Quarterly report pursuant to Section 13 or 15(d)

SHAREHOLDERS' EQUITY (Narrative) (Details)

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SHAREHOLDERS' EQUITY (Narrative) (Details) - USD ($)
$ / shares in Units, $ in Millions
 3 Months Ended
Mar. 31, 2020
Mar. 31, 2019
Mar. 13, 2020
Dec. 31, 2019
Apr. 24, 2019
Nov. 26, 2018
Class of Stock [Line Items]            
Common shares, par value $ 0.125     $ 0.125    
Fair value of Cliffs common shares issued for AK Steel outstanding common stock     $ 617.6      
Stock Repurchase Program, Authorized Amount         $ 100.0 $ 200.0
Stock Repurchased During Period, Shares   11,500,000        
Stock Repurchase Program, Remaining Authorized Repurchase Amount   $ 124.3        
Preferred Class A [Member]            
Class of Stock [Line Items]            
Preferred stock, shares authorized (in shares) 3,000,000          
Preferred Stock, Shares Issued 0          
Preferred Stock, Shares Outstanding 0          
Preferred Class B [Member]            
Class of Stock [Line Items]            
Preferred stock, shares authorized (in shares) 4,000,000          
Preferred Stock, Shares Issued 0          
Preferred Stock, Shares Outstanding 0          
Common Stock [Member]            
Class of Stock [Line Items]            
Stock Issued During Period, Shares, Acquisitions 126,800,000          
Fair value of Cliffs common shares issued for AK Steel outstanding common stock   $ 0.0 $ 617.6      
Stock Repurchased During Period, Shares   11,500,000