Quarterly report pursuant to Section 13 or 15(d)

ACQUISITION OF AK STEEL - Consideration (Details)

v3.20.2
ACQUISITION OF AK STEEL - Consideration (Details)
$ / shares in Units, $ in Millions
Mar. 13, 2020
USD ($)
$ / shares
shares
Sep. 30, 2020
shares
Jun. 30, 2020
shares
Mar. 31, 2020
shares
Dec. 31, 2019
shares
Sep. 30, 2019
shares
Jun. 30, 2019
shares
Mar. 31, 2019
shares
Dec. 31, 2018
shares
Business Acquisition [Line Items]                  
Fair value of Cliffs common shares issued for AK Steel outstanding common stock $ 617.6                
Common shares, outstanding (in shares) | shares   399,229,917     270,084,005        
AK Steel Holding Corporation [Member]                  
Business Acquisition [Line Items]                  
Fair value of Cliffs common shares issued for AK Steel outstanding common stock 617.6                
Fair value of debt included in consideration 913.6                
Business Combination, Consideration Transferred $ 1,535.1                
Common shares, outstanding (in shares) | shares 316,900,000                
Business Acquisition, Shares Exchange Ratio 0.400                
Shares of Cliffs common shares issued to AK Steel stockholders | shares 126,800,000                
Price per share of Cliffs common shares | $ / shares $ 4.87                
Credit Facility | AK Steel Holding Corporation [Member]                  
Business Acquisition [Line Items]                  
Fair value of debt included in consideration $ 590.0                
7.50% 2023 AK Senior Notes [Member] | AK Steel Holding Corporation [Member]                  
Business Acquisition [Line Items]                  
Fair value of debt included in consideration 323.6                
Share-based Payment Arrangement [Member] | AK Steel Holding Corporation [Member]                  
Business Acquisition [Line Items]                  
Fair value of Cliffs common shares issued for AK Steel outstanding common stock 3.9                
Common Stock [Member]                  
Business Acquisition [Line Items]                  
Common shares, outstanding (in shares) | shares   399,200,000 399,200,000 398,600,000 270,100,000 270,100,000 270,000,000.0 282,800,000 292,600,000
Common Stock [Member] | AK Steel Holding Corporation [Member]                  
Business Acquisition [Line Items]                  
Fair value of Cliffs common shares issued for AK Steel outstanding common stock $ 617.6