v3.26.3
Revenue Recognition - Revenue by Product Type and Market (Details) - USD ($)
$ in Millions
 3 Months Ended 9 Months Ended
Sep. 30, 2026
Sep. 30, 2025
Sep. 30, 2026
Sep. 30, 2025
Disaggregation of Revenue [Line Items]        
Revenues $ 1,096.3 $ 1,012.1 $ 3,072.8 $ 2,839.2
Thermal coal        
Disaggregation of Revenue [Line Items]        
Revenues 715.5 735.8 2,004.4 2,070.8
Metallurgical coal        
Disaggregation of Revenue [Line Items]        
Revenues 365.4 258.5 1,005.9 730.1
Other        
Disaggregation of Revenue [Line Items]        
Revenues 15.4 17.8 62.5 38.3
Domestic | Thermal coal        
Disaggregation of Revenue [Line Items]        
Revenues 514.1 527.0 1,449.3 1,475.4
Export | Thermal coal        
Disaggregation of Revenue [Line Items]        
Revenues 201.4 208.8 555.1 595.4
Export | Metallurgical coal        
Disaggregation of Revenue [Line Items]        
Revenues 365.4 258.5 1,005.9 730.1
Seaborne Thermal        
Disaggregation of Revenue [Line Items]        
Revenues 241.6 242.7 669.7 702.9
Seaborne Thermal | Thermal coal        
Disaggregation of Revenue [Line Items]        
Revenues 241.4 242.4 669.3 702.5
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 0.2 0.3 0.4 0.4
Seaborne Thermal | Domestic | Thermal coal        
Disaggregation of Revenue [Line Items]        
Revenues 40.0 33.6 115.7 107.1
Seaborne Thermal | Export | Thermal coal        
Disaggregation of Revenue [Line Items]        
Revenues 201.4 208.8 553.6 595.4
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 366.0 258.9 1,007.3 731.2
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 365.4 258.5 1,005.9 730.1
Seaborne Metallurgical | Other        
Disaggregation of Revenue [Line Items]        
Revenues 0.6 0.4 1.4 1.1
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 365.4 258.5 1,005.9 730.1
Powder River Basin        
Disaggregation of Revenue [Line Items]        
Revenues 268.1 301.4 781.4 852.7
Powder River Basin | Thermal coal        
Disaggregation of Revenue [Line Items]        
Revenues 268.0 301.4 781.3 852.6
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 0.1 0.0 0.1 0.1
Powder River Basin | Domestic | Thermal coal        
Disaggregation of Revenue [Line Items]        
Revenues 268.0 301.4 779.8 852.6
Powder River Basin | Export | Thermal coal        
Disaggregation of Revenue [Line Items]        
Revenues 0.0 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 206.1 192.0 553.8 515.8
Other U.S. Thermal | Thermal coal        
Disaggregation of Revenue [Line Items]        
Revenues 206.1 192.0 553.8 515.7
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 0.0 0.0 0.1
Other U.S. Thermal | Domestic | Thermal coal        
Disaggregation of Revenue [Line Items]        
Revenues 206.1 192.0 553.8 515.7
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 14.5 17.1 60.6 36.6
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 14.5 17.1 60.6 36.6
Corporate and Other (1) | Coal sales (2)        
Disaggregation of Revenue [Line Items]        
Revenues 8.6 14.0 44.0 25.8
Corporate and Other (1) | Other revenue        
Disaggregation of Revenue [Line Items]        
Revenues 5.9 3.1 16.6 10.8
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