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Percentage points added to the reference rate to compute the variable rate on the debt instrument.

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Description of the interest rate as being fixed or variable, and, if variable, identification of the index or rate on which the interest rate is based and the number of points or percentage added to that index or rate to set the rate, and other pertinent information, such as frequency of rate resets.

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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 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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Amount of investments, and noncurrent assets classified as other.

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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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Investments Textuals (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended	12 Months Ended
Investments Textuals (Details) - USD ($) $ in Millions	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