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 6 Months Ended
Mar. 31, 2020
Jun. 30, 2019
Mar. 31, 2019
Jun. 30, 2020
Jun. 30, 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         24,400,000        
Payments for Repurchase of common shares       $ 0.0 $ 252.9        
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     126,800,000          
Stock Repurchased During Period, Shares   12,900,000 11,500,000