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Fair Value Disclosure (Narrative) (Details) - USD ($) $ in Thousands | 3 Months Ended | 12 Months Ended |
--- | --- | --- | ---
Sep. 30, 2022 | Dec. 31, 2023 | Dec. 31, 2022 | Dec. 31, 2021
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
(Gain) loss on equity securities |  | $ 1,958 | $ (283) |
Long-term debt |  | 35,844 | 18,900 |
Fair Value, Nonrecurring | MLMC |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Non-cash asset impairment charge |  | 65,900 |  |
Fair Value, Nonrecurring | Coal Mining customer |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Non-cash asset impairment charge |  | 60,800 |  |
Fair Value, Nonrecurring | Mineral Management |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Non-cash asset impairment charge |  | 5,100 |  |
Quoted Prices in Active Markets for Identical Assets (Level 1) | Fair Value, Measurements, Recurring |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
(Gain) loss on equity securities |  | 400 | 1,900 |
Quoted Prices in Active Markets for Identical Assets (Level 1) | Fair Value, Measurements, Recurring | Reported Value Measurement |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Equity securities cost |  |  |  | $ 2,000
Quoted Prices in Active Markets for Identical Assets (Level 1) | Fair Value, Measurements, Recurring | Bellaire |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
(Gain) loss on equity securities |  | 1,600 | (2,200) |
Significant Unobservable Inputs (Level 3) | Fair Value, Nonrecurring |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Carrying amount of long-lived asset after impairment | $ 0 |  |  |
Significant Unobservable Inputs (Level 3) | Fair Value, Nonrecurring | Mineral Management |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Non-cash asset impairment charge | $ 3,900 |  |  |
Significant Other Observable Inputs (Level 2) |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Long-term debt fair value |  | $ 35,300 | $ 18,100 |

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