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Revenue Recognition - Revenue by Product Type and Market (Details) - USD ($)
$ in Millions
 3 Months Ended 6 Months Ended
Jun. 30, 2019
Jun. 30, 2018
Jun. 30, 2019
Jun. 30, 2018
Disaggregation of Revenue [Line Items]        
Revenues $ 1,149.0 $ 1,309.4 $ 2,399.6 $ 2,772.1
Thermal coal        
Disaggregation of Revenue [Line Items]        
Revenues 804.9 921.6 1,671.2 1,851.7
Metallurgical coal        
Disaggregation of Revenue [Line Items]        
Revenues 290.3 417.2 614.0 882.5
Other        
Disaggregation of Revenue [Line Items]        
Revenues 53.8 (29.4) 114.4 37.9
Domestic | Thermal coal        
Disaggregation of Revenue [Line Items]        
Revenues 618.4 688.0 1,265.8 1,444.5
Export | Thermal coal        
Disaggregation of Revenue [Line Items]        
Revenues 186.5 233.6 405.4 407.2
Export | Metallurgical coal        
Disaggregation of Revenue [Line Items]        
Revenues 290.3 417.2 614.0 882.5
Seaborne Thermal Mining | Operating Segments        
Disaggregation of Revenue [Line Items]        
Revenues 220.2 267.4 471.2 468.8
Seaborne Thermal Mining | Operating Segments | Thermal coal        
Disaggregation of Revenue [Line Items]        
Revenues 219.9 267.2 470.2 468.2
Seaborne Thermal Mining | Operating Segments | Metallurgical coal        
Disaggregation of Revenue [Line Items]        
Revenues 0.0 0.0 0.0 0.0
Seaborne Thermal Mining | Operating Segments | Other        
Disaggregation of Revenue [Line Items]        
Revenues 0.3 0.2 1.0 0.6
Seaborne Thermal Mining | Operating Segments | Domestic | Thermal coal        
Disaggregation of Revenue [Line Items]        
Revenues 37.7 38.5 76.1 74.6
Seaborne Thermal Mining | Operating Segments | Export | Thermal coal        
Disaggregation of Revenue [Line Items]        
Revenues 182.2 228.7 394.1 393.6
Seaborne Thermal Mining | Operating Segments | Export | Metallurgical coal        
Disaggregation of Revenue [Line Items]        
Revenues 0.0 0.0 0.0 0.0
Seaborne Metallurgical Mining | Operating Segments        
Disaggregation of Revenue [Line Items]        
Revenues 290.9 417.5 615.4 883.7
Seaborne Metallurgical Mining | Operating Segments | Thermal coal        
Disaggregation of Revenue [Line Items]        
Revenues 0.0 0.0 0.0 0.0
Seaborne Metallurgical Mining | Operating Segments | Metallurgical coal        
Disaggregation of Revenue [Line Items]        
Revenues 290.3 417.2 614.0 882.5
Seaborne Metallurgical Mining | Operating Segments | Other        
Disaggregation of Revenue [Line Items]        
Revenues 0.6 0.3 1.4 1.2
Seaborne Metallurgical Mining | Operating Segments | Domestic | Thermal coal        
Disaggregation of Revenue [Line Items]        
Revenues 0.0 0.0 0.0 0.0
Seaborne Metallurgical Mining | Operating Segments | Export | Thermal coal        
Disaggregation of Revenue [Line Items]        
Revenues 0.0 0.0 0.0 0.0
Seaborne Metallurgical Mining | Operating Segments | Export | Metallurgical coal        
Disaggregation of Revenue [Line Items]        
Revenues 290.3 417.2 614.0 882.5
Powder River Basin Mining | Operating Segments        
Disaggregation of Revenue [Line Items]        
Revenues 282.6 321.5 569.9 710.8
Powder River Basin Mining | Operating Segments | Thermal coal        
Disaggregation of Revenue [Line Items]        
Revenues 282.5 321.5 569.8 710.7
Powder River Basin Mining | Operating Segments | Metallurgical coal        
Disaggregation of Revenue [Line Items]        
Revenues 0.0 0.0 0.0 0.0
Powder River Basin Mining | Operating Segments | Other        
