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Revenue Recognition - Revenue by Product Type and Market (Details) - USD ($)
$ in Millions
 3 Months Ended
Mar. 31, 2025
Mar. 31, 2024
Disaggregation of Revenue [Line Items]    
Revenues $ 937.0 $ 983.6
Thermal coal    
Disaggregation of Revenue [Line Items]    
Revenues 709.2 715.8
Metallurgical coal    
Disaggregation of Revenue [Line Items]    
Revenues 219.7 244.0
Other    
Disaggregation of Revenue [Line Items]    
Revenues 8.1 23.8
Domestic | Thermal coal    
Disaggregation of Revenue [Line Items]    
Revenues 481.9 472.7
Export | Thermal coal    
Disaggregation of Revenue [Line Items]    
Revenues 227.3 243.1
Export | Metallurgical coal    
Disaggregation of Revenue [Line Items]    
Revenues 219.7 244.0
Seaborne Thermal    
Disaggregation of Revenue [Line Items]    
Revenues 265.1 283.9
Seaborne Thermal | Thermal coal    
Disaggregation of Revenue [Line Items]    
Revenues 265.0 283.7
Seaborne Thermal | Metallurgical coal    
Disaggregation of Revenue [Line Items]    
Revenues 0.0 0.0
Seaborne Thermal | Other    
Disaggregation of Revenue [Line Items]    
Revenues 0.1 0.2
Seaborne Thermal | Domestic | Thermal coal    
Disaggregation of Revenue [Line Items]    
Revenues 37.7 40.6
Seaborne Thermal | Export | Thermal coal    
Disaggregation of Revenue [Line Items]    
Revenues 227.3 243.1
Seaborne Thermal | Export | Metallurgical coal    
Disaggregation of Revenue [Line Items]    
Revenues 0.0 0.0
Seaborne Metallurgical    
Disaggregation of Revenue [Line Items]    
Revenues 220.1 247.0
Seaborne Metallurgical | Thermal coal    
Disaggregation of Revenue [Line Items]    
Revenues 0.0 0.0
Seaborne Metallurgical | Metallurgical coal    
Disaggregation of Revenue [Line Items]    
Revenues 219.7 244.0
Seaborne Metallurgical | Other    
Disaggregation of Revenue [Line Items]    
Revenues 0.4 3.0
Seaborne Metallurgical | Domestic | Thermal coal    
Disaggregation of Revenue [Line Items]    
Revenues 0.0 0.0
Seaborne Metallurgical | Export | Thermal coal    
Disaggregation of Revenue [Line Items]    
Revenues 0.0 0.0
Seaborne Metallurgical | Export | Metallurgical coal    
Disaggregation of Revenue [Line Items]    
Revenues 219.7 244.0
Powder River Basin    
Disaggregation of Revenue [Line Items]    
Revenues 275.6 254.1
Powder River Basin | Thermal coal    
Disaggregation of Revenue [Line Items]    
Revenues 275.5 254.1
Powder River Basin | Metallurgical coal    
Disaggregation of Revenue [Line Items]    
Revenues 0.0 0.0
Powder River Basin | Other    
Disaggregation of Revenue [Line Items]    
Revenues 0.1 0.0
Powder River Basin | Domestic | Thermal coal    
Disaggregation of Revenue [Line Items]    
Revenues 275.5 254.1
Powder River Basin | Export | Thermal coal    
Disaggregation of Revenue [Line Items]    
Revenues 0.0 0.0
Powder River Basin | Export | Metallurgical coal    
Disaggregation of Revenue [Line Items]    
Revenues 0.0 0.0
Other U.S. Thermal    
Disaggregation of Revenue [Line Items]    
Revenues 168.7 191.6
Other U.S. Thermal | Thermal coal    
Disaggregation of Revenue [Line Items]    
Revenues 168.7 178.0
Other U.S. Thermal | Metallurgical coal    
Disaggregation of Revenue [Line Items]    
Revenues 0.0 0.0
Other U.S. Thermal | Other    
Disaggregation of Revenue [Line Items]    
Revenues 0.0 13.6
Other U.S. Thermal | Domestic | Thermal coal    
Disaggregation of Revenue [Line Items]    
Revenues 168.7 178.0
Other U.S. Thermal | Export | Thermal coal    
Disaggregation of Revenue [Line Items]    
Revenues 0.0 0.0
Other U.S. Thermal | Export | Metallurgical coal    
Disaggregation of Revenue [Line Items]    
Revenues 0.0 0.0
Corporate and Other (1)    
Disaggregation of Revenue [Line Items]    
Revenues 7.5 7.0
Corporate and Other (1) | Thermal coal    
Disaggregation of Revenue [Line Items]    
Revenues 0.0 0.0
Corporate and Other (1) | Metallurgical coal    
Disaggregation of Revenue [Line Items]    
Revenues 0.0 0.0
Corporate and Other (1) | Other    
Disaggregation of Revenue [Line Items]    
Revenues 7.5 7.0
Corporate and Other (1) | Trading and Brokerage Coal Deliveries    
Disaggregation of Revenue [Line Items]    
Revenues 4.0 2.1
Corporate and Other (1) | Other revenue    
Disaggregation of Revenue [Line Items]    
Revenues 3.5 4.9
Corporate and Other (1) | Domestic | Thermal coal    
Disaggregation of Revenue [Line Items]    
Revenues 0.0 0.0
Corporate and Other (1) | Export | Thermal coal    
Disaggregation of Revenue [Line Items]    
Revenues 0.0 0.0
Corporate and Other (1) | Export | Metallurgical coal    
Disaggregation of Revenue [Line Items]    
Revenues $ 0.0 $ 0.0