Note 8 - Stockholders' Equity - Summary of Warrant Activity (Details) - Warrant [Member] - $ / shares	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Warrants outstanding, beginning of year (in shares)	5,960	6,291
Warrants outstanding, beginning of year, weighted average exercise price (in dollars per share)	$ 4.46	$ 5.34
Warrants granted (in shares)	1,113	1,258
Warrants granted, weighted average exercise price (in dollars per share)	$ 4.24	$ 4.77
Warrants exercised (in shares)
Warrants exercised, weighted average exercise price (in dollars per share)
Warrants expired (in shares)	(2,183)	(1,589)
Warrants expired, weighted average exercise price (in dollars per share)	$ 4.06	$ 8.19
Warrants outstanding, end of year (in shares)	4,890	5,960
Warrants outstanding, end of year, weighted average exercise price (in dollars per share)	$ 4.58	$ 4.46
Warrants exercisable (in shares)	4,890	5,960
Warrants exercisable, weighted average exercise price (in dollars per share)