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Revenues - Revenues by Contract Duration (Details) - USD ($) $ in Millions | 3 Months Ended | 6 Months Ended | 9 Months Ended |
--- | --- | --- | --- | ---
Sep. 30, 2018 | Sep. 30, 2017 | Apr. 01, 2017 | Sep. 30, 2017 | Sep. 30, 2018 | Dec. 31, 2017
Successor |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |
Revenues |  | $ 1,412.6 | $ 1,477.2 |  | $ 2,735.5 | $ 4,184.7 |
Unrealized Loss (Gain) on Price Risk Fair Value Hedging Instruments |  | 26.8 | 10.8 |  | 1.4 | 36.3 |
Loss (Gain) on Price Risk Fair Value Hedge Ineffectiveness |  | 11.6 | 12.3 |  | 20.0 | 41.1 |
Successor | One year or longer |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |
Revenues |  | 1,116.9 | 1,152.1 |  | 2,177.8 | 3,409.7 |
Successor | Short-term contracts |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |
Revenues |  | 292.3 | 314.4 |  | 495.2 | 733.7 |
Successor | Other |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |
Revenues | [1] | 3.4 | 10.7 |  | 62.5 | 41.3 |
Successor | Operating Segments | Powder River Basin Mining |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |
Revenues |  | 373.7 | 420.9 |  | 786.3 | 1,084.5 |
Successor | Operating Segments | Powder River Basin Mining | One year or longer |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |
Revenues |  | 330.2 | 373.3 |  | 700.0 | 984.4 |
Successor | Operating Segments | Powder River Basin Mining | Short-term contracts |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |
Revenues |  | 43.5 | 47.6 |  | 82.0 | 100.0 |
Successor | Operating Segments | Powder River Basin Mining | Other |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |
Revenues | [1] | 0.0 | 0.0 |  | 4.3 | 0.1 |
Successor | Operating Segments | Midwestern U.S. Mining |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |
Revenues |  | 208.5 | 207.7 |  | 402.6 | 607.7 |
Successor | Operating Segments | Midwestern U.S. Mining | One year or longer |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |
Revenues |  | 205.3 | 199.0 |  | 386.3 | 587.0 |
Successor | Operating Segments | Midwestern U.S. Mining | Short-term contracts |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |
Revenues |  | 3.1 | 8.5 |  | 16.0 | 20.5 |
Successor | Operating Segments | Midwestern U.S. Mining | Other |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |
Revenues | [1] | 0.1 | 0.2 |  | 0.3 | 0.2 |
Successor | Operating Segments | Western U.S. Mining |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |
Revenues |  | 156.1 | 155.7 |  | 281.1 | 439.4 |
Successor | Operating Segments | Western U.S. Mining | One year or longer |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |
Revenues |  | 146.6 | 144.2 |  | 266.9 | 400.4 |
Successor | Operating Segments | Western U.S. Mining | Short-term contracts |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |
Revenues |  | 6.0 | 14.1 |  | 15.2 | 25.9 |
Successor | Operating Segments | Western U.S. Mining | Other |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |
Revenues | [1] | 3.5 | (2.6) |  | (1.0) | 13.1 |
Successor | Operating Segments | Australian Metallurgical Mining |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |
Revenues |  | 370.3 | 415.9 |  | 703.7 | 1,254.0 |
Successor | Operating Segments | Australian Metallurgical Mining | One year or longer |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |
Revenues |  | 199.5 | 270.9 |  | 524.3 | 852.1 |
Successor | Operating Segments | Australian Metallurgical Mining | Short-term contracts |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |
Revenues |  | 169.9 | 143.2 |  | 177.6 | 399.8 |
Successor | Operating Segments | Australian Metallurgical Mining | Other |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |
Revenues | [1] | 0.9 | 1.8 |  | 1.8 | 2.1 |
Successor | Operating Segments | Australian Thermal Mining |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |
Revenues |  | 305.1 | 265.8 |  | 505.0 | 773.9 |
Successor | Operating Segments | Australian Thermal Mining | One year or longer |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |
Revenues |  | 235.3 | 164.7 |  | 300.3 | 585.8 |
Successor | Operating Segments | Australian Thermal Mining | Short-term contracts |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |
Revenues |  | 69.8 | 101.0 |  | 204.4 | 187.5 |
