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