Note 4 - Investments (Details Textual) - USD ($)	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Available-for-sale Securities, Current, Total	$ 278,000		$ 278,000		$ 343,000	[1]
Proceeds from Sale and Maturity of Marketable Securities, Total			0		$ 0
Available-for-sale Securities, Gross Unrealized Loss, Total	97,000		65,000
Other Comprehensive Income (Loss), Securities, Available-for-Sale, Unrealized Holding Gain (Loss) Arising During Period, after Tax		43,000		(35,000)
Other Comprehensive Income (Loss), Securities, Available-for-Sale, Unrealized Holding Gain (Loss) Arising During Period, Tax		$ 22,000		$ (8,000)

[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.