Successor | Operating Segments | Australian Thermal Mining | Other |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |
Revenues | [1] | 0.0 | 0.1 |  | 0.3 | 0.6 |
Successor | Corporate, Non-Segment | Trading and Brokerage |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |
Revenues |  | 22.6 | 19.4 |  | 24.6 | 52.7 |
Successor | Corporate, Non-Segment | Trading and Brokerage | One year or longer |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |
Revenues |  | 0.0 | 0.0 |  | 0.0 | 0.0 |
Successor | Corporate, Non-Segment | Trading and Brokerage | Short-term contracts |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |
Revenues |  | 0.0 | 0.0 |  | 0.0 | 0.0 |
Successor | Corporate, Non-Segment | Trading and Brokerage | Other |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |
Revenues | [1] | 22.6 | 19.4 |  | 24.6 | 52.7 |
Successor | Corporate, Non-Segment | Corporate and Other |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |
Revenues | [2] | (23.7) | (8.2) |  | 32.2 | (27.5) |
Successor | Corporate, Non-Segment | Corporate and Other | One year or longer |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |
Revenues | [2] | 0.0 | 0.0 |  | 0.0 | 0.0 |
Successor | Corporate, Non-Segment | Corporate and Other | Short-term contracts |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |
Revenues | [2] | 0.0 | 0.0 |  | 0.0 | 0.0 |
Successor | Corporate, Non-Segment | Corporate and Other | Other |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |
Revenues | [1],[2] | (23.7) | $ (8.2) |  | $ 32.2 | (27.5) |
Predecessor |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |
Revenues |  |  |  | $ 1,326.2 |  |  |
Unrealized Loss (Gain) on Price Risk Fair Value Hedging Instruments |  |  |  | (16.6) |  |  |
Loss (Gain) on Price Risk Fair Value Hedge Ineffectiveness |  |  |  | 11.1 |  |  |
Predecessor | One year or longer |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |
Revenues |  |  |  | 1,054.9 |  |  |
Predecessor | Short-term contracts |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |
Revenues |  |  |  | 215.2 |  |  |
Predecessor | Other |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |
Revenues | [1] |  |  | 56.1 |  |  |
Predecessor | Operating Segments | Powder River Basin Mining |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |
Revenues |  |  |  | 394.3 |  |  |
Predecessor | Operating Segments | Powder River Basin Mining | One year or longer |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |
Revenues |  |  |  | 357.7 |  |  |
Predecessor | Operating Segments | Powder River Basin Mining | Short-term contracts |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |
Revenues |  |  |  | 36.6 |  |  |
Predecessor | Operating Segments | Powder River Basin Mining | Other |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |
Revenues | [1] |  |  | 0.0 |  |  |
Predecessor | Operating Segments | Midwestern U.S. Mining |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |
Revenues |  |  |  | 193.2 |  |  |
Predecessor | Operating Segments | Midwestern U.S. Mining | One year or longer |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |
Revenues |  |  |  | 193.2 |  |  |
Predecessor | Operating Segments | Midwestern U.S. Mining | Short-term contracts |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |
Revenues |  |  |  | 0.0 |  |  |
Predecessor | Operating Segments | Midwestern U.S. Mining | Other |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |
Revenues | [1] |  |  | 0.0 |  |  |
Predecessor | Operating Segments | Western U.S. Mining |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |
Revenues |  |  |  | 149.7 |  |  |
Predecessor | Operating Segments | Western U.S. Mining | One year or longer |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |
Revenues |  |  |  | 129.3 |  |  |
Predecessor | Operating Segments | Western U.S. Mining | Short-term contracts |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |
Revenues |  |  |  | 4.2 |  |  |
Predecessor | Operating Segments | Western U.S. Mining | Other |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |
Revenues | [1] |  |  | 16.2 |  |  |
Predecessor | Operating Segments | Australian Metallurgical Mining |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |
Revenues |  |  |  | 328.9 |  |  |
Predecessor | Operating Segments | Australian Metallurgical Mining | One year or longer |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |
