Quarterly report pursuant to Section 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
Mar. 31, 2020
Dec. 31, 2019
Debt Instrument [Line Items]    
Long-term Debt, Gross $ 4,570.5  
LONG-TERM DEBT 4,357.1 $ 2,113.8
6.375% 2025 Senior Notes [Member]    
Debt Instrument [Line Items]    
Long-term Debt, Gross 231.8  
7.00% 2027 Senior Notes [Member]    
Debt Instrument [Line Items]    
Long-term Debt, Gross 335.4  
Credit Facility    
Debt Instrument [Line Items]    
Long-term Debt, Gross 2,000.0  
Long-term Line of Credit $ 800.0  
Cleveland-Cliffs Inc. [Member] | 4.875% 2024 Senior Secured Notes [Member]    
Debt Instrument [Line Items]    
Debt Instrument, Interest Rate, Effective Percentage 5.00% 5.00%
Long-term Debt, Gross $ 400.0 $ 400.0
Unamortized Debt Issuance Expense (4.3) (4.6)
Debt Instrument, Unamortized Discount (1.7) (1.8)
Long-term Debt $ 394.0 $ 393.6
Cleveland-Cliffs Inc. [Member] | 6.75% 2026 Senior Secured Notes [Member]    
Debt Instrument [Line Items]    
Debt Instrument, Interest Rate, Effective Percentage 7.00%  
Long-term Debt, Gross $ 725.0  
Unamortized Debt Issuance Expense (20.3)  
Debt Instrument, Unamortized Discount (8.8)  
Long-term Debt $ 695.9  
Cleveland-Cliffs Inc. [Member] | 6.375% 2025 Senior Notes [Member]    
Debt Instrument [Line Items]    
Debt Instrument, Interest Rate, Effective Percentage 8.11%  
Long-term Debt, Gross $ 231.8  
Unamortized Debt Issuance Expense (0.9)  
Debt Instrument, Unamortized Discount (17.9)  
Long-term Debt $ 213.0  
Cleveland-Cliffs Inc. [Member] | 1.50% 2025 Convertible Senior Notes [Member]    
Debt Instrument [Line Items]    
Debt Instrument, Interest Rate, Effective Percentage 6.26% 6.26%
Long-term Debt, Gross $ 316.3 $ 316.3
Unamortized Debt Issuance Expense (4.4) (4.6)
Debt Instrument, Unamortized Discount (62.2) (65.0)
Long-term Debt $ 249.7 $ 246.7
Cleveland-Cliffs Inc. [Member] | 5.75% 2025 Senior Notes [Member]    
Debt Instrument [Line Items]    
Debt Instrument, Interest Rate, Effective Percentage 6.01% 6.01%
Long-term Debt, Gross $ 473.3 $ 473.3
Unamortized Debt Issuance Expense (3.5) (3.6)
Debt Instrument, Unamortized Discount (5.3) (5.5)
Long-term Debt $ 464.5 $ 464.2
Cleveland-Cliffs Inc. [Member] | 7.00% 2027 Senior Notes [Member]    
Debt Instrument [Line Items]    
Debt Instrument, Interest Rate, Effective Percentage 9.24%  
Long-term Debt, Gross $ 335.4  
Unamortized Debt Issuance Expense (1.3)  
Debt Instrument, Unamortized Discount (38.4)  
Long-term Debt $ 295.7  
Cleveland-Cliffs Inc. [Member] | 5.875% 2027 Senior Notes [Member]    
Debt Instrument [Line Items]    
Debt Instrument, Interest Rate, Effective Percentage 6.49% 6.49%
Long-term Debt, Gross $ 750.0 $ 750.0
Unamortized Debt Issuance Expense (6.2) (6.3)
Debt Instrument, Unamortized Discount (26.6) (27.3)
Long-term Debt $ 717.2 $ 716.4
Cleveland-Cliffs Inc. [Member] | 6.25% 2040 Senior Notes [Member]    
Debt Instrument [Line Items]    
Debt Instrument, Interest Rate, Effective Percentage 6.34% 6.34%
Long-term Debt, Gross $ 298.4 $ 298.4
Unamortized Debt Issuance Expense (2.1) (2.2)
Debt Instrument, Unamortized Discount (3.2) (3.3)
Long-term Debt $ 293.1 292.9
Cleveland-Cliffs Inc. [Member] | Credit Facility    
Debt Instrument [Line Items]    
Debt Instrument, Interest Rate, Effective Percentage 2.33%  
Long-term Debt, Gross $ 2,000.0 450.0
Long-term Line of Credit $ 800.0 $ 0.0
AK Steel Holding Corporation [Member] | 7.625% 2021 Senior Notes [Member]    
Debt Instrument [Line Items]    
Debt Instrument, Interest Rate, Effective Percentage 7.33%  
Long-term Debt, Gross $ 33.5  
Unamortized Debt Issuance Expense 0.0  
Debt Instrument, Unamortized Premium 0.1  
Long-term Debt $ 33.6  
AK Steel Holding Corporation [Member] | 7.50% 2023 AK Senior Notes [Member]    
Debt Instrument [Line Items]    
Debt Instrument, Interest Rate, Effective Percentage 6.17%  
Long-term Debt, Gross $ 12.8  
Unamortized Debt Issuance Expense 0.0  
Debt Instrument, Unamortized Premium 0.5  
Long-term Debt $ 13.3  
AK Steel Holding Corporation [Member] | 6.375% 2025 AK Senior Notes [Member]    
Debt Instrument [Line Items]    
Debt Instrument, Interest Rate, Effective Percentage 8.11%  
Long-term Debt, Gross $ 38.4  
Unamortized Debt Issuance Expense 0.0  
Debt Instrument, Unamortized Discount (3.0)  
Long-term Debt $ 35.4  
AK Steel Holding Corporation [Member] | 7.00% 2027 AK Senior Notes [Member]    
Debt Instrument [Line Items]    
Debt Instrument, Interest Rate, Effective Percentage 9.24%  
Long-term Debt, Gross $ 56.3  
Unamortized Debt Issuance Expense 0.0  
Debt Instrument, Unamortized Discount (6.4)  
Long-term Debt 49.9  
AK Steel Holding Corporation [Member] | Industrial Revenue Bonds [Member]    
Debt Instrument [Line Items]    
Long-term Debt, Gross 99.3  
Unamortized Debt Issuance Expense 0.0  
Debt Instrument, Unamortized Premium 2.5  
Long-term Debt $ 101.8