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Revenue Recognition - Revenue by Contract Duration (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Disaggregation of Revenue [Line Items]      
Revenues $ 2,881.1 $ 4,623.4 $ 5,581.8
Seaborne Thermal Mining      
Disaggregation of Revenue [Line Items]      
Revenues 711.8 971.7 1,099.2
Seaborne Metallurgical Mining      
Disaggregation of Revenue [Line Items]      
Revenues 486.5 1,033.1 1,553.0
Powder River Basin Mining      
Disaggregation of Revenue [Line Items]      
Revenues 991.1 1,228.7 1,424.8
Other U.S. Thermal Mining      
Disaggregation of Revenue [Line Items]      
Revenues 707.3 1,309.4 1,393.0
Corporate and Other      
Disaggregation of Revenue [Line Items]      
Revenues (15.6) 80.5 111.8
One year or longer      
Disaggregation of Revenue [Line Items]      
Revenues 2,336.6 3,759.6 4,427.3
One year or longer | Seaborne Thermal Mining      
Disaggregation of Revenue [Line Items]      
Revenues 349.2 589.2 799.5
One year or longer | Seaborne Metallurgical Mining      
Disaggregation of Revenue [Line Items]      
Revenues 371.0 828.6 1,036.7
One year or longer | Powder River Basin Mining      
Disaggregation of Revenue [Line Items]      
Revenues 945.0 1,087.6 1,283.9
One year or longer | Other U.S. Thermal Mining      
Disaggregation of Revenue [Line Items]      
Revenues 671.4 1,254.2 1,307.2
One year or longer | Corporate and Other      
Disaggregation of Revenue [Line Items]      
Revenues 0.0 0.0 0.0
Less than one year      
Disaggregation of Revenue [Line Items]      
Revenues 527.1 735.1 1,010.0
Less than one year | Seaborne Thermal Mining      
Disaggregation of Revenue [Line Items]      
Revenues 361.1 381.1 298.5
Less than one year | Seaborne Metallurgical Mining      
Disaggregation of Revenue [Line Items]      
Revenues 113.3 201.4 511.9
Less than one year | Powder River Basin Mining      
Disaggregation of Revenue [Line Items]      
Revenues 48.9 121.3 140.9
Less than one year | Other U.S. Thermal Mining      
Disaggregation of Revenue [Line Items]      
Revenues 3.8 31.3 58.7
Less than one year | Corporate and Other      
Disaggregation of Revenue [Line Items]      
Revenues 0.0 0.0 0.0
Other      
Disaggregation of Revenue [Line Items]      
Revenues 17.4 128.7 144.5
Other | Seaborne Thermal Mining      
Disaggregation of Revenue [Line Items]      
Revenues 1.5 1.4 1.2
Other | Seaborne Metallurgical Mining      
Disaggregation of Revenue [Line Items]      
Revenues 2.2 3.1 4.4
Other | Powder River Basin Mining      
Disaggregation of Revenue [Line Items]      
Revenues (2.8) 19.8 0.0
Other | Other U.S. Thermal Mining      
Disaggregation of Revenue [Line Items]      
Revenues 32.1 23.9 27.1
Other | Corporate and Other      
Disaggregation of Revenue [Line Items]      
Revenues $ (15.6) $ 80.5 $ 111.8