       $ 26.0      
AK Steel Holding Corporation [Member] | 7.625% 2021 AK Senior Notes [Member]              
Debt Instrument [Line Items]              
Long-term Debt, Gross       33.5      
AK Steel Holding Corporation [Member] | 7.50% 2023 AK Senior Notes [Member]              
Debt Instrument [Line Items]              
Long-term Debt, Gross       $ 12.8      
Subsequent Event [Member]              
Debt Instrument [Line Items]              
Extinguishment of Debt, Amount $ 736.4            
Reduction of Principal Long-Term Debt 181.3            
Subsequent Event [Member] | 9.875% 2025 Senior Secured Notes [Member]              
Debt Instrument [Line Items]              
Long-term Debt, Gross $ 555.2         $ 400.0  
Debt issuance, discount rate 99.00%         94.50%  
Debt Instrument, Interest Rate, Stated Percentage           9.875%