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Revenue Recognition - Revenue by Product Type and Market (Details) - USD ($)
$ in Millions
 3 Months Ended 9 Months Ended
Sep. 30, 2024
Sep. 30, 2023
Sep. 30, 2024
Sep. 30, 2023
Disaggregation of Revenue [Line Items]        
Revenues $ 1,088.0 $ 1,078.9 $ 3,113.6 $ 3,711.7
Designated as Hedging Instrument        
Disaggregation of Revenue [Line Items]        
Other Comprehensive Income (Loss), Cash Flow Hedge, Gain (Loss), Reclassification, before Tax 3.7 19.1 0.4 147.6
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net (0.1) (21.9) (2.5) (77.3)
Derivative, Gain (Loss) on Derivative, Net 3.6 (2.8) (2.1) 70.3
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 0.0 0.0 159.0
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       159.0
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net 0.0 0.0 0.0 (80.9)
Derivative, Gain (Loss) on Derivative, Net       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   19.6   (11.5)
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net   (19.7)   11.5
Derivative, Gain (Loss) on Derivative, Net 0.0 (0.1) 0.0 0.0
Thermal coal        
Disaggregation of Revenue [Line Items]        
Revenues 829.4 837.6 2,274.4 2,594.2
Metallurgical coal        
Disaggregation of Revenue [Line Items]        
Revenues 240.4 246.0 778.4 904.8
Other (2)        
Disaggregation of Revenue [Line Items]        
Revenues 18.2 (4.7) 60.8 212.7
Domestic | Thermal coal        
Disaggregation of Revenue [Line Items]        
Revenues 557.0 571.7 1,488.3 1,652.0
Export | Thermal coal        
Disaggregation of Revenue [Line Items]        
Revenues 272.4 265.9 786.1 942.2
Export | Metallurgical coal        
Disaggregation of Revenue [Line Items]        
Revenues 240.4 246.0 778.4 904.8
Seaborne Thermal        
Disaggregation of Revenue [Line Items]        
Revenues 313.2 297.4 904.6 1,043.4
Seaborne Thermal | Thermal coal        
Disaggregation of Revenue [Line Items]        
Revenues 311.2 297.2 902.4 1,042.9
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 2.0 0.2 2.2 0.5
Seaborne Thermal | Domestic | Thermal coal        
Disaggregation of Revenue [Line Items]        
Revenues 38.8 31.3 116.3 100.7
Seaborne Thermal | Export | Thermal coal        
Disaggregation of Revenue [Line Items]        
Revenues 272.4 265.9 786.1 942.2
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 242.5 247.0 783.8 907.9
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 240.4 246.0 778.4 904.8
Seaborne Metallurgical | Other (2)        
Disaggregation of Revenue [Line Items]        
Revenues 2.1 1.0 5.4 3.1
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 240.4 246.0 778.4 904.8
Powder River Basin        
Disaggregation of Revenue [Line Items]        
Revenues 305.3 313.0 781.3 878.0
Powder River Basin | Thermal coal        
Disaggregation of Revenue [Line Items]        
Revenues 304.2 313.3 779.7 878.1
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 1.1 (0.3) 1.6 (0.1)
Powder River Basin | Domestic | Thermal coal        
Disaggregation of Revenue [Line Items]        
Revenues 304.2 313.3 779.7 878.1
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 216.7 228.2 610.3 677.5
Other U.S. Thermal | Thermal coal        
Disaggregation of Revenue [Line Items]        
Revenues 214.0 227.1 592.3 673.2
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 2.7 1.1 18.0 4.3
Other U.S. Thermal | Domestic | Thermal coal        
Disaggregation of Revenue [Line Items]        
Revenues 214.0 227.1 592.3 673.2
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 10.3 (6.7) 33.6 204.9
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 5.9 (10.4) 20.1 97.4
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 10.3 (6.7) 33.6 204.9
Corporate and Other (1) | Other revenue        
Disaggregation of Revenue [Line Items]        
Revenues 4.4 3.8 13.5 29.4
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