| Document and Entity Information | Feb. 25, 2021 |
| Cover [Abstract] | |
| Document Type | 8-K |
| Document Period End Date | Feb. 25, 2021 |
| Entity Registrant Name | CLEVELAND-CLIFFS INC. |
| Entity Incorporation, State or Country Code | OH |
| Entity File Number | 1-8944 |
| Entity Tax Identification Number | 34-1464672 |
| Entity Address, Address Line One | 200 Public Square, |
| Entity Address, City or Town | Cleveland, |
| Entity Address, State or Province | OH |
| Entity Address, Postal Zip Code | 44114-2315 |
| City Area Code | 216 |
| Local Phone Number | 694-5700 |
| Written Communications | false |
| Soliciting Material | false |
| Pre-commencement Tender Offer | false |
| Pre-commencement Issuer Tender Offer | false |
| Title of 12(b) Security | Common Shares, par value $0.125 per share |
| Trading Symbol | CLF |
| Security Exchange Name | NYSE |
| Entity Emerging Growth Company | false |
| Entity Central Index Key | 0000764065 |
| Amendment Flag | false |
