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Note 8 - Mineral Rights, Net - Composition of Mineral Rights (Details) - USD ($)
$ in Thousands
Sep. 30, 2023
Dec. 31, 2022
Mineral Properties, carrying value $ 695,833 $ 695,971
Mineral Properties, accumulated depletion (295,285) (283,659)
Mineral Properties, net book value 400,548 412,312
Coal Properties [Member]    
Mineral Properties, carrying value 661,680 661,812
Mineral Properties, accumulated depletion (279,988) (269,037)
Mineral Properties, net book value 381,692 392,775
Aggregates Properties [Member]    
Mineral Properties, carrying value 8,655 8,655
Mineral Properties, accumulated depletion (3,723) (3,410)
Mineral Properties, net book value 4,932 5,245
Oil and Gas Royalty Properties [Member]    
Mineral Properties, carrying value 12,354 12,354
Mineral Properties, accumulated depletion (9,962) (9,600)
Mineral Properties, net book value 2,392 2,754
Other [Member]    
Mineral Properties, carrying value 13,144 13,150
Mineral Properties, accumulated depletion (1,612) (1,612)
Mineral Properties, net book value $ 11,532 $ 11,538