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Nature of Operations and Basis of Presentation (Narrative) (Details) T in Millions, $ in Millions | 3 Months Ended | 6 Months Ended | 12 Months Ended |
--- | --- | --- | --- | ---
Jun. 30, 2021 USD ($) a | Jun. 30, 2021 USD ($) operatingSegment | Dec. 31, 2020 T | Jun. 17, 2021 USD ($)
Organization, Consolidation and Presentation of Financial Statements [Abstract] |  |  |  |
Number of operating segments | operatingSegment |  | 3 |  |
Midwest AgEnergy |  |  |  |
Long-term Purchase Commitment [Line Items] |  |  |  |
Investment | $ 5.0 | $ 5.0 |  |
Coal Mining | Bisti |  |  |  |
Long-term Purchase Commitment [Line Items] |  |  |  |
Early contract termination fee |  |  |  | $ 10.0
Coal Mining | Falkirk |  |  |  |
Long-term Purchase Commitment [Line Items] |  |  |  |
Cash received from sale |  |  |  | $ 14.0
Coal Mining | Falkirk | Maximum |  |  |  |
Long-term Purchase Commitment [Line Items] |  |  |  |
Possible additional payment from cancelation of agreement | 8.0 | 8.0 |  |
Coal Mining | Falkirk | Minimum |  |  |  |
Long-term Purchase Commitment [Line Items] |  |  |  |
Possible additional payment from cancelation of agreement | $ 2.0 | $ 2.0 |  |
Coal Mining | Falkirk | Long-term Contract with Customer |  |  |  |
Long-term Purchase Commitment [Line Items] |  |  |  |
Historical lignite coal annual delivery | T |  |  | 0.3 |
Minerals Management |  |  |  |
Long-term Purchase Commitment [Line Items] |  |  |  |
Gross acres acquired | a | 14,100 |  |  |
Net royalty acres acquired | a | 1,700 |  |  |
Minerals Management | Eagle Ford Basin |  |  |  |
Long-term Purchase Commitment [Line Items] |  |  |  |
Payments to acquire royalty interests | $ 4.7 |  |  |
Additional payment to acquire assets | 0.6 |  |  |
Total additional payment to acquire assets | 1.2 |  |  |
Minerals Management | Delaware Basin |  |  |  |
Long-term Purchase Commitment [Line Items] |  |  |  |
Payments to acquire royalty interests | $ 0.3 |  |  |

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