Note 4 - Investments (Details Textual) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019	Dec. 31, 2019	[1]
Available-for-sale Securities, Current, Total	$ 270,000		$ 270,000		$ 335,000
Proceeds from Sale and Maturity of Marketable Securities, Total	0	$ 0
Debt Securities, Available-for-sale, Realized Gain (Loss), Total		439,000	17,000	$ 55,000
Debt Securities, Available-for-sale, Unrealized Gain (Loss), Total		387,000	35,000	554,000
Debt Securities, Available-for-sale, Realized Gain		52,000
Debt Securities, Available-for-sale, Gain (Loss), Total	$ 41,000	$ 439,000	$ (18,000)	$ 609,000

[1] The condensed consolidated balance sheet as of December 31, 2019 has been prepared using information from the audited consolidated balance sheet as of that date.