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Summary of Significant Accounting Policies Discussion Textuals (Details) - USD ($) $ in Millions |  | 3 Months Ended | 9 Months Ended | 12 Months Ended
--- | --- | --- | --- | ---
Nov. 28, 2017 | Apr. 01, 2017 | Dec. 31, 2017 | Dec. 31, 2018 | Dec. 31, 2016 | Dec. 31, 2015
Summary of Significant Accounting Policies Textuals |  |  |  |  |  |
Period considered to treat short-term, highly liquid investments as cash equivalents |  |  | three months or less |  |  |
Accounts Receivable Invoicing Terms |  |  | Customers are invoiced as coal is shipped or at periodic intervals in accordance with contractual terms |  |  |
Recognition of tax benefits from uncertain tax positions |  |  | greater than fifty percent likelihood of being realized upon ultimate settlement |  |  |
Buildings and improvements [Member] |  |  |  |  |  |
Summary of Significant Accounting Policies Textuals |  |  |  |  |  |
Estimated useful life |  |  | 2 to 28 |  |  |
Machinery and equipment |  |  |  |  |  |
Summary of Significant Accounting Policies Textuals |  |  |  |  |  |
Estimated useful life |  |  | 2 to 28 |  |  |
Leasehold improvements [Member] |  |  |  |  |  |
Summary of Significant Accounting Policies Textuals |  |  |  |  |  |
Estimated useful life |  |  | Shorter of Useful Life or Remaining Life of Lease |  |  |
Minimum | Buildings and improvements [Member] |  |  |  |  |  |
Summary of Significant Accounting Policies Textuals |  |  |  |  |  |
Estimated useful life |  |  | 2 |  |  |
Minimum | Machinery and equipment |  |  |  |  |  |
Summary of Significant Accounting Policies Textuals |  |  |  |  |  |
Estimated useful life |  |  | 2 |  |  |
Maximum | Buildings and improvements [Member] |  |  |  |  |  |
Summary of Significant Accounting Policies Textuals |  |  |  |  |  |
Estimated useful life |  |  | 28 |  |  |
Maximum | Machinery and equipment |  |  |  |  |  |
Summary of Significant Accounting Policies Textuals |  |  |  |  |  |
Estimated useful life |  |  | 28 |  |  |
Scenario, Forecast |  |  |  |  |  |
Summary of Significant Accounting Policies Textuals |  |  |  |  |  |
New Accounting Pronouncement or Change in Accounting Principle, Effect of Change on Net Income |  |  |  | $ 15.0 |  |
Predecessor |  |  |  |  |  |
Summary of Significant Accounting Policies Textuals |  |  |  |  |  |
Other-than-temporary impairment losses on equity and cost method investments |  | $ 0.0 |  |  |  | $ 276.5
Restructuring charges for voluntary and involuntary workforce reductions |  | 0.0 |  |  | $ 15.5 | 23.5
Asset impairment charges related to long-lived assets |  | 30.5 |  |  | 247.9 | 1,001.3
Foreign currency remeasurement gain (loss) |  | 10.6 |  |  | (7.4) | (6.4)
Foreign currency translation adjustment |  | $ 5.5 |  |  | $ (1.8) | $ (34.9)
Successor |  |  |  |  |  |
Summary of Significant Accounting Policies Textuals |  |  |  |  |  |
Other-than-temporary impairment losses on equity and cost method investments |  |  | $ 0.0 |  |  |
Restructuring charges for voluntary and involuntary workforce reductions | $ 3.0 |  | 7.6 |  |  |
Asset impairment charges related to long-lived assets |  |  | 0.0 |  |  |
Foreign currency remeasurement gain (loss) |  |  | 0.7 |  |  |
Foreign currency translation adjustment |  |  | $ 1.4 |  |  |
Successor | Interest In Middlemount Coal Pty Limited [Member] |  |  |  |  |  |
Summary of Significant Accounting Policies Textuals |  |  |  |  |  |
Ownership percentage of equity method investment |  |  | 50.00% |  |  |

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