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Revenue Recognition - Revenue by Product Type and Market (Details) - USD ($)
$ in Millions
 3 Months Ended 9 Months Ended
Sep. 30, 2020
Sep. 30, 2019
Sep. 30, 2020
Sep. 30, 2019
Disaggregation of Revenue [Line Items]        
Revenues $ 671.0 $ 1,106.4 $ 2,143.9 $ 3,506.0
Thermal coal        
Disaggregation of Revenue [Line Items]        
Revenues 599.9 902.4 1,763.0 2,573.6
Metallurgical coal        
Disaggregation of Revenue [Line Items]        
Revenues 78.4 215.4 362.3 829.4
Other        
Disaggregation of Revenue [Line Items]        
Revenues (7.3) (11.4) 18.6 103.0
Domestic | Thermal coal        
Disaggregation of Revenue [Line Items]        
Revenues 474.2 687.8 1,350.1 1,953.6
Export | Thermal coal        
Disaggregation of Revenue [Line Items]        
Revenues 125.7 214.6 412.9 620.0
Export | Metallurgical coal        
Disaggregation of Revenue [Line Items]        
Revenues 78.4 215.4 362.3 829.4
Seaborne Thermal Mining        
Disaggregation of Revenue [Line Items]        
Revenues 163.0 249.5 526.1 720.7
Seaborne Thermal Mining | Thermal coal        
Disaggregation of Revenue [Line Items]        
Revenues 162.8 249.4 524.8 719.6
Seaborne Thermal Mining | Metallurgical coal        
Disaggregation of Revenue [Line Items]        
Revenues 0.0 0.0 0.0 0.0
Seaborne Thermal Mining | Other        
Disaggregation of Revenue [Line Items]        
Revenues 0.2 0.1 1.3 1.1
Seaborne Thermal Mining | Domestic | Thermal coal        
Disaggregation of Revenue [Line Items]        
Revenues 37.1 34.8 111.9 110.9
Seaborne Thermal Mining | Export | Thermal coal        
Disaggregation of Revenue [Line Items]        
Revenues 125.7 214.6 412.9 608.7
Seaborne Thermal Mining | Export | Metallurgical coal        
Disaggregation of Revenue [Line Items]        
Revenues 0.0 0.0 0.0 0.0
Seaborne Metallurgical Mining        
Disaggregation of Revenue [Line Items]        
Revenues 78.8 216.3 363.6 831.7
Seaborne Metallurgical Mining | Thermal coal        
Disaggregation of Revenue [Line Items]        
Revenues 0.0 0.0 0.0 0.0
Seaborne Metallurgical Mining | Metallurgical coal        
Disaggregation of Revenue [Line Items]        
Revenues 78.4 215.4 362.3 829.4
Seaborne Metallurgical Mining | Other        
Disaggregation of Revenue [Line Items]        
Revenues 0.4 0.9 1.3 2.3
Seaborne Metallurgical Mining | Domestic | Thermal coal        
Disaggregation of Revenue [Line Items]        
Revenues 0.0 0.0 0.0 0.0
Seaborne Metallurgical Mining | Export | Thermal coal        
Disaggregation of Revenue [Line Items]        
Revenues 0.0 0.0 0.0 0.0
Seaborne Metallurgical Mining | Export | Metallurgical coal        
Disaggregation of Revenue [Line Items]        
Revenues 78.4 215.4 362.3 829.4
Powder River Basin Mining        
Disaggregation of Revenue [Line Items]        
Revenues 264.8 333.6 737.2 903.5
Powder River Basin Mining | Thermal coal        
Disaggregation of Revenue [Line Items]        
Revenues 264.3 333.6 736.7 903.4
Powder River Basin Mining | Metallurgical coal        
Disaggregation of Revenue [Line Items]        
Revenues 0.0 0.0 0.0 0.0
Powder River Basin Mining | Other        
Disaggregation of Revenue [Line Items]        
Revenues 0.5 0.0 0.5 0.1
Powder River Basin Mining | Domestic | Thermal coal        
Disaggregation of Revenue [Line Items]        
Revenues 264.3 333.6 736.7 903.4
Powder River Basin Mining | Export | Thermal coal        
Disaggregation of Revenue [Line Items]        
Revenues 0.0 0.0 0.0 0.0
Powder River Basin Mining | Export | Metallurgical coal        
Disaggregation of Revenue [Line Items]        
Revenues 0.0 0.0 0.0 0.0
Other U.S. Thermal Mining        
Disaggregation of Revenue [Line Items]        
Revenues 179.8 326.4 524.1 970.8
Other U.S. Thermal Mining | Thermal coal        
Disaggregation of Revenue [Line Items]        
Revenues 172.8 319.4 501.5 950.6
Other U.S. Thermal Mining | Metallurgical coal        
Disaggregation of Revenue [Line Items]        
Revenues 0.0 0.0 0.0 0.0
Other U.S. Thermal Mining | Other        
Disaggregation of Revenue [Line Items]        
Revenues 7.0 7.0 22.6 20.2
Other U.S. Thermal Mining | Domestic | Thermal coal        
Disaggregation of Revenue [Line Items]        
Revenues 172.8 319.4 501.5 939.3
Other U.S. Thermal Mining | Export | Thermal coal        
Disaggregation of Revenue [Line Items]        
Revenues 0.0 0.0 0.0 11.3
Other U.S. Thermal Mining | Export | Metallurgical coal        
Disaggregation of Revenue [Line Items]        
Revenues 0.0 0.0 0.0 0.0
Corporate and Other        
Disaggregation of Revenue [Line Items]        
Revenues (15.4) (19.4) (7.1) 79.3
Corporate and Other | Thermal coal        
Disaggregation of Revenue [Line Items]        
Revenues 0.0 0.0 0.0 0.0
Corporate and Other | Metallurgical coal        
Disaggregation of Revenue [Line Items]        
Revenues 0.0 0.0 0.0 0.0
Corporate and Other | Other        
Disaggregation of Revenue [Line Items]        
Revenues (15.4) (19.4) (7.1) 79.3
Corporate and Other | Domestic | Thermal coal        
Disaggregation of Revenue [Line Items]        
Revenues 0.0 0.0 0.0 0.0
Corporate and Other | Export | Thermal coal        
Disaggregation of Revenue [Line Items]        
Revenues 0.0 0.0 0.0 0.0
Corporate and Other | Export | Metallurgical coal        
Disaggregation of Revenue [Line Items]        
Revenues $ 0.0 $ 0.0 $ 0.0 $ 0.0