Note 8 - Stock-based Compensation - Summary of Outstanding Stock Options by Exercise Price Range (Details)	12 Months Ended
	Dec. 31, 2020 $ / shares shares
Number of Options (in shares) | shares	484,500
Weighted- Average Remaining Contractual Life (Year)	3 years 255 days
Weighted-Average Exercise Price (in dollars per share)	$ 3.57
Range One [Member]
Lower Range Limit (in dollars per share)	0