Disaggregation of Revenue [Line Items]        
Revenues 0.1 0.0 0.1 0.1
Powder River Basin Mining | Operating Segments | Domestic | Thermal coal        
Disaggregation of Revenue [Line Items]        
Revenues 282.5 321.5 569.8 710.7
Powder River Basin Mining | Operating Segments | Export | Thermal coal        
Disaggregation of Revenue [Line Items]        
Revenues 0.0 0.0 0.0 0.0
Powder River Basin Mining | Operating Segments | Export | Metallurgical coal        
Disaggregation of Revenue [Line Items]        
Revenues 0.0 0.0 0.0 0.0
Midwestern U.S. Mining | Operating Segments        
Disaggregation of Revenue [Line Items]        
Revenues 167.5 197.5 346.6 399.2
Midwestern U.S. Mining | Operating Segments | Thermal coal        
Disaggregation of Revenue [Line Items]        
Revenues 167.5 197.5 346.5 399.1
Midwestern U.S. Mining | Operating Segments | Metallurgical coal        
Disaggregation of Revenue [Line Items]        
Revenues 0.0 0.0 0.0 0.0
Midwestern U.S. Mining | Operating Segments | Other        
Disaggregation of Revenue [Line Items]        
Revenues 0.0 0.0 0.1 0.1
Midwestern U.S. Mining | Operating Segments | Domestic | Thermal coal        
Disaggregation of Revenue [Line Items]        
Revenues 167.5 196.9 346.5 397.8
Midwestern U.S. Mining | Operating Segments | Export | Thermal coal        
Disaggregation of Revenue [Line Items]        
Revenues 0.0 0.6 0.0 1.3
Midwestern U.S. Mining | Operating Segments | Export | Metallurgical coal        
Disaggregation of Revenue [Line Items]        
Revenues 0.0 0.0 0.0 0.0
Western U.S. Mining | Operating Segments        
Disaggregation of Revenue [Line Items]        
Revenues 142.1 139.6 297.8 283.3
Western U.S. Mining | Operating Segments | Thermal coal        
Disaggregation of Revenue [Line Items]        
Revenues 135.0 135.4 284.7 273.7
Western U.S. Mining | Operating Segments | Metallurgical coal        
Disaggregation of Revenue [Line Items]        
Revenues 0.0 0.0 0.0 0.0
Western U.S. Mining | Operating Segments | Other        
Disaggregation of Revenue [Line Items]        
Revenues 7.1 4.2 13.1 9.6
Western U.S. Mining | Operating Segments | Domestic | Thermal coal        
Disaggregation of Revenue [Line Items]        
Revenues 130.7 131.1 273.4 261.4
Western U.S. Mining | Operating Segments | Export | Thermal coal        
Disaggregation of Revenue [Line Items]        
Revenues 4.3 4.3 11.3 12.3
Western U.S. Mining | Operating Segments | Export | Metallurgical coal        
Disaggregation of Revenue [Line Items]        
Revenues 0.0 0.0 0.0 0.0
Corporate and Other | Corporate, Non-Segment        
Disaggregation of Revenue [Line Items]        
Revenues 45.7 (34.1) 98.7 26.3
Corporate and Other | Corporate, Non-Segment | Thermal coal        
Disaggregation of Revenue [Line Items]        
Revenues 0.0 0.0 0.0 0.0
Corporate and Other | Corporate, Non-Segment | Metallurgical coal        
Disaggregation of Revenue [Line Items]        
Revenues 0.0 0.0 0.0 0.0
Corporate and Other | Corporate, Non-Segment | Other        
Disaggregation of Revenue [Line Items]        
Revenues 45.7 (34.1) 98.7 26.3
Corporate and Other | Corporate, Non-Segment | Domestic | Thermal coal        
Disaggregation of Revenue [Line Items]        
Revenues 0.0 0.0 0.0 0.0
Corporate and Other | Corporate, Non-Segment | Export | Thermal coal        
Disaggregation of Revenue [Line Items]        
Revenues 0.0 0.0 0.0 0.0
Corporate and Other | Corporate, Non-Segment | Export | Metallurgical coal        
Disaggregation of Revenue [Line Items]        
Revenues $ 0.0 $ 0.0 $ 0.0 $ 0.0