v3.26.1
Revenue Recognition - Revenue by Product Type and Market (Details) - USD ($)
$ in Millions
 3 Months Ended 6 Months Ended
Jun. 30, 2026
Jun. 30, 2025
Jun. 30, 2026
Jun. 30, 2025
Disaggregation of Revenue [Line Items]        
Revenues $ 1,003.2 $ 890.1 $ 1,976.5 $ 1,827.1
Thermal coal        
Disaggregation of Revenue [Line Items]        
Revenues 617.6 625.8 1,288.9 1,335.0
Metallurgical coal        
Disaggregation of Revenue [Line Items]        
Revenues 358.2 251.9 640.5 471.6
Other        
Disaggregation of Revenue [Line Items]        
Revenues 27.4 12.4 47.1 20.5
Domestic | Thermal coal        
Disaggregation of Revenue [Line Items]        
Revenues 425.7 466.5 935.2 948.4
Export | Thermal coal        
Disaggregation of Revenue [Line Items]        
Revenues 191.9 159.3 353.7 386.6
Export | Metallurgical coal        
Disaggregation of Revenue [Line Items]        
Revenues 358.2 251.9 640.5 471.6
Seaborne Thermal        
Disaggregation of Revenue [Line Items]        
Revenues 230.6 195.1 428.1 460.2
Seaborne Thermal | Thermal coal        
Disaggregation of Revenue [Line Items]        
Revenues 230.6 195.1 427.9 460.1
Seaborne Thermal | Metallurgical coal        
Disaggregation of Revenue [Line Items]        
Revenues 0.0 0.0 0.0 0.0
Seaborne Thermal | Other        
Disaggregation of Revenue [Line Items]        
Revenues 0.0 0.0 0.2 0.1
Seaborne Thermal | Domestic | Thermal coal        
Disaggregation of Revenue [Line Items]        
Revenues 40.2 35.8 75.7 73.5
Seaborne Thermal | Export | Thermal coal        
Disaggregation of Revenue [Line Items]        
Revenues 190.4 159.3 352.2 386.6
Seaborne Thermal | Export | Metallurgical coal        
Disaggregation of Revenue [Line Items]        
Revenues 0.0 0.0 0.0 0.0
Seaborne Metallurgical        
Disaggregation of Revenue [Line Items]        
Revenues 358.3 252.2 641.3 472.3
Seaborne Metallurgical | Thermal coal        
Disaggregation of Revenue [Line Items]        
Revenues 0.0 0.0 0.0 0.0
Seaborne Metallurgical | Metallurgical coal        
Disaggregation of Revenue [Line Items]        
Revenues 358.2 251.9 640.5 471.6
Seaborne Metallurgical | Other        
Disaggregation of Revenue [Line Items]        
Revenues 0.1 0.3 0.8 0.7
Seaborne Metallurgical | Domestic | Thermal coal        
Disaggregation of Revenue [Line Items]        
Revenues 0.0 0.0 0.0 0.0
Seaborne Metallurgical | Export | Thermal coal        
Disaggregation of Revenue [Line Items]        
Revenues 0.0 0.0 0.0 0.0
Seaborne Metallurgical | Export | Metallurgical coal        
Disaggregation of Revenue [Line Items]        
Revenues 358.2 251.9 640.5 471.6
Powder River Basin        
Disaggregation of Revenue [Line Items]        
Revenues 223.8 275.7 513.3 551.3
Powder River Basin | Thermal coal        
Disaggregation of Revenue [Line Items]        
Revenues 223.8 275.7 513.3 551.2
Powder River Basin | Metallurgical coal        
Disaggregation of Revenue [Line Items]        
Revenues 0.0 0.0 0.0 0.0
Powder River Basin | Other        
Disaggregation of Revenue [Line Items]        
Revenues 0.0 0.0 0.0 0.1
Powder River Basin | Domestic | Thermal coal        
Disaggregation of Revenue [Line Items]        
Revenues 222.3 275.7 511.8 551.2
Powder River Basin | Export | Thermal coal        
Disaggregation of Revenue [Line Items]        
Revenues 1.5 0.0 1.5 0.0
Powder River Basin | Export | Metallurgical coal        
Disaggregation of Revenue [Line Items]        
Revenues 0.0 0.0 0.0 0.0
Other U.S. Thermal        
Disaggregation of Revenue [Line Items]        
Revenues 163.2 155.1 347.7 323.8
Other U.S. Thermal | Thermal coal        
Disaggregation of Revenue [Line Items]        
Revenues 163.2 155.0 347.7 323.7
Other U.S. Thermal | Metallurgical coal        
Disaggregation of Revenue [Line Items]        
Revenues 0.0 0.0 0.0 0.0
Other U.S. Thermal | Other        
Disaggregation of Revenue [Line Items]        
Revenues 0.0 0.1 0.0 0.1
Other U.S. Thermal | Domestic | Thermal coal        
Disaggregation of Revenue [Line Items]        
Revenues 163.2 155.0 347.7 323.7
Other U.S. Thermal | Export | Thermal coal        
Disaggregation of Revenue [Line Items]        
Revenues 0.0 0.0 0.0 0.0
Other U.S. Thermal | Export | Metallurgical coal        
Disaggregation of Revenue [Line Items]        
Revenues 0.0 0.0 0.0 0.0
Corporate and Other (1)        
Disaggregation of Revenue [Line Items]        
Revenues 27.3 12.0 46.1 19.5
Corporate and Other (1) | Thermal coal        
Disaggregation of Revenue [Line Items]        
Revenues 0.0 0.0 0.0 0.0
Corporate and Other (1) | Metallurgical coal        
Disaggregation of Revenue [Line Items]        
Revenues 0.0 0.0 0.0 0.0
Corporate and Other (1) | Other        
Disaggregation of Revenue [Line Items]        
Revenues 27.3 12.0 46.1 19.5
Corporate and Other (1) | Trading and Brokerage Coal Deliveries        
Disaggregation of Revenue [Line Items]        
Revenues 21.7 7.8 35.4 11.8
Corporate and Other (1) | Other revenue        
Disaggregation of Revenue [Line Items]        
Revenues 5.6 4.2 10.7 7.7
Corporate and Other (1) | Domestic | Thermal coal        
Disaggregation of Revenue [Line Items]        
Revenues 0.0 0.0 0.0 0.0
Corporate and Other (1) | Export | Thermal coal        
Disaggregation of Revenue [Line Items]        
Revenues 0.0 0.0 0.0 0.0
Corporate and Other (1) | Export | Metallurgical coal        
Disaggregation of Revenue [Line Items]        
Revenues $ 0.0 $ 0.0 $ 0.0 $ 0.0