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Note 8 - Mineral Rights, Net - Composition of Mineral Rights (Details) - USD ($)
$ in Thousands
Mar. 31, 2025
Dec. 31, 2024
Mineral Properties, carrying value $ 687,959 $ 695,113
Mineral Properties, accumulated depletion (310,714) (315,475)
Mineral Properties, net book value 377,245 379,638
Coal Properties [Member]    
Mineral Properties, carrying value 653,808 660,961
Mineral Properties, accumulated depletion (294,522) (299,404)
Mineral Properties, net book value 359,286 361,557
Aggregates Properties [Member]    
Mineral Properties, carrying value 8,655 8,655
Mineral Properties, accumulated depletion (4,139) (4,065)
Mineral Properties, net book value 4,516 4,590
Oil and Gas Royalty Properties [Member]    
Mineral Properties, carrying value 12,354 12,354
Mineral Properties, accumulated depletion (10,441) (10,394)
Mineral Properties, net book value 1,913 1,960
Other [Member]    
Mineral Properties, carrying value 13,142 13,143
Mineral Properties, accumulated depletion (1,612) (1,612)
Mineral Properties, net book value $ 11,530 $ 11,531