Note 2 - Revenues from Contracts with Customers - Disaggregation of Revenue (Details) - Mineral Rights Segment [Member] - USD ($) $ in Thousands	3 Months Ended	6 Months Ended
Jun. 30, 2026	Jun. 30, 2025	Jun. 30, 2026	Jun. 30, 2025
Operating Segments [Member]
Revenue from contract with customers	$ 52,508	$ 46,324	$ 99,213	$ 101,469
Coal Royalty Revenues [Member]
Revenue from contract with customers	34,793	31,543	64,367	67,041
Production Lease Minimum Revenues [Member]
Revenue from contract with customers	251	123	809	2,848
Minimum Lease Straight-line Revenues [Member]
Revenue from contract with customers	4,019	4,050	8,038	8,100
Oil and Gas Royalty Revenues [Member]
Revenue from contract with customers	2,447	1,981	3,833	4,425
Carbon Neutral Revenues [Member]
Revenue from contract with customers	94	290	279	885
Property Tax Revenues [Member]
Revenue from contract with customers	1,710	1,519	3,421	3,156
Wheelage Revenues [Member]
Revenue from contract with customers	1,959	2,543	3,949	4,281
Coal Overriding Royalty Revenues [Member]
Revenue from contract with customers	1,040	456	2,426	1,336
Lease Amendment Revenues [Member]
Revenue from contract with customers	1,242	656	2,442	1,311
Aggregates Royalty Revenues [Member]
Revenue from contract with customers	1,246	906	2,364	1,759
Other Revenues [Member]
Revenue from contract with customers	318	228	488	413
Royalty and Other Mineral Rights [Member]
Revenue from contract with customers	49,119	44,295	92,416	95,555
Transportation and Processing Services [Member]
Revenue from contract with customers	$ 3,389	$ 2,029	$ 6,797	$ 5,914