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Revenue Recognition - Revenue by Product Type and Market (Details) - USD ($)
$ in Millions
 3 Months Ended 9 Months Ended
Sep. 30, 2019
Sep. 30, 2018
Sep. 30, 2019
Sep. 30, 2018
Disaggregation of Revenue [Line Items]        
Revenues $ 1,106.4 $ 1,412.6 $ 3,506.0 $ 4,184.7
Thermal coal        
Disaggregation of Revenue [Line Items]        
Revenues 902.4 1,039.8 2,573.6 2,891.5
Metallurgical coal        
Disaggregation of Revenue [Line Items]        
Revenues 215.4 369.4 829.4 1,251.9
Other        
Disaggregation of Revenue [Line Items]        
Revenues (11.4) 3.4 103.0 41.3
Domestic | Thermal coal        
Disaggregation of Revenue [Line Items]        
Revenues 687.8 769.0 1,953.6 2,213.5
Export | Thermal coal        
Disaggregation of Revenue [Line Items]        
Revenues 214.6 270.8 620.0 678.0
Export | Metallurgical coal        
Disaggregation of Revenue [Line Items]        
Revenues 215.4 369.4 829.4 1,251.9
Seaborne Thermal Mining | Operating Segments        
Disaggregation of Revenue [Line Items]        
Revenues 249.5 305.1 720.7 773.9
Seaborne Thermal Mining | Operating Segments | Thermal coal        
Disaggregation of Revenue [Line Items]        
Revenues 249.4 305.1 719.6 773.3
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 0.1 0.0 1.1 0.6
Seaborne Thermal Mining | Operating Segments | Domestic | Thermal coal        
Disaggregation of Revenue [Line Items]        
Revenues 34.8 37.4 110.9 112.0
Seaborne Thermal Mining | Operating Segments | Export | Thermal coal        
Disaggregation of Revenue [Line Items]        
Revenues 214.6 267.7 608.7 661.3
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 216.3 370.3 831.7 1,254.0
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 215.4 369.4 829.4 1,251.9
Seaborne Metallurgical Mining | Operating Segments | Other        
Disaggregation of Revenue [Line Items]        
Revenues 0.9 0.9 2.3 2.1
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 215.4 369.4 829.4 1,251.9
Powder River Basin Mining | Operating Segments        
Disaggregation of Revenue [Line Items]        
Revenues 333.6 373.7 903.5 1,084.5
Powder River Basin Mining | Operating Segments | Thermal coal        
Disaggregation of Revenue [Line Items]        
Revenues 333.6 373.7 903.4 1,084.4
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 0.0 0.0 0.1 0.1
Powder River Basin Mining | Operating Segments | Domestic | Thermal coal        
Disaggregation of Revenue [Line Items]        
Revenues 333.6 373.7 903.4 1,084.4
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 176.0 208.5 522.6 607.7
Midwestern U.S. Mining | Operating Segments | Thermal coal        
Disaggregation of Revenue [Line Items]        
Revenues 175.8 208.4 522.3 607.5
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 0.2 0.1 0.3 0.2
Midwestern U.S. Mining | Operating Segments | Domestic | Thermal coal        
Disaggregation of Revenue [Line Items]        
Revenues 175.8 208.4 522.3 606.2
Midwestern U.S. Mining | Operating Segments | Export | Thermal coal        
Disaggregation of Revenue [Line Items]        
Revenues 0.0 0.0 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 150.4 156.1 448.2 439.4
Western U.S. Mining | Operating Segments | Thermal coal        
Disaggregation of Revenue [Line Items]        
Revenues 143.6 152.6 428.3 426.3
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 6.8 3.5 19.9 13.1
Western U.S. Mining | Operating Segments | Domestic | Thermal coal        
Disaggregation of Revenue [Line Items]        
Revenues 143.6 149.5 417.0 410.9
Western U.S. Mining | Operating Segments | Export | Thermal coal        
Disaggregation of Revenue [Line Items]        
Revenues 0.0 3.1 11.3 15.4
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 (19.4) (1.1) 79.3 25.2
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 (19.4) (1.1) 79.3 25.2
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