Note 1 - Significant Accounting Policies and Recent Accounting Pronouncements (Details Textual) - USD ($) $ / shares in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Dec. 31, 2019
Equity Securities, FV-NI	$ 499,610		$ 500,642
Increase (Decrease) in Accrued Royalty Income, Percentage	(33.00%)
Accounts Receivable, Allowance for Credit Loss, Current	$ 0
Impairment of Long-Lived Assets Held-for-use	0	$ 0
Accrued Royalty Income	71,162		84,880
Deposit Assets, Stumpage Agreements	$ 87,300		$ 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	80,025		$ 77,857
Mutual Fund [Member]
Equity Securities, FV-NI	$ 499,610		$ 500,642