Annual report pursuant to Section 13 and 15(d)

ACQUISITIONS - Narrative (Details)

v3.20.4
ACQUISITIONS - Narrative (Details)
$ in Millions
10 Months Ended 12 Months Ended
Dec. 09, 2020
USD ($)
Mar. 13, 2020
Dec. 31, 2020
USD ($)
Dec. 31, 2020
USD ($)
Dec. 31, 2019
USD ($)
Dec. 31, 2018
USD ($)
Business Acquisition [Line Items]            
Revenues       $ 5,354 $ 1,990 $ 2,332
Net income (loss) attributable to Cliffs shareholders       (122) 293 1,128
Amortization of Inventory Step-up       96 0 0
Severance Costs       38 2 0
Business Combination, Acquisition Related Costs       90 $ 7 $ 0
ArcelorMittal USA [Member]            
Business Acquisition [Line Items]            
Revenues $ 446          
Net income (loss) attributable to Cliffs shareholders (40)          
Amortization of Inventory Step-up $ 21          
Business Combination, Acquisition Related Costs       26    
AK Steel [Member]            
Business Acquisition [Line Items]            
Revenues     $ 3,573      
Net income (loss) attributable to Cliffs shareholders     (302)      
Amortization of Inventory Step-up     74      
Severance Costs     $ 35      
Business Combination, Acquisition Related Costs       $ 26    
Business Acquisition, Shares Exchange Ratio   0.400        
AK Steel [Member] | Common Stock [Member]            
Business Acquisition [Line Items]            
Business Acquisition, Shares Exchange Ratio       0.400