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