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Customer Concentrations (Sales and Receivables to Significant Customers) (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2022
Dec. 31, 2021
Dec. 31, 2020
Concentration Risk [Line Items]      
Sales and other operating revenue $ 1,972.5 $ 1,456.0 $ 1,333.0
Cliffs Steel | Sales Revenue, Net | Customer Concentration Risk      
Concentration Risk [Line Items]      
Sales and other operating revenue $ 1,182.8 $ 994.6  
Percent of Company sales and other operating revenue 60.00% 68.30%  
U.S. Steel | Sales Revenue, Net | Customer Concentration Risk      
Concentration Risk [Line Items]      
Sales and other operating revenue $ 284.8 $ 210.0 $ 208.2
Percent of Company sales and other operating revenue 14.40% 14.40% 15.60%
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%