Annual report pursuant to Section 13 and 15(d)


Cover - USD ($)
12 Months Ended
Dec. 31, 2020
Feb. 24, 2021
Jun. 30, 2020
Cover [Abstract]      
Document Type 10-K    
Document Annual Report true    
Document Period End Date Dec. 31, 2020    
Document Transition Report false    
Entity File Number 1-8944    
Entity Registrant Name CLEVELAND-CLIFFS INC.    
Entity Incorporation, State or Country Code OH    
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    
Title of 12(b) Security Common Shares, par value $0.125 per share    
Trading Symbol CLF    
Security Exchange Name NYSE    
Entity Well-known Seasoned Issuer Yes    
Entity Voluntary Filers No    
Entity Current Reporting Status Yes    
Entity Interactive Data Current Yes    
Entity Filer Category Large Accelerated Filer    
Entity Small Business false    
Entity Emerging Growth Company false    
ICFR Auditor Attestation Flag true    
Entity Shell Company false    
Entity Public Float     $ 2,171,029,299
Entity Common Stock, Shares Outstanding   498,885,558  
Entity Central Index Key 0000764065    
Current Fiscal Year End Date --12-31    
Document Fiscal Year Focus 2020    
Document Fiscal Period Focus FY    
Amendment Flag false    
Documents Incorporated by Reference [Text Block] Portions of the registrant’s proxy statement for its 2021 annual meeting of shareholders are incorporated by reference into Part III.