Revenues |  |  |  | 240.6 |  |  |
Predecessor | Operating Segments | Australian Metallurgical Mining | Short-term contracts |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |
Revenues |  |  |  | 84.0 |  |  |
Predecessor | Operating Segments | Australian Metallurgical Mining | Other |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |
Revenues | [1] |  |  | 4.3 |  |  |
Predecessor | Operating Segments | Australian Thermal Mining |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |
Revenues |  |  |  | 224.8 |  |  |
Predecessor | Operating Segments | Australian Thermal Mining | One year or longer |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |
Revenues |  |  |  | 134.1 |  |  |
Predecessor | Operating Segments | Australian Thermal Mining | Short-term contracts |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |
Revenues |  |  |  | 90.4 |  |  |
Predecessor | Operating Segments | Australian Thermal Mining | Other |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |
Revenues | [1] |  |  | 0.3 |  |  |
Predecessor | Corporate, Non-Segment | Trading and Brokerage |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |
Revenues |  |  |  | 15.0 |  |  |
Predecessor | Corporate, Non-Segment | Trading and Brokerage | One year or longer |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |
Revenues |  |  |  | 0.0 |  |  |
Predecessor | Corporate, Non-Segment | Trading and Brokerage | Short-term contracts |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |
Revenues |  |  |  | 0.0 |  |  |
Predecessor | Corporate, Non-Segment | Trading and Brokerage | Other |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |
Revenues | [1] |  |  | 15.0 |  |  |
Predecessor | Corporate, Non-Segment | Corporate and Other |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |
Revenues | [2] |  |  | 20.3 |  |  |
Predecessor | Corporate, Non-Segment | Corporate and Other | One year or longer |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |
Revenues | [2] |  |  | 0.0 |  |  |
Predecessor | Corporate, Non-Segment | Corporate and Other | Short-term contracts |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |
Revenues | [2] |  |  | 0.0 |  |  |
Predecessor | Corporate, Non-Segment | Corporate and Other | Other |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |
Revenues | [1],[2] |  |  | $ 20.3 |  |  |
Coal Trading Positions | Successor |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |
Derivative Liability, Fair Value, Gross Liability | [3] | $ (75.2) |  |  |  | $ (75.2) | $ (38.9)
[1] | Other includes revenues from arrangements such as customer contract-related payments, royalties related to coal lease agreements, sales agency commissions, farm income and property and facility rentals, for which contract duration is not meaningful.
[2] | Corporate and Other revenue includes unrealized gains and losses related to mark-to-market activity from economic hedge activities intended to hedge future coal sales. Such net unrealized losses were $26.8 million and $10.8 million during the three months ended September 30, 2018 and 2017, respectively, and $36.3 million and $1.4 million during the nine months ended September 30, 2018 and the period April 2 through September 30, 2017, respectively. During the period January 1 through April 1, 2017, such net unrealized gains were $16.6 million. When such gains and losses are realized in connection with recognition of the underlying transaction, they are reclassified to realized gains and losses and are then reflected in Trading and Brokerage revenue (realized losses of $11.6 million and $12.3 million during the three months ended September 30, 2018 and 2017, respectively, and $41.1 million, $20.0 million and $11.1 million during the nine months ended September 30, 2018 and the periods April 2 through September 30, 2017 and January 1 through April 1, 2017, respectively). At September 30, 2018 and December 31, 2017, the financial contracts’ fair values resulted in net liabilities, excluding margin, of $75.2 million and $38.9 million, respectively.
[3] | Amounts include net liabilities of $75.2 million and $38.9 million at September 30, 2018 and December 31, 2017, respectively, representing the fair value of financial contracts used to hedge future coal sales, as further described in Note 3. “Revenue Recognition.”

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