Note 4 - Investments - Available-for-sale Marketable Securities (Details) - 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
Unrealized loss recognized in earnings since January 1, 2018	$ (97,000)		$ (65,000)
Available-for-sale Securities, Current, Total	278,000		278,000		$ 343,000	[1]
Marketable Securities [Member]
Cost basis	543,000		543,000		543,000
Gains previously recognized on warrants			380,000		380,000
Loss included in accumulated other comprehensive loss or reclassified to retained earnings on Jaunuary 1, 2018			(580,000)		(580,000)
Unrealized loss recognized in earnings since January 1, 2018			(65,000)
Available-for-sale Securities, Current, Total	$ 278,000		$ 278,000		$ 343,000

[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.