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Revenues - Revenue by Contract Duration (Details) - USD ($) $ in Millions |  | 3 Months Ended | 9 Months Ended | 12 Months Ended
--- | --- | --- | --- | ---
Apr. 01, 2017 | Dec. 31, 2018 | Sep. 30, 2018 | Jun. 30, 2018 | Mar. 31, 2018 | Dec. 31, 2017 | Sep. 30, 2017 | Jun. 30, 2017 | Apr. 01, 2017 | Mar. 31, 2017 | Dec. 31, 2017 | Dec. 31, 2018 | Dec. 31, 2016
Successor |  |  |  |  |  |  |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Revenues |  | $ 1,397.1 | $ 1,412.6 | $ 1,309.4 | $ 1,462.7 | $ 1,517.1 | $ 1,477.2 | $ 1,258.3 |  |  | $ 4,252.6 | $ 5,581.8 |
Successor | Powder River Basin Mining |  |  |  |  |  |  |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Revenues |  |  |  |  |  |  |  |  |  |  | 1,178.7 | 1,424.8 |
Successor | Midwestern U.S. Mining |  |  |  |  |  |  |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Revenues |  |  |  |  |  |  |  |  |  |  | 592.3 | 801.0 |
Successor | Western U.S. Mining |  |  |  |  |  |  |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Revenues |  |  |  |  |  |  |  |  |  |  | 440.7 | 592.0 |
Successor | Seaborne Metallurgical Mining |  |  |  |  |  |  |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Revenues |  |  |  |  |  |  |  |  |  |  | 1,221.0 | 1,553.0 |
Successor | Seaborne Thermal Mining |  |  |  |  |  |  |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Revenues |  |  |  |  |  |  |  |  |  |  | 772.5 | 1,099.2 |
Successor | Corporate and Other |  |  |  |  |  |  |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Revenues |  |  |  |  |  |  |  |  |  |  | 47.4 | 111.8 |
Successor | One year or longer |  |  |  |  |  |  |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Revenues |  |  |  |  |  |  |  |  |  |  | 3,358.3 | 4,427.3 |
Successor | Less than one year |  |  |  |  |  |  |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Revenues |  |  |  |  |  |  |  |  |  |  | 830.0 | 1,010.0 |
Successor | Other |  |  |  |  |  |  |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Revenues |  |  |  |  |  |  |  |  |  |  | 64.3 | 144.5 |
Successor | Operating Segments | Powder River Basin Mining |  |  |  |  |  |  |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Revenues |  |  |  |  |  |  |  |  |  |  | 1,178.7 | 1,424.8 |
Successor | Operating Segments | Midwestern U.S. Mining |  |  |  |  |  |  |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Revenues |  |  |  |  |  |  |  |  |  |  | 592.3 | 801.0 |
Successor | Operating Segments | Western U.S. Mining |  |  |  |  |  |  |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Revenues |  |  |  |  |  |  |  |  |  |  | 440.7 | 592.0 |
Successor | Operating Segments | Seaborne Metallurgical Mining |  |  |  |  |  |  |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Revenues |  |  |  |  |  |  |  |  |  |  | 1,221.0 | 1,553.0 |
Successor | Operating Segments | Seaborne Thermal Mining |  |  |  |  |  |  |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Revenues |  |  |  |  |  |  |  |  |  |  | 772.5 | 1,099.2 |
Successor | Operating Segments | One year or longer | Powder River Basin Mining |  |  |  |  |  |  |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Revenues |  |  |  |  |  |  |  |  |  |  | 1,023.1 | 1,283.9 |
Successor | Operating Segments | One year or longer | Midwestern U.S. Mining |  |  |  |  |  |  |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Revenues |  |  |  |  |  |  |  |  |  |  | 560.5 | 775.4 |
Successor | Operating Segments | One year or longer | Western U.S. Mining |  |  |  |  |  |  |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Revenues |  |  |  |  |  |  |  |  |  |  | 404.6 | 531.8 |
Successor | Operating Segments | One year or longer | Seaborne Metallurgical Mining |  |  |  |  |  |  |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Revenues |  |  |  |  |  |  |  |  |  |  | 867.1 | 1,036.7 |
Successor | Operating Segments | One year or longer | Seaborne Thermal Mining |  |  |  |  |  |  |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Revenues |  |  |  |  |  |  |  |  |  |  | 503.0 | 799.5 |
Successor | Operating Segments | Less than one year | Powder River Basin Mining |  |  |  |  |  |  |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Revenues |  |  |  |  |  |  |  |  |  |  | 151.2 | 140.9 |
Successor | Operating Segments | Less than one year | Midwestern U.S. Mining |  |  |  |  |  |  |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Revenues |  |  |  |  |  |  |  |  |  |  | 31.5 | 25.3 |
Successor | Operating Segments | Less than one year | Western U.S. Mining |  |  |  |  |  |  |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Revenues |  |  |  |  |  |  |  |  |  |  | 24.4 | 33.4 |
