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Customer Concentrations (Narrative) (Details) - Customer Concentration Risk
T in Millions, $ in Millions
12 Months Ended
Dec. 31, 2022
Dec. 31, 2021
Dec. 31, 2020
USD ($)
T
Dec. 31, 2019
USD ($)
Concentration Risk [Line Items]        
Quantity of coke sold to three primary customers (in tons) | T     3.8  
Cliffs | Accounts Receivable        
Concentration Risk [Line Items]        
Accounts receivable due, net     $ 22.9  
U.S. Steel | Accounts Receivable        
Concentration Risk [Line Items]        
Accounts receivable due, net     $ 6.3 $ 7.3
AM USA | Accounts Receivable        
Concentration Risk [Line Items]        
Accounts receivable due, net       19.5
AK Steel | Accounts Receivable        
Concentration Risk [Line Items]        
Accounts receivable due, net       $ 13.2
Forecast | Cliffs        
Concentration Risk [Line Items]        
Percent of cokemaking capacity 65.00% 75.00%