Quarterly report pursuant to Section 13 or 15(d)

SHAREHOLDERS' EQUITY - Narrative (Details)

v3.20.2
SHAREHOLDERS' EQUITY - Narrative (Details) - USD ($)
$ / shares in Units, $ in Millions
 3 Months Ended 9 Months Ended
Mar. 13, 2020
Dec. 31, 2020
Mar. 31, 2020
Jun. 30, 2019
Mar. 31, 2019
Sep. 30, 2020
Sep. 30, 2019
Apr. 24, 2019
Nov. 26, 2018
Class of Stock [Line Items]                  
Common shares, par value           $ 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             24,400,000    
Payments for Repurchase of common shares           $ 0.0 $ 252.9    
Forecast [Member] | ArcelorMittal USA [Member] | Series B Participating Redeemable Preferred Stock [Member]                  
Class of Stock [Line Items]                  
Shares of Cliffs common shares issued to AK Steel stockholders   583,273              
Preferred Class A [Member]                  
Class of Stock [Line Items]                  
Preferred Stock, Shares Authorized           3,000,000      
Preferred Stock, Shares Outstanding           0      
Preferred Stock, Shares Issued           0      
Preferred Class B [Member]                  
Class of Stock [Line Items]                  
Preferred Stock, Shares Authorized           4,000,000      
Preferred Stock, Shares Outstanding           0      
Preferred Stock, Shares Issued           0      
Common Stock [Member]                  
Class of Stock [Line Items]                  
Stock Issued During Period, Shares, Acquisitions 126,800,000   126,800,000            
Stock Repurchased During Period, Shares       12,900,000 11,500,000