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Principles of Consolidation and Nature of Operations (Narrative) (Details) $ in Thousands |  |  | 3 Months Ended | 12 Months Ended
--- | --- | --- | --- | ---
Dec. 01, 2022 USD ($) | May 02, 2022 USD ($) | Dec. 31, 2023 USD ($) a | Dec. 31, 2023 USD ($) a segment | Dec. 31, 2022 USD ($) a
Long-term Purchase Commitment [Line Items] |  |  |  |  |
Number of operating segments | segment |  |  |  | 3 |
Long-lived asset impairment charge |  |  | $ 65,900 | $ 65,887 | $ 3,939
Acquisition of mineral and royalty interest |  |  |  | $ 36,714 | $ 11,924
Gross acreage | a |  |  |  | 43,400 | 13,600
Net acreage | a |  |  |  | 2,500 | 880
Gross acreage of mineral and royalty interest | a |  |  | 184,700 | 184,700 |
Net acreage of mineral and royalty interest | a |  |  | 63,300 | 63,300 |
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) Excluding Service Cost, Statement of Income or Comprehensive Income [Extensible Enumeration] |  |  |  | Other Nonoperating Income (Expense) |
Other contract termination settlements |  |  |  | $ 0 | $ 14,000
Proceeds from the sale of private company equity units |  |  |  | 3,574 | 18,628
Pension Plan |  |  |  |  |
Long-term Purchase Commitment [Line Items] |  |  |  |  |
Settlements |  |  |  | 1,815 | 0
Private Equity Funds |  |  |  |  |
Long-term Purchase Commitment [Line Items] |  |  |  |  |
Income from equity method investee |  |  |  |  | 2,200
HLCP |  |  |  |  |
Long-term Purchase Commitment [Line Items] |  |  |  |  |
Proceeds from the sale of private company equity units | $ 18,600 |  |  |  |
Loss on disposal of equity method investment |  |  |  |  | 1,300
Post-closing purchase price adjustment |  |  |  | 3,600 |
Coal Mining customer | Falkirk |  |  |  |  |
Long-term Purchase Commitment [Line Items] |  |  |  |  |
Total gain on contract termination |  |  |  |  | 30,900
Other contract termination settlements |  | $ 14,000 |  |  |
Fair value gained from contract termination |  | 4,100 |  |  |
Fair value of equity securities without readily determinable fair value, gained as a result of terminated contract |  | 12,800 |  |  |
Investment |  | $ 5,000 |  |  |
Mineral and Royalty Interest |  |  |  |  |
Long-term Purchase Commitment [Line Items] |  |  |  |  |
Acquisition of mineral and royalty interest |  |  |  | $ 36,700 | $ 11,400

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