Changes in tax reserves.

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Cash received from Equity Interest during the period.

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This item represents an other than temporary decline in value that has been recognized against an investment accounted for under the equity method of accounting. The excess of the carrying amount over the fair value of the investment represents the amount of the write down which is or was reflected in earnings. The written down value is a new cost basis with the adjusted value of the investment becoming its new carrying value subject to the equity accounting method. Evidence of a loss in value might include, but would not necessarily be limited to, absence of an ability to recover the carrying amount of the investment or inability of the investee to sustain an earnings capacity which would justify the carrying amount of the investment.

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The percentage of ownership of common stock or equity participation in the investee accounted for under the equity method of accounting.

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The amount of current assets reported by an equity method investment of the entity.

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The amount of current liabilities reported by an equity method investment of the entity.

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The amount of net income (loss) reported by an equity method investment of the entity.

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The amount of noncurrent assets reported by an equity method investment of the entity.

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The amount of noncurrent liabilities reported by an equity method investment of the entity.

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The amount of revenue from sale of goods and services reduced by sales returns, allowances, and discounts reported by an equity method investment of the entity.

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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.

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No definition available.

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Investments Textuals (Details) - USD ($)	3 Months Ended	9 Months Ended	12 Months Ended
Investments Textuals (Details) - USD ($)	Apr. 01, 2017	Dec. 31, 2017	Dec. 31, 2018	Dec. 31, 2016
Successor
Schedule of Equity Method Investments [Line Items]
Changes in tax reserves		$ 7,200,000	$ 2,100,000
Equity method investment		0	$ 0
Successor \| Middlemount Mine
Schedule of Equity Method Investments [Line Items]
Ownership percentage of equity method investment			50.00%
Successor \| Financing Receivable \| Middlemount Mine
Schedule of Equity Method Investments [Line Items]
Financing Receivable, Cash received from Equity Interest		48,100,000	$ 106,700,000
Successor \| Equity method investment and financing receivables related to Middlemount
Schedule of Equity Method Investments [Line Items]
Ownership percentage of equity method investment			50.00%
Successor \| Middlemount Mine
Schedule of Equity Method Investments [Line Items]
Equity Method Investment, Summarized Financial Information, Revenue		193,000,000	$ 271,000,000
Changes in tax reserves			17,000,000
Equity Method Investment, Summarized Financial Information, Net Income (Loss)			9,000,000
Predecessor
Schedule of Equity Method Investments [Line Items]
Changes in tax reserves	$ (9,200,000)			$ 2,300,000
Equity method investment	0			0
Predecessor \| Financing Receivable \| Middlemount Mine
Schedule of Equity Method Investments [Line Items]
Financing Receivable, Cash received from Equity Interest	32,700,000
Predecessor \| Middlemount Mine
Schedule of Equity Method Investments [Line Items]
Equity Method Investment, Summarized Financial Information, Current Assets		61,700,000	33,600,000
Equity Method Investment, Summarized Financial Information, Revenue	$ 60,000,000			$ 183,000,000
Equity Method Investment, Summarized Financial Information, Noncurrent Assets		232,200,000	181,800,000
Equity Method Investment, Summarized Financial Information, Current Liabilities		313,900,000	187,900,000
Equity Method Investment, Summarized Financial Information, Noncurrent Liabilities		$ 41,200,000	$ 35,800,000