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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

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