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Revenue Recognition - Revenue by Product Type and Market (Details) - USD ($)
$ in Millions
 3 Months Ended
Mar. 31, 2022
Mar. 31, 2021
Disaggregation of Revenue [Line Items]    
Revenue $ 691.4 $ 651.3
Thermal coal    
Disaggregation of Revenue [Line Items]    
Revenue 702.3 551.2
Metallurgical coal    
Disaggregation of Revenue [Line Items]    
Revenue 318.0 86.6
Coal [Member]    
Disaggregation of Revenue [Line Items]    
Revenue 1,039.0 656.0
Other (2)    
Disaggregation of Revenue [Line Items]    
Revenue (328.9) 13.5
Domestic | Thermal coal    
Disaggregation of Revenue [Line Items]    
Revenue 491.8 419.3
Export | Thermal coal    
Disaggregation of Revenue [Line Items]    
Revenue 210.5 131.9
Export | Metallurgical coal    
Disaggregation of Revenue [Line Items]    
Revenue 318.0 86.6
Seaborne Thermal Mining    
Disaggregation of Revenue [Line Items]    
Revenue 251.2 176.4
Seaborne Thermal Mining | Thermal coal    
Disaggregation of Revenue [Line Items]    
Revenue 250.9 176.0
Seaborne Thermal Mining | Metallurgical coal    
Disaggregation of Revenue [Line Items]    
Revenue 0.0 0.0
Seaborne Thermal Mining | Other (2)    
Disaggregation of Revenue [Line Items]    
Revenue 0.3 0.4
Seaborne Thermal Mining | Domestic | Thermal coal    
Disaggregation of Revenue [Line Items]    
Revenue 40.4 44.1
Seaborne Thermal Mining | Export | Thermal coal    
Disaggregation of Revenue [Line Items]    
Revenue 210.5 131.9
Seaborne Thermal Mining | Export | Metallurgical coal    
Disaggregation of Revenue [Line Items]    
Revenue 0.0 0.0
Seaborne Metallurgical Mining    
Disaggregation of Revenue [Line Items]    
Revenue 321.3 87.5
Seaborne Metallurgical Mining | Thermal coal    
Disaggregation of Revenue [Line Items]    
Revenue 0.0 0.0
Seaborne Metallurgical Mining | Metallurgical coal    
Disaggregation of Revenue [Line Items]    
Revenue 318.0 86.6
Seaborne Metallurgical Mining | Other (2)    
Disaggregation of Revenue [Line Items]    
Revenue 3.3 0.9
Seaborne Metallurgical Mining | Domestic | Thermal coal    
Disaggregation of Revenue [Line Items]    
Revenue 0.0 0.0
Seaborne Metallurgical Mining | Export | Thermal coal    
Disaggregation of Revenue [Line Items]    
Revenue 0.0 0.0
Seaborne Metallurgical Mining | Export | Metallurgical coal    
Disaggregation of Revenue [Line Items]    
Revenue 318.0 86.6
Powder River Basin Mining    
Disaggregation of Revenue [Line Items]    
Revenue 251.2 228.4
Powder River Basin Mining | Thermal coal    
Disaggregation of Revenue [Line Items]    
Revenue 251.5 228.4
Powder River Basin Mining | Metallurgical coal    
Disaggregation of Revenue [Line Items]    
Revenue 0.0 0.0
Powder River Basin Mining | Other (2)    
Disaggregation of Revenue [Line Items]    
Revenue (0.3) 0.0
Powder River Basin Mining | Domestic | Thermal coal    
Disaggregation of Revenue [Line Items]    
Revenue 251.5 228.4
Powder River Basin Mining | Export | Thermal coal    
Disaggregation of Revenue [Line Items]    
Revenue 0.0 0.0
Powder River Basin Mining | Export | Metallurgical coal    
Disaggregation of Revenue [Line Items]    
Revenue 0.0 0.0
Other U.S. Thermal Mining    
Disaggregation of Revenue [Line Items]    
Revenue 203.1 149.3
Other U.S. Thermal Mining | Thermal coal    
Disaggregation of Revenue [Line Items]    
Revenue 199.9 146.8
Other U.S. Thermal Mining | Metallurgical coal    
Disaggregation of Revenue [Line Items]    
Revenue 0.0 0.0
Other U.S. Thermal Mining | Other (2)    
Disaggregation of Revenue [Line Items]    
Revenue 3.2 2.5
Other U.S. Thermal Mining | Domestic | Thermal coal    
Disaggregation of Revenue [Line Items]    
Revenue 199.9 146.8
Other U.S. Thermal Mining | Export | Thermal coal    
Disaggregation of Revenue [Line Items]    
Revenue 0.0 0.0
Other U.S. Thermal Mining | Export | Metallurgical coal    
Disaggregation of Revenue [Line Items]    
Revenue 0.0 0.0
Corporate and Other (1)    
Disaggregation of Revenue [Line Items]    
Revenue (335.4) 9.7
Corporate and Other (1) | Thermal coal    
Disaggregation of Revenue [Line Items]    
Revenue 0.0 0.0
Corporate and Other (1) | Metallurgical coal    
Disaggregation of Revenue [Line Items]    
Revenue 0.0 0.0
Corporate and Other (1) | Other (2)    
Disaggregation of Revenue [Line Items]    
Revenue 335.4 (9.7)
Corporate and Other (1) | Domestic | Thermal coal    
Disaggregation of Revenue [Line Items]    
Revenue 0.0 0.0
Corporate and Other (1) | Export | Thermal coal    
Disaggregation of Revenue [Line Items]    
Revenue 0.0 0.0
Corporate and Other (1) | Export | Metallurgical coal    
Disaggregation of Revenue [Line Items]    
Revenue $ 0.0 $ 0.0