Note 4 - Investments (Details Textual) - USD ($)	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Available-for-sale Securities, Current, Total	$ 289,000		$ 289,000		$ 343,000	[1]
Proceeds from Sale and Maturity of Marketable Securities, Total					$ 0
Debt Securities, Available-for-sale, Realized Gain			3,000
Debt Securities, Available-for-sale, Unrealized Gain	22,000
Other Comprehensive Income (Loss), Securities, Available-for-Sale, Unrealized Holding Gain (Loss) Arising During Period, after Tax		$ (208,000)		$ (243,000)
Other Comprehensive Income (Loss), Securities, Available-for-Sale, Unrealized Holding Gain (Loss) Arising During Period, Tax		$ (105,000)		(113,000)
Available-for-sale Securities, Gross Unrealized Loss, Total			$ 43,000

[1] The condensed consolidated balance sheet as of December 31, 2017 has been prepared using information from the audited consolidated balance sheet as of that date.