Quarterly report pursuant to Section 13 or 15(d)

ACQUISITION OF AK STEEL - Transaction Consideration (Details)

v3.20.2
ACQUISITION OF AK STEEL - Transaction Consideration (Details)
$ / shares in Units, $ in Millions
Mar. 13, 2020
USD ($)
$ / shares
shares
Jun. 30, 2020
shares
Mar. 31, 2020
shares
Dec. 31, 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            
Number of shares of AK Steel common stock issued and outstanding | shares   399,159,988   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            
Number of shares of AK Steel common stock issued and outstanding | 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]              
Number of shares of AK Steel common stock issued and outstanding | shares   399,200,000 398,600,000 270,100,000 270,000,000.0 282,800,000 292,600,000
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