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Summary of Significant Accounting Policies Discussion Textuals (Details) - USD ($) |  | 3 Months Ended | 9 Months Ended | 12 Months Ended | 36 Months Ended |
--- | --- | --- | --- | --- | --- | ---
Nov. 28, 2017 | Dec. 31, 2019 | Sep. 30, 2019 | Apr. 01, 2017 | Dec. 31, 2017 | Dec. 31, 2019 | Dec. 31, 2018 | Dec. 31, 2019 | Apr. 02, 2017
Summary of Significant Accounting Policies Textuals |  |  |  |  |  |  |  |  |
Royalty Expense |  |  |  | $ 115,200,000 | $ 364,600,000 | $ 388,600,000 | $ 474,300,000 |  |
Minimum Annual Production Of Federal Coal Mining Leases On Original Amount |  |  |  |  |  |  | 1.00% |  |
Monthly federal royalties payable from sale using surface mining methods |  |  |  |  |  |  | 12.50% |  |
Operating Lease, Liability |  | $ 82,400,000 |  |  |  | $ 82,400,000 |  | $ 82,400,000 |
Period considered to treat short-term, highly liquid investments as cash equivalents |  |  |  |  |  | three months or less |  |  |
Other-than-temporary impairment losses on equity and cost method investments |  |  |  |  | 0 |  | $ 0 | 0 |
Recognition of tax benefits from uncertain tax positions |  |  |  |  |  | greater than fifty percent likelihood of being realized upon ultimate settlement |  |  |
Restructuring charges for voluntary and involuntary workforce reductions | $ 3,000,000.0 | 23,000,000.0 |  | 0 | 7,600,000 | $ 24,300,000 | 1,200,000 |  |
Asset impairment charges related to long-lived assets |  |  |  | 30,500,000 |  | 261,200,000 | 0 |  |
Foreign currency remeasurement gain (loss) |  |  |  | 10,600,000 | 700,000 | (2,700,000) | 1,400,000 |  |
Foreign currency translation adjustment |  |  |  | 5,500,000 | 1,400,000 | 200,000 | (5,900,000) |  |
Interest Costs Capitalized |  |  |  | 0 | 0 | 0 | 0 |  |
Equity Method Investment, Other than Temporary Impairment |  |  |  | 0 | 0 | 9,000,000.0 | 0 |  |
Asset impairment |  | 250,200,000 | $ 20,000,000.0 | $ 30,500,000 | $ 0 | 270,200,000 | $ 0 |  |
Monthly federal royalties payable of production using underground mining methods |  |  |  |  |  |  | 8.00% |  |
Restructuring Reserve, Current |  | $ 7,400,000 |  |  |  | $ 7,400,000 |  | $ 7,400,000 |
Equity method investment and financing receivables related to Middlemount |  |  |  |  |  |  |  |  |
Summary of Significant Accounting Policies Textuals |  |  |  |  |  |  |  |  |
Ownership percentage of equity method investment |  | 50.00% |  |  |  | 50.00% |  | 50.00% |
Buildings and improvements |  |  |  |  |  |  |  |  |
Summary of Significant Accounting Policies Textuals |  |  |  |  |  |  |  |  |
Estimated useful life |  |  |  |  |  | up to 29 |  |  |
Machinery and equipment |  |  |  |  |  |  |  |  |
Summary of Significant Accounting Policies Textuals |  |  |  |  |  |  |  |  |
Estimated useful life |  |  |  |  |  | 1 - 15 |  |  |
Leasehold improvements [Member] |  |  |  |  |  |  |  |  |
Summary of Significant Accounting Policies Textuals |  |  |  |  |  |  |  |  |
Estimated useful life |  |  |  |  |  | Shorter of Useful Life or Remaining Life of Lease |  |  |
Maximum | Buildings and improvements |  |  |  |  |  |  |  |  |
Summary of Significant Accounting Policies Textuals |  |  |  |  |  |  |  |  |
Estimated useful life |  |  |  |  |  |  | 27 |  |
Maximum | Machinery and equipment |  |  |  |  |  |  |  |  |
Summary of Significant Accounting Policies Textuals |  |  |  |  |  |  |  |  |
Estimated useful life |  |  |  |  |  |  | 27 |  |
Treasury Stock |  |  |  |  |  |  |  |  |
Summary of Significant Accounting Policies Textuals |  |  |  |  |  |  |  |  |
Voting Shares Received Upon Reorganization, Percent |  |  |  |  |  |  |  |  | 50.00%

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