Note 8 - Stockholders' Equity - Summary of Warrants Outstanding and Exercisable (Details) - Warrant [Member] - $ / shares	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Exercise prices (in dollars per share)		$ 4.46	$ 5.34
Warrants Outstanding (in shares)	4,890	5,960	6,291
Weighted Avg. Remaining Life (Year)	1 year 259 days
Weighted Average [Member]
Exercise prices (in dollars per share)	$ 4.58
Minimum [Member]
Exercise prices (in dollars per share)	4.24
Maximum [Member]
Exercise prices (in dollars per share)	4.77
Exercise Price Range 1 [Member]
Exercise prices (in dollars per share)	$ 4.24
Warrants Outstanding (in shares)	1,113
Weighted Avg. Remaining Life (Year)	3 years
Exercise Price Range 1 [Member] | Weighted Average [Member]
Exercise prices (in dollars per share)	$ 4.24
Exercise Price Range 2 [Member]
Exercise prices (in dollars per share)	$ 4.64
Warrants Outstanding (in shares)	2,519
Weighted Avg. Remaining Life (Year)	1 year
Exercise Price Range 2 [Member] | Weighted Average [Member]
Exercise prices (in dollars per share)	$ 4.64
Exercise Price Range 3 [Member]
Exercise prices (in dollars per share)	$ 4.77
Warrants Outstanding (in shares)	1,258
Weighted Avg. Remaining Life (Year)	2 years
Exercise Price Range 3 [Member] | Weighted Average [Member]
Exercise prices (in dollars per share)	$ 4.77