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Note 2 - Revenue From Contracts With Customers - Assets and Liabilities (Details) - USD ($)
$ in Thousands
 9 Months Ended
Sep. 30, 2023
Sep. 30, 2022
Dec. 31, 2022
Current portion of deferred revenue $ 6,399   $ 6,256
Deferred revenue 35,076   40,181
Balance 46,437 $ 61,862  
Increase due to minimums and lease amendment fees 11,525 11,309  
Recognition of previously deferred revenue (16,487) (28,403)  
Balance 41,475 $ 44,768  
Coal Royalty Revenues [Member]      
Current portion of deferred revenue 6,399   6,256
Deferred revenue 35,076   40,181
Coal Royalty Revenues [Member] | Accounts Receivable [Member]      
Contract with customer, current 34,775   39,004
Coal Royalty Revenues [Member] | Other Current Assets [Member]      
Contract with customer, current [1] 2,382   0
Coal Royalty Revenues [Member] | Other Noncurrent Assets [Member]      
Contract with customer, noncurrent [2] $ 0   $ 75
[1] Other current assets, net includes short-term notes receivables from contracts with customers.
[2] Other long-term assets, net includes long-term lease amendment fee receivables from contracts with customers.