Note 3 - Investments (Details Textual) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Equity Securities, FV-NI	$ 15,100	$ 6,200
Equity Securities, FV-NI, Cost	9,900	9,800
Available-for-sale Equity Securities, Acquired During Period, Cost Basis	2,200	2,700
Marketable Equity Securities Received for Sale of Mineral Interest	0	2,257	$ 0
Long-term Investments, Total	15,148	6,207
Equity Securities, FV-NI, Unrealized Gain (Loss), Total	10,300	(2,400)
Other Assets, Current	19,114	$ 12,038
Revision of Prior Period, Adjustment [Member]
Long-term Investments, Total	(4,200)
Other Assets, Current	$ 4,200