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Coal Trading - Trading Revenues and Balance Sheet Offsetting (Details) - Coal Trading - USD ($) $ in Millions | 3 Months Ended |
--- | --- | ---
Mar. 31, 2018 | Mar. 31, 2017 | Dec. 31, 2017
Cash Flow Hedging |  |  |  |
Coal trading derivative instruments and balance sheet offsetting disclosures: |  |  |  |
Net variation margin (held) posted |  | $ 0.0 |  |
Successor |  |  |  |
Trading Revenues by Type of Instrument [Line Items] |  |  |  |
Trading Revenues by Type of Instrument |  | 20.1 |  |
Coal trading derivative instruments and balance sheet offsetting disclosures: |  |  |  |
Assets from coal trading activities, gross amounts of recognized assets |  | 53.6 |  | $ 77.1
Liabilities from coal trading activities, gross amounts of recognized liabilities |  | (57.9) |  | (122.0)
Assets and (liabilities) from coal trading activities, net amounts recognized before the application of variation margin |  | (4.3) |  | (44.9)
Gross amounts of coal trading liabilities offset against associated coal trading assets |  | (42.4) |  | (74.5)
Gross amounts of coal trading assets offset against associated coal trading liabilities |  | 42.4 |  | 74.5
Net coal trading (liabilities) assets offset against associated assets (liabilities) |  | 0.0 |  | 0.0
Variation margin held offset against assets from coal trading activities | [1] | 0.0 |  | 0.0
Variation margin posted offset against liabilities from coal trading activities | [1] | 6.7 |  | 35.8
Net variation margin (held) posted | [1] | 6.7 |  | 35.8
Assets from coal trading activities, net |  | 11.2 |  | 2.6
Liabilities from coal trading activities, net |  | (8.8) |  | (11.7)
Net assets (liabilities) from coal trading activities |  | 2.4 |  | $ (9.1)
Successor | Futures, swaps and options |  |  |  |
Trading Revenues by Type of Instrument [Line Items] |  |  |  |
Trading Revenues by Type of Instrument |  | (16.9) |  |
Successor | Physical purchase/sale contracts |  |  |  |
Trading Revenues by Type of Instrument [Line Items] |  |  |  |
Trading Revenues by Type of Instrument |  | $ 37.0 |  |
Predecessor |  |  |  |
Trading Revenues by Type of Instrument [Line Items] |  |  |  |
Trading Revenues by Type of Instrument |  |  | $ 15.0 |
Predecessor | Futures, swaps and options |  |  |  |
Trading Revenues by Type of Instrument [Line Items] |  |  |  |
Trading Revenues by Type of Instrument |  |  | (10.2) |
Predecessor | Physical purchase/sale contracts |  |  |  |
Trading Revenues by Type of Instrument [Line Items] |  |  |  |
Trading Revenues by Type of Instrument |  |  | $ 25.2 |
[1] | None of the net variation margin posted at March 31, 2018 and December 31, 2017 related to cash flow hedges.

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