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Note 1 - Significant Accounting Policies and Recent Accounting Pronouncements (Details Textual) - USD ($)
$ / shares in Thousands
 6 Months Ended 12 Months Ended
Jun. 30, 2020
Jun. 30, 2019
Dec. 31, 2019
Equity Securities, FV-NI $ 502,314   $ 500,642
Accounts Receivable, Allowance for Credit Loss, Current 0    
Impairment of Long-Lived Assets Held-for-use 0    
Accrued Royalty Income 78,753   84,880
Deposit Assets, Stumpage Agreements $ 137,300   $ 87,300
Earnings Per Share, Diluted, Total (in dollars per share) $ 0   $ 0
Payments of Ordinary Dividends, Common Stock $ 0 $ 0  
Oil and Gas [Member]      
Capitalized Contract Cost, Net, Total 0    
Surface Leases [Member] | Accounts Payable and Accrued Liabilities [Member]      
Contract with Customer, Liability, Total 94,706   $ 77,857
Mutual Fund [Member]      
Equity Securities, FV-NI $ 502,314   $ 500,642