Note 2 - Investments (Details Textual) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Equity Securities, FV-NI	$ 6,200	$ 6,600
Equity Securities, FV-NI, Cost	9,800	7,700
Available-for-sale Equity Securities, Acquired During Period, Cost Basis	2,700	2,400
Marketable Equity Securities Received for Sale of Mineral Interest	2,257	0	$ 0
Proceeds from Sale of Available-for-sale Securities, Equity	900
Equity Securities, FV-NI, Unrealized Gain (Loss), Total	$ 2,400	$ 2,800