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Nature of Operations and Basis of Presentation (Narrative) (Details) $ in Thousands, T in Millions |  | 3 Months Ended | 9 Months Ended | 12 Months Ended
--- | --- | --- | --- | ---
Jun. 17, 2021 USD ($) | Sep. 30, 2021 USD ($) | Jun. 30, 2021 USD ($) a | Sep. 30, 2020 USD ($) | Sep. 30, 2021 USD ($) operatingSegment | Sep. 30, 2020 USD ($) | Dec. 31, 2020 T
Long-term Purchase Commitment [Line Items] |  |  |  |  |  |  |
Number of operating segments | operatingSegment |  |  |  |  | 3 |  |
Contract termination settlement |  | $ 10,333 |  | $ 0 | $ 10,333 | $ 0 |
Coal Mining | Midwest AgEnergy |  |  |  |  |  |  |
Long-term Purchase Commitment [Line Items] |  |  |  |  |  |  |
Investment |  | 5,000 |  |  | 5,000 |  |
Coal Mining | Falkirk |  |  |  |  |  |  |
Long-term Purchase Commitment [Line Items] |  |  |  |  |  |  |
Contract termination settlement | $ 14,000 |  |  |  |  |  |
Coal Mining | Falkirk | Maximum |  |  |  |  |  |  |
Long-term Purchase Commitment [Line Items] |  |  |  |  |  |  |
Possible additional payment from cancelation of agreement |  | 8,000 |  |  | 8,000 |  |
Coal Mining | Falkirk | Minimum |  |  |  |  |  |  |
Long-term Purchase Commitment [Line Items] |  |  |  |  |  |  |
Possible additional payment from cancelation of agreement |  | $ 2,000 |  |  | 2,000 |  |
Coal Mining | Falkirk | Long-term Contract with Customer |  |  |  |  |  |  |
Long-term Purchase Commitment [Line Items] |  |  |  |  |  |  |
Historical lignite coal annual delivery | T |  |  |  |  |  |  | 0.3
Coal Mining | Bisti |  |  |  |  |  |  |
Long-term Purchase Commitment [Line Items] |  |  |  |  |  |  |
Contract termination settlement |  |  |  |  | $ 10,300 |  |
Minerals Management | Eagle Ford Basin |  |  |  |  |  |  |
Long-term Purchase Commitment [Line Items] |  |  |  |  |  |  |
Gross acres acquired | a |  |  | 14,100 |  |  |  |
Net royalty acres acquired | a |  |  | 1,700 |  |  |  |
Payments to acquire royalty interests |  |  | $ 4,700 |  |  |  |
Additional payment to acquire assets |  |  | 600 |  |  |  |
Total additional payment to acquire assets |  |  | 1,200 |  |  |  |
Minerals Management | Delaware Basin |  |  |  |  |  |  |
Long-term Purchase Commitment [Line Items] |  |  |  |  |  |  |
Payments to acquire royalty interests |  |  | $ 300 |  |  |  |

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