v3.26.1
Revenue Recognition - Revenue by Product Type and Market (Details) - USD ($)
$ in Millions
 3 Months Ended
Mar. 31, 2026
Mar. 31, 2025
Disaggregation of Revenue [Line Items]    
Revenues $ 973.3 $ 937.0
Thermal coal    
Disaggregation of Revenue [Line Items]    
Revenues 671.3 709.2
Metallurgical coal    
Disaggregation of Revenue [Line Items]    
Revenues 282.3 219.7
Other    
Disaggregation of Revenue [Line Items]    
Revenues 19.7 8.1
Domestic | Thermal coal    
Disaggregation of Revenue [Line Items]    
Revenues 509.5 481.9
Export | Thermal coal    
Disaggregation of Revenue [Line Items]    
Revenues 161.8 227.3
Export | Metallurgical coal    
Disaggregation of Revenue [Line Items]    
Revenues 282.3 219.7
Seaborne Thermal    
Disaggregation of Revenue [Line Items]    
Revenues 197.5 265.1
Seaborne Thermal | Thermal coal    
Disaggregation of Revenue [Line Items]    
Revenues 197.3 265.0
Seaborne Thermal | Metallurgical coal    
Disaggregation of Revenue [Line Items]    
Revenues 0.0 0.0
Seaborne Thermal | Other    
Disaggregation of Revenue [Line Items]    
Revenues 0.2 0.1
Seaborne Thermal | Domestic | Thermal coal    
Disaggregation of Revenue [Line Items]    
Revenues 35.5 37.7
Seaborne Thermal | Export | Thermal coal    
Disaggregation of Revenue [Line Items]    
Revenues 161.8 227.3
Seaborne Thermal | Export | Metallurgical coal    
Disaggregation of Revenue [Line Items]    
Revenues 0.0 0.0
Seaborne Metallurgical    
Disaggregation of Revenue [Line Items]    
Revenues 283.0 220.1
Seaborne Metallurgical | Thermal coal    
Disaggregation of Revenue [Line Items]    
Revenues 0.0 0.0
Seaborne Metallurgical | Metallurgical coal    
Disaggregation of Revenue [Line Items]    
Revenues 282.3 219.7
Seaborne Metallurgical | Other    
Disaggregation of Revenue [Line Items]    
Revenues 0.7 0.4
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 282.3 219.7
Powder River Basin    
Disaggregation of Revenue [Line Items]    
Revenues 289.5 275.6
Powder River Basin | Thermal coal    
Disaggregation of Revenue [Line Items]    
Revenues 289.5 275.5
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 0.0 0.1
Powder River Basin | Domestic | Thermal coal    
Disaggregation of Revenue [Line Items]    
Revenues 289.5 275.5
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 184.5 168.7
Other U.S. Thermal | Thermal coal    
Disaggregation of Revenue [Line Items]    
Revenues 184.5 168.7
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 0.0
Other U.S. Thermal | Domestic | Thermal coal    
Disaggregation of Revenue [Line Items]    
Revenues 184.5 168.7
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 18.8 7.5
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 18.8 7.5
Corporate and Other (1) | Trading and Brokerage Coal Deliveries    
Disaggregation of Revenue [Line Items]    
Revenues 13.7 4.0
Corporate and Other (1) | Other revenue    
Disaggregation of Revenue [Line Items]    
Revenues 5.1 3.5
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