Note 7 - Stock-based Compensation - Shares Authorized Under Stock Option Plan (Details)	12 Months Ended
	Dec. 31, 2018 $ / shares shares
Exercise price, upper range (in dollars per share)
Number of shares outstanding (in shares) | shares	2,043,402
Weighted average remaining contractual life (Year)	5 years 215 days
Weighted average exercise price (in dollars per share)	$ 1.91
Price Range 1 [Member]
Exercise price, lower range (in dollars per share)	0.49
Exercise price, upper range (in dollars per share)	$ 1
Number of shares outstanding (in shares) | shares	3,571
Weighted average remaining contractual life (Year)	2 years 332 days
Weighted average exercise price (in dollars per share)	$ 0.49
Price Range 2 [Member]
Exercise price, lower range (in dollars per share)	1.01
Exercise price, upper range (in dollars per share)	$ 1.50
Number of shares outstanding (in shares) | shares	412,073
Weighted average remaining contractual life (Year)	1 year 332 days
Weighted average exercise price (in dollars per share)	$ 1.24
Price Range 3 [Member]
Exercise price, lower range (in dollars per share)	1.51
Exercise price, upper range (in dollars per share)	$ 2.50
Number of shares outstanding (in shares) | shares	1,549,367
Weighted average remaining contractual life (Year)	6 years 189 days
Weighted average exercise price (in dollars per share)	$ 1.94
Price Range 4 [Member]
Exercise price, lower range (in dollars per share)	2.51
Exercise price, upper range (in dollars per share)	$ 8.60
Number of shares outstanding (in shares) | shares	78,391
Weighted average remaining contractual life (Year)	6 years 251 days
Weighted average exercise price (in dollars per share)	$ 4.99