| DEBT AND CREDIT FACILITIES (Tables)
 | 3 Months Ended | 
| Mar. 31, 2020 | Mar. 31, 2019 | 
| Debt Instrument, Redemption [Line Items] |  |  | 
| Schedule Of Long-Term Debt | The following represents a summary of our long-term debt: 
|  |  
|  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | (In Millions) |  | March 31, 2020 |  
| Debt Instrument |   | 
Issuer1
 |   | Annual Effective  Interest Rate |   | Total Principal Amount |   | Debt Issuance Costs |   | Unamortized Premiums (Discounts) |   | Total Debt |  
| Senior Secured Notes: |   |   |   |   |   |   |   |   |   |   |   |   |  
| 4.875% 2024 Senior Secured Notes |   | Cliffs |   | 5.00% |   | $ | 400.0 | 
 |   | $ | (4.3 | ) |   | $ | (1.7 | ) |   | $ | 394.0 | 
 |  
| 6.75% 2026 Senior Secured Notes |   | Cliffs |   | 7.00% |   | 725.0 | 
 |   | (20.3 | ) |   | (8.8 | ) |   | 695.9 | 
 |  
| Senior Unsecured Notes: |   |   |   |   |   |   |   |   |   |   |   |   |  
| 7.625% 2021 AK Senior Notes |   | AK Steel |   | 7.33% |   | 33.5 | 
 |   | — | 
 |   | 0.1 | 
 |   | 33.6 | 
 |  
| 7.50% 2023 AK Senior Notes |   | AK Steel |   | 6.17% |   | 12.8 | 
 |   | — | 
 |   | 0.5 | 
 |   | 13.3 | 
 |  
| 6.375% 2025 Senior Notes |   | Cliffs |   | 8.11% |   | 231.8 | 
 |   | (0.9 | ) |   | (17.9 | ) |   | 213.0 | 
 |  
| 6.375% 2025 AK Senior Notes |   | AK Steel |   | 8.11% |   | 38.4 | 
 |   | — | 
 |   | (3.0 | ) |   | 35.4 | 
 |  
| 1.50% 2025 Convertible Senior Notes |   | Cliffs |   | 6.26% |   | 316.3 | 
 |   | (4.4 | ) |   | (62.2 | ) |   | 249.7 | 
 |  
| 5.75% 2025 Senior Notes |   | Cliffs |   | 6.01% |   | 473.3 | 
 |   | (3.5 | ) |   | (5.3 | ) |   | 464.5 | 
 |  
| 7.00% 2027 Senior Notes |   | Cliffs |   | 9.24% |   | 335.4 | 
 |   | (1.3 | ) |   | (38.4 | ) |   | 295.7 | 
 |  
| 7.00% 2027 AK Senior Notes |   | AK Steel |   | 9.24% |   | 56.3 | 
 |   | — | 
 |   | (6.4 | ) |   | 49.9 | 
 |  
| 5.875% 2027 Senior Notes |   | Cliffs |   | 6.49% |   | 750.0 | 
 |   | (6.2 | ) |   | (26.6 | ) |   | 717.2 | 
 |  
| 6.25% 2040 Senior Notes |   | Cliffs |   | 6.34% |   | 298.4 | 
 |   | (2.1 | ) |   | (3.2 | ) |   | 293.1 | 
 |  
| IRBs due 2020 to 2028 |   | AK Steel |   | Various |   | 99.3 | 
 |   | — | 
 |   | 2.5 | 
 |   | 101.8 | 
 |  
| ABL Facility |   | 
Cliffs2
 |   | 2.33% |   | 2,000.0 | 
 |   | — | 
 |   | — | 
 |   | 800.0 | 
 |  
| Total long-term debt |   |   |   |   |   |   |   |   |   |   |   | $ | 4,357.1 | 
 |  
|   |   |   |   |   |   |   |   |   |   |   |   |   |  | 
1 Unless otherwise noted, references in this column to "Cliffs" are to Cleveland-Cliffs Inc., and references to "AK Steel" are to AK Steel Corporation.
