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Customer Concentrations (Sales and Receivables to Significant Customers) (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Concentration Risk [Line Items]      
Sales and other operating revenue $ 1,333.0 $ 1,600.3 $ 1,450.9
Cliffs/AM USA | Sales Revenue, Net | Customer Concentration Risk      
Concentration Risk [Line Items]      
Sales and other operating revenue $ 687.3    
Percent of Company sales and other operating revenue 51.60%    
Cliffs/AK Steel | Sales Revenue, Net | Customer Concentration Risk      
Concentration Risk [Line Items]      
Sales and other operating revenue $ 355.8    
Percent of Company sales and other operating revenue 26.70%    
U.S. Steel | Sales Revenue, Net | Customer Concentration Risk      
Concentration Risk [Line Items]      
Sales and other operating revenue $ 208.2 $ 255.4 $ 206.8
Percent of Company sales and other operating revenue 15.60% 16.00% 14.30%
AM USA | Sales Revenue, Net | Customer Concentration Risk      
Concentration Risk [Line Items]      
Sales and other operating revenue   $ 786.4 $ 695.6
Percent of Company sales and other operating revenue   49.10% 47.90%
AK Steel | Sales Revenue, Net | Customer Concentration Risk      
Concentration Risk [Line Items]      
Sales and other operating revenue   $ 433.3 $ 377.9
Percent of Company sales and other operating revenue   27.10% 26.00%
Cliffs Steel/AM USA and Cliffs Steel/AK Steel | Sales Revenue, Net | Customer Concentration Risk      
Concentration Risk [Line Items]      
Sales and other operating revenue   $ 1,043.1  
Percent of Company sales and other operating revenue   78.30%