Successor | Operating Segments | Less than one year | Seaborne Metallurgical Mining |  |  |  |  |  |  |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Revenues |  |  |  |  |  |  |  |  |  |  | 353.9 | 511.9 |
Successor | Operating Segments | Less than one year | Seaborne Thermal Mining |  |  |  |  |  |  |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Revenues |  |  |  |  |  |  |  |  |  |  | 269.0 | 298.5 |
Successor | Operating Segments | Other | Powder River Basin Mining |  |  |  |  |  |  |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Revenues |  |  |  |  |  |  |  |  |  |  | 4.4 | 0.0 |
Successor | Operating Segments | Other | Midwestern U.S. Mining |  |  |  |  |  |  |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Revenues |  |  |  |  |  |  |  |  |  |  | 0.3 | 0.3 |
Successor | Operating Segments | Other | Western U.S. Mining |  |  |  |  |  |  |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Revenues |  |  |  |  |  |  |  |  |  |  | 11.7 | 26.8 |
Successor | Operating Segments | Other | Seaborne Metallurgical Mining |  |  |  |  |  |  |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Revenues |  |  |  |  |  |  |  |  |  |  | 0.0 | 4.4 |
Successor | Operating Segments | Other | Seaborne Thermal Mining |  |  |  |  |  |  |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Revenues |  |  |  |  |  |  |  |  |  |  | 0.5 | 1.2 |
Successor | Corporate, Non-Segment | Corporate and Other |  |  |  |  |  |  |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Revenues |  |  |  |  |  |  |  |  |  |  | 47.4 | 111.8 |
Successor | Corporate, Non-Segment | One year or longer | Corporate and Other |  |  |  |  |  |  |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Revenues |  |  |  |  |  |  |  |  |  |  | 0.0 | 0.0 |
Successor | Corporate, Non-Segment | Less than one year | Corporate and Other |  |  |  |  |  |  |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Revenues |  |  |  |  |  |  |  |  |  |  | 0.0 | 0.0 |
Successor | Corporate, Non-Segment | Other | Corporate and Other |  |  |  |  |  |  |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Revenues |  |  |  |  |  |  |  |  |  |  | $ 47.4 | $ 111.8 |
Predecessor |  |  |  |  |  |  |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Revenues | $ 0.0 |  |  |  |  |  |  |  | $ 1,326.2 | $ 1,326.2 |  |  | $ 4,715.3
Predecessor | Powder River Basin Mining |  |  |  |  |  |  |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Revenues |  |  |  |  |  |  |  |  | 394.3 |  |  |  | 1,473.3
Predecessor | Midwestern U.S. Mining |  |  |  |  |  |  |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Revenues |  |  |  |  |  |  |  |  | 193.2 |  |  |  | 792.5
Predecessor | Western U.S. Mining |  |  |  |  |  |  |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Revenues |  |  |  |  |  |  |  |  | 149.7 |  |  |  | 526.0
Predecessor | Seaborne Metallurgical Mining |  |  |  |  |  |  |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Revenues |  |  |  |  |  |  |  |  | 328.9 |  |  |  | 1,090.4
Predecessor | Seaborne Thermal Mining |  |  |  |  |  |  |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Revenues |  |  |  |  |  |  |  |  | 224.8 |  |  |  | 824.9
Predecessor | Corporate and Other |  |  |  |  |  |  |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Revenues |  |  |  |  |  |  |  |  | 35.3 |  |  |  | 8.2
Predecessor | One year or longer |  |  |  |  |  |  |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Revenues |  |  |  |  |  |  |  |  | 1,054.9 |  |  |  | 3,784.9
Predecessor | Less than one year |  |  |  |  |  |  |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Revenues |  |  |  |  |  |  |  |  | 215.2 |  |  |  | 910.1
Predecessor | Other |  |  |  |  |  |  |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Revenues |  |  |  |  |  |  |  |  | 56.1 |  |  |  | 20.3
Predecessor | Operating Segments | Powder River Basin Mining |  |  |  |  |  |  |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Revenues |  |  |  |  |  |  |  |  | 394.3 |  |  |  | 1,473.3
Predecessor | Operating Segments | Midwestern U.S. Mining |  |  |  |  |  |  |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Revenues |  |  |  |  |  |  |  |  | 193.2 |  |  |  | 792.5
Predecessor | Operating Segments | Western U.S. Mining |  |  |  |  |  |  |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Revenues |  |  |  |  |  |  |  |  | 149.7 |  |  |  | 526.0
Predecessor | Operating Segments | Seaborne Metallurgical Mining |  |  |  |  |  |  |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Revenues |  |  |  |  |  |  |  |  | 328.9 |  |  |  | 1,090.4
Predecessor | Operating Segments | Seaborne Thermal Mining |  |  |  |  |  |  |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Revenues |  |  |  |  |  |  |  |  | 224.8 |  |  |  | 824.9
