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Revenue Recognition - Revenue by Product Type and Market (Details) - USD ($)
$ in Millions
 3 Months Ended 6 Months Ended
Jun. 30, 2024
Jun. 30, 2023
Jun. 30, 2024
Jun. 30, 2023
Disaggregation of Revenue [Line Items]        
Revenues $ 1,042.0 $ 1,268.8 $ 2,025.6 $ 2,632.8
Other Income 4.2 3.8 9.1 25.6
Designated as Hedging Instrument        
Disaggregation of Revenue [Line Items]        
Other Comprehensive Income (Loss), Cash Flow Hedge, Gain (Loss), Reclassification, before Tax 2.4 29.3 (3.3) 128.5
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net (1.6) (19.3) (2.4) (55.5)
Derivative, Gain (Loss) on Derivative, Net 0.8 10.0 (5.7) 73.0
Physical commodity purchase / sale contracts [Member] | Designated as Hedging Instrument        
Disaggregation of Revenue [Line Items]        
Other Comprehensive Income (Loss), Cash Flow Hedge, Gain (Loss), Reclassification, before Tax   40.3   159.0
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net 0.0 (30.3) 0.0 (80.9)
Derivative, Gain (Loss) on Derivative, Net   10.0   78.1
Coal Contract [Member] | Designated as Hedging Instrument        
Disaggregation of Revenue [Line Items]        
Other Comprehensive Income (Loss), Cash Flow Hedge, Gain (Loss), Reclassification, before Tax   (13.8)   (31.1)
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net   13.9   31.2
Derivative, Gain (Loss) on Derivative, Net 0.0 0.1 0.0 0.1
Coal Contract and Physical commodity purchase / sale contracts | Designated as Hedging Instrument        
Disaggregation of Revenue [Line Items]        
Other Comprehensive Income (Loss), Cash Flow Hedge, Gain (Loss), Reclassification, before Tax 0.0 40.3 0.0 159.0
Thermal coal        
Disaggregation of Revenue [Line Items]        
Revenues 729.2 857.4 1,445.0 1,756.6
Metallurgical coal        
Disaggregation of Revenue [Line Items]        
Revenues 294.0 371.6 538.0 658.8
Other (2)        
Disaggregation of Revenue [Line Items]        
Revenues 18.8 39.8 42.6 217.4
Domestic | Thermal coal        
Disaggregation of Revenue [Line Items]        
Revenues 458.6 489.9 931.3 1,080.3
Export | Thermal coal        
Disaggregation of Revenue [Line Items]        
Revenues 270.6 367.5 513.7 676.3
Export | Metallurgical coal        
Disaggregation of Revenue [Line Items]        
Revenues 294.0 371.6 538.0 658.8
Seaborne Thermal        
Disaggregation of Revenue [Line Items]        
Revenues 307.5 399.5 591.4 746.0
Seaborne Thermal | Thermal coal        
Disaggregation of Revenue [Line Items]        
Revenues 307.5 399.4 591.2 745.7
Seaborne Thermal | Metallurgical coal        
Disaggregation of Revenue [Line Items]        
Revenues 0.0 0.0 0.0 0.0
Seaborne Thermal | Other (2)        
Disaggregation of Revenue [Line Items]        
Revenues 0.0 0.1 0.2 0.3
Seaborne Thermal | Domestic | Thermal coal        
Disaggregation of Revenue [Line Items]        
Revenues 36.9 31.9 77.5 69.4
Seaborne Thermal | Export | Thermal coal        
Disaggregation of Revenue [Line Items]        
Revenues 270.6 367.5 513.7 676.3
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 294.3 372.5 541.3 660.9
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 294.0 371.6 538.0 658.8
Seaborne Metallurgical | Other (2)        
Disaggregation of Revenue [Line Items]        
Revenues 0.3 0.9 3.3 2.1
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 294.0 371.6 538.0 658.8
Powder River Basin        
Disaggregation of Revenue [Line Items]        
Revenues 221.9 259.7 476.0 565.0
Powder River Basin | Thermal coal        
Disaggregation of Revenue [Line Items]        
Revenues 221.4 259.8 475.5 564.8
Powder River Basin | Metallurgical coal        
Disaggregation of Revenue [Line Items]        
Revenues 0.0 0.0 0.0 0.0
Powder River Basin | Other (2)        
Disaggregation of Revenue [Line Items]        
Revenues 0.5 (0.1) 0.5 0.2
Powder River Basin | Domestic | Thermal coal        
Disaggregation of Revenue [Line Items]        
Revenues 221.4 259.8 475.5 564.8
Powder River Basin | Export | Thermal coal        
Disaggregation of Revenue [Line Items]        
Revenues 0.0 0.0 0.0 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 202.0 199.9 393.6 449.3
Other U.S. Thermal | Thermal coal        
Disaggregation of Revenue [Line Items]        
Revenues 200.3 198.2 378.3 446.1
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 (2)        
Disaggregation of Revenue [Line Items]        
Revenues 1.7 1.7 15.3 3.2
Other U.S. Thermal | Domestic | Thermal coal        
Disaggregation of Revenue [Line Items]        
Revenues 200.3 198.2 378.3 446.1
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 16.3 37.2 23.3 211.6
Corporate and Other (1) | Thermal coal        
Disaggregation of Revenue [Line Items]        
Revenues 0.0 0.0 0.0 0.0
Corporate and Other (1) | Trading and Brokerage Coal Deliveries        
Disaggregation of Revenue [Line Items]        
Revenues 12.1 23.3 14.2 107.8
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 (2)        
Disaggregation of Revenue [Line Items]        
Revenues 16.3 37.2 23.3 211.6
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