 |  | 
2 Refers to Cleveland-Cliffs Inc. as borrower under our ABL Facility.
 | 
|  |  
|  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | (In Millions) |  | December 31, 2019 |  
| Debt Instrument |   | 
Issuer1
 |   | Annual Effective  Interest Rate |   | Total Principal Amount |   | Debt Issuance Costs |   | Unamortized Discounts |   | Total Debt |  
| Senior Secured Notes: |   |   |   |   |   |   |   |   |   |   |   |   |  
| 4.875% 2024 Senior Notes |   | Cliffs |   | 5.00% |   | $ | 400.0 | 
 |   | $ | (4.6 | ) |   | $ | (1.8 | ) |   | $ | 393.6 | 
 |  
| Senior Unsecured Notes: |   |   |   |   |   |   |   |   |   |   |   |   |  
| 1.50% 2025 Convertible Senior Notes |   | Cliffs |   | 6.26% |   | 316.3 | 
 |   | (4.6 | ) |   | (65.0 | ) |   | 246.7 | 
 |  
| 5.75% 2025 Senior Notes |   | Cliffs |   | 6.01% |   | 473.3 | 
 |   | (3.6 | ) |   | (5.5 | ) |   | 464.2 | 
 |  
| 5.875% 2027 Senior Notes |   | Cliffs |   | 6.49% |   | 750.0 | 
 |   | (6.3 | ) |   | (27.3 | ) |   | 716.4 | 
 |  
| 6.25% 2040 Senior Notes |   | Cliffs |   | 6.34% |   | 298.4 | 
 |   | (2.2 | ) |   | (3.3 | ) |   | 292.9 | 
 |  
| Former ABL Facility |   | 
Cliffs2
 |   | N/A |   | 450.0 | 
 |   | N/A | 
 |   | N/A | 
 |   | — | 
 |  
| Total long-term debt |   |   |   |   |   |   |   |   |   |   |   | $ | 2,113.8 | 
 |  
|   |   |   |   |   |   |   |   |   |   |   |   |   |  | 
1 Unless otherwise noted, references in this column to "Cliffs" are to Cleveland-Cliffs Inc.
 |  | 
2 Refers to Cleveland-Cliffs Inc. and certain of its subsidiaries as borrowers under our Former ABL Facility.
 | 
 |  | 
| Schedule of Extinguishment of Debt | The following is a summary of the debt extinguished and the respective gain on extinguishment: 
|  |  
|  |  |  |  |  |  |  |  |  |  
|   |   | (In Millions) |  
|   |   | Three Months Ended  March 31, 2020 |  
| Debt Instrument |   | Debt Extinguished |   | 
Gain on Extinguishment1
 |  
| 7.625% 2021 AK Senior Notes |   | $ | 372.7 | 
 |   | $ | 0.4 | 
 |  
| 7.50% 2023 AK Senior Notes |   | 367.2 | 
 |   | 2.8 | 
 |  
|   |   | $ | 739.9 | 
 |   | $ | 3.2 | 
 |  
|   |   |   |   |   |  | 
1 The gain on extinguishment relates to the March 27, 2020 purchases.
 | 
 | The following is a summary of the debt extinguished with cash and the respective loss on extinguishment: 
|  |  
|  |  |  |  |  |  |  |  |  |  
|   |   | (In Millions) |  
|   |   | Three Months Ended  March 31, 2019 |  
| Debt Instrument |   | Debt Extinguished |   | (Loss) on Extinguishment |  
| 4.875% 2021 Senior Notes |   | $ | 10.0 | 
 |   | $ | (0.3 | ) |  
|   |   | $ | 10.0 | 
 |   | $ | (0.3 | ) | 
 | 
| Schedule of Line of Credit Facilities | The following represents a summary of our borrowing capacity under the ABL Facility: 
|  |  
|  |  |  |  |  |  
|   |   | (In Millions) |  
|   |   | March 31, 2020
 |  
| 
Available borrowing base on ABL Facility1
 |   | $ | 1,789.3 | 
 |  
| Borrowings |   | (800.0 | ) |  
| 
Letter of credit obligations2
 |   | (199.3 | ) |  
| Borrowing capacity available |   | $ | 790.0 | 
 | 
 |  | 
| Schedule of Maturities of Long-term Debt | 
The following represents a summary of our maturities of debt instruments based on the principal amounts outstanding at March 31, 2020: 
 
|  |  
|  |  |  |  |  |  
|   |   | (In Millions) |  
|   |   | Maturities of Debt |  
| 
2020 (remaining period of year)1
 |   | $ | 7.3 | 
 |  
| 2021 |   | 33.5 | 
 |  
| 2022 |   | — | 
 |  
| 2023 |   | 12.8 | 
 |  
| 2024 |   | 462.0 | 
 |  
| Thereafter |   | 4,054.9 | 
 |  
| Total maturities of debt |   | $ | 4,570.5 | 
 |  
|   |   |   |  | 
1 Amounts maturing in 2020 are classified as Long-term debt based on our ability and intent to refinance on a long-term basis.
 | 
 |  | 
| 6.375% 2025 Senior Notes [Member] |  |  | 
| Debt Instrument, Redemption [Line Items] |  |  | 
| Debt Instrument Redemption | The following is a summary of redemption prices for our 6.375% 2025 Senior Notes: 
|  |  
|  |  |  |  |  |  |  
| Redemption Period |   | 
Redemption Price1
 |   | Restricted Amount |  
| Prior to October 15, 2020 - using proceeds of equity issuance |   | 106.375 | % |   | Up to 35% of original aggregate principal |  
| 
Prior to October 15, 20202
 |   | 100.000 |   |   |   |  
| Beginning on October 15, 2020 |   | 103.188 |   |   |   |  
| Beginning on October 15, 2021 |   | 101.594 |   |   |   |  
| Beginning on October 15, 2022 and thereafter |   | 100.000 |   |   |   |  
|   |   |   |   |   |   |  | 
1  Plus accrued and unpaid interest, if any, up to but excluding the redemption date.
 |  | 
2  Plus a "make-whole" premium.
 | 
 |  | 
| 7.00% 2027 Senior Notes [Member] |  |  | 
| Debt Instrument, Redemption [Line Items] |  |  | 
| Debt Instrument Redemption | The following is a summary of redemption prices for our 7.00% 2027 Senior Notes: 
|  |  
|  |  |  |  |  
| Redemption Period |   | 
Redemption Price1
 |  
| 
Prior to March 15, 20222
 |   | 100.000 | % |  
| Beginning on March 15, 2022 |   | 103.500 |   |  
| Beginning on March 15, 2023 |   | 102.333 |   |  
| Beginning on March 15, 2024 |   | 101.167 |   |  
| Beginning on March 15, 2025 and thereafter |   | 100.000 |   |  
|   |   |   |   |  | 
1  Plus accrued and unpaid interest, if any, up to but excluding the redemption date.
 |  | 
2  Plus a "make-whole" premium.
 | 
 |  | 
| 7.50% 2023 AK Senior Notes [Member] |  |  | 
| Debt Instrument, Redemption [Line Items] |  |  | 
| Debt Instrument Redemption | The following is a summary of redemption prices for the 7.50% 2023 AK Senior Notes: 
|  |  
|  |  |  |  |  
| Redemption Period |   | 
Redemption Price1
 |  
| Prior to July 15, 2020 |   | 103.750 | % |  
| Beginning on July 15, 2020 |   | 101.875 |   |  
| Beginning on July 15, 2021 and thereafter |   | 100.000 |   |  
|   |   |   |   |  | 
1  Plus accrued and unpaid interest, if any, up to but excluding the redemption date.
 | 
 |  | 
| 6.375% 2025 AK Senior Notes [Member] |  |  | 
| Debt Instrument, Redemption [Line Items] |  |  | 
| Debt Instrument Redemption | The following is a summary of redemption prices for the 6.375% 2025 AK Senior Notes: 
|  |  
|  |  |  |  |  
| Redemption Period |   | 
Redemption Price1
 |  
| 
Prior to October 15, 20202
 |   | 100.000 | % |  
| Beginning on October 15, 2020 |   | 103.188 |   |  
| Beginning on October 15, 2021 |   | 101.594 |   |  
| Beginning on October 15, 2022 and thereafter |   | 100.000 |   |  
|   |   |   |   |  | 
1  Plus accrued and unpaid interest, if any, up to but excluding the redemption date.
 |  | 
2  Plus a "make-whole" premium.
 | 
 |  | 
| 6.75% 2026 Senior Notes [Member] |  |  | 
| Debt Instrument, Redemption [Line Items] |  |  | 
| Debt Instrument Redemption | The following is a summary of redemption prices for our 6.75% 2026 Senior Secured Notes: 
|  |  
|  |  |  |  |  |  |  
| Redemption Period |   | 
Redemption Price1
 |   | Restricted Amount |  
| Prior to March 15, 2022 - using proceeds of equity issuance |   | 106.750 | % |   | Up to 35% of original aggregate principal |  
| 
Prior to March 15, 20222
 |   | 100.000 |   |   |   |  
| Beginning on March 15, 2022 |   | 105.063 |   |   |   |  
| Beginning on March 15, 2023 |   | 103.375 |   |   |   |  
| Beginning on March 15, 2024 |   | 101.688 |   |   |   |  
| Beginning on March 15, 2025 and thereafter |   | 100.000 |   |   |   |  
|   |   |   |   |   |   |  | 
1  Plus accrued and unpaid interest, if any, up to, but excluding, the redemption date.
 |  | 
2  Plus a "make-whole" premium.
 | 
 |  | 
| 7.00% 2027 AK Senior Notes [Member] |  |  | 
| Debt Instrument, Redemption [Line Items] |  |  | 
| Debt Instrument Redemption | The following is a summary of redemption prices for the 7.00% 2027 AK Senior Notes: 
|  |  
|  |  |  |  |  
| Redemption Period |   | 
Redemption Price1
 |  
| 
Prior to March 15, 20222
 |   | 100.000 | % |  
| Beginning on March 15, 2022 |   | 103.500 |   |  
| Beginning on March 15, 2023 |   | 102.333 |   |  
| Beginning on March 15, 2024 |   | 101.167 |   |  
| Beginning on March 15, 2025 and thereafter |   | 100.000 |   |  
|   |   |   |   |  | 
1  Plus accrued and unpaid interest, if any, up to but excluding the redemption date.
 |  | 
2  Plus a "make-whole" premium.
 | 
 |  |