Note 9 - Stock-based Compensation - Shares Authorized Under Stock Option Plan (Details)	12 Months Ended
	Dec. 31, 2020 $ / shares shares
Number of shares outstanding (in shares) | shares	844,455
Weighted average remaining contractual life (Year)	3 years 7 months 28 days
Weighted average exercise price (in dollars per share)	$ 2.00
Price Range 1 [Member]
Exercise price, lower range (in dollars per share)	0.49
Exercise price, upper range (in dollars per share)	$ 1.00
Number of shares outstanding (in shares) | shares	3,571
Weighted average remaining contractual life (Year)	10 months 28 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	110,449
Weighted average remaining contractual life (Year)	9 months 10 days
Weighted average exercise price (in dollars per share)	$ 1.19
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	676,367
Weighted average remaining contractual life (Year)	4 years 25 days
Weighted average exercise price (in dollars per share)	$ 1.95
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	54,068
Weighted average remaining contractual life (Year)	4 years 7 months 20 days
Weighted average exercise price (in dollars per share)	$ 4.49