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Investments Textuals (Details) - USD ($) $ in Millions | 3 Months Ended | 9 Months Ended | 12 Months Ended
--- | --- | --- | ---
Apr. 01, 2017 | Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Predecessor |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |
Equity method investment |  |  |  | $ 276.5
Predecessor | Subordinated Borrowing, Name [Domain] |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |
Equity method investment |  |  | $ 229.9 | 229.9
Predecessor | Interest In Middlemount Coal Pty Limited [Member] |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |
Equity method investment |  |  |  | 276.5
Predecessor | Middlemount Mine [Member] |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |
Equity Method Investment, Summarized Financial Information, Current Assets |  |  | 47.3 |
Equity Method Investment, Summarized Financial Information, Revenue | $ 60.0 |  | 183.0 | 160.0
Equity method investment |  |  |  | 46.6
Equity Method Investment, Summarized Financial Information, Noncurrent Assets |  |  | 263.4 |
Equity Method Investment, Summarized Financial Information, Current Liabilities |  |  | 363.5 |
Equity Method Investment, Summarized Financial Information, Noncurrent Liabilities |  |  | 50.3 |
Successor | Interest In Middlemount Coal Pty Limited [Member] |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |
Ownership percentage of equity method investment |  | 50.00% |  |
Successor | Middlemount Mine [Member] |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |
Equity Method Investment, Summarized Financial Information, Current Assets |  | $ 61.7 |  |
Equity Method Investment, Summarized Financial Information, Revenue |  | 193.0 |  |
Equity Method Investment, Summarized Financial Information, Noncurrent Assets |  | 232.2 |  |
Equity Method Investment, Summarized Financial Information, Current Liabilities |  | 313.9 |  |
Equity Method Investment, Summarized Financial Information, Noncurrent Liabilities |  | $ 41.2 |  |
Middlemount Mine [Member] | Successor |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |
Ownership percentage of equity method investment |  | 50.00% |  |
Middlemount Mine [Member] | Successor |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |
Basis spread on variable rate |  | 3.50% |  |
Debt Instrument, Interest Rate Terms |  | 0.15 |  |
Financing Receivable [Member] | Middlemount Mine [Member] | Predecessor |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |
Investments and Other Noncurrent Assets |  |  | 84.8 | $ 65.2
Financing Receivable, Cash received from Equity Interest | $ 31.0 |  | $ 40.6 |
Financing Receivable [Member] | Middlemount Mine [Member] | Successor |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |
Financing Receivable, Cash received from Equity Interest |  | $ 48.0 |  |
Intercompany Loans |  | 50.0 |  |
Intercompany Loans, Carrying Value |  | $ 0.0 |  |

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