Predecessor | Operating Segments | One year or longer | Powder River Basin Mining |  |  |  |  |  |  |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Revenues |  |  |  |  |  |  |  |  | 357.7 |  |  |  | 1,364.0
Predecessor | Operating Segments | One year or longer | Midwestern U.S. Mining |  |  |  |  |  |  |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Revenues |  |  |  |  |  |  |  |  | 193.2 |  |  |  | 791.8
Predecessor | Operating Segments | One year or longer | Western U.S. Mining |  |  |  |  |  |  |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Revenues |  |  |  |  |  |  |  |  | 129.3 |  |  |  | 478.8
Predecessor | Operating Segments | One year or longer | Seaborne Metallurgical Mining |  |  |  |  |  |  |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Revenues |  |  |  |  |  |  |  |  | 240.6 |  |  |  | 686.1
Predecessor | Operating Segments | One year or longer | Seaborne Thermal Mining |  |  |  |  |  |  |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Revenues |  |  |  |  |  |  |  |  | 134.1 |  |  |  | 464.2
Predecessor | Operating Segments | Less than one year | Powder River Basin Mining |  |  |  |  |  |  |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Revenues |  |  |  |  |  |  |  |  | 36.6 |  |  |  | 108.5
Predecessor | Operating Segments | Less than one year | Midwestern U.S. Mining |  |  |  |  |  |  |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Revenues |  |  |  |  |  |  |  |  | 0.0 |  |  |  | 1.6
Predecessor | Operating Segments | Less than one year | Western U.S. Mining |  |  |  |  |  |  |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Revenues |  |  |  |  |  |  |  |  | 4.2 |  |  |  | 37.3
Predecessor | Operating Segments | Less than one year | Seaborne Metallurgical Mining |  |  |  |  |  |  |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Revenues |  |  |  |  |  |  |  |  | 84.0 |  |  |  | 403.5
Predecessor | Operating Segments | Less than one year | Seaborne Thermal Mining |  |  |  |  |  |  |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Revenues |  |  |  |  |  |  |  |  | 90.4 |  |  |  | 359.2
Predecessor | Operating Segments | Other | Powder River Basin Mining |  |  |  |  |  |  |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Revenues |  |  |  |  |  |  |  |  | 0.0 |  |  |  | 0.8
Predecessor | Operating Segments | Other | Midwestern U.S. Mining |  |  |  |  |  |  |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Revenues |  |  |  |  |  |  |  |  | 0.0 |  |  |  | (0.9)
Predecessor | Operating Segments | Other | Western U.S. Mining |  |  |  |  |  |  |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Revenues |  |  |  |  |  |  |  |  | 16.2 |  |  |  | 9.9
Predecessor | Operating Segments | Other | Seaborne Metallurgical Mining |  |  |  |  |  |  |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Revenues |  |  |  |  |  |  |  |  | 4.3 |  |  |  | 0.8
Predecessor | Operating Segments | Other | Seaborne Thermal Mining |  |  |  |  |  |  |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Revenues |  |  |  |  |  |  |  |  | 0.3 |  |  |  | 1.5
Predecessor | Corporate, Non-Segment | Corporate and Other |  |  |  |  |  |  |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Revenues |  |  |  |  |  |  |  |  | 35.3 |  |  |  | 8.2
Predecessor | Corporate, Non-Segment | One year or longer | Corporate and Other |  |  |  |  |  |  |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Revenues |  |  |  |  |  |  |  |  | 0.0 |  |  |  | 0.0
Predecessor | Corporate, Non-Segment | Less than one year | Corporate and Other |  |  |  |  |  |  |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Revenues |  |  |  |  |  |  |  |  | 0.0 |  |  |  | 0.0
Predecessor | Corporate, Non-Segment | Other | Corporate and Other |  |  |  |  |  |  |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Revenues |  |  |  |  |  |  |  |  | $ 35.3 |  |  |  | $ 8.2

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- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + References No definition available. + Details
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- Definition Amount of revenue recognized from goods sold, services rendered, insurance premiums, or other activities that constitute an earning process. Includes, but is not limited to, investment and interest income before deduction of interest expense when recognized as a component of revenue, and sales and trading gain (loss). + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 225 -SubTopic 10 -Section S99 -Paragraph 2 -Subparagraph (SX 210.5-03.1) -URI http://asc.fasb.org/extlink&oid=63488584&loc=d3e20235-122688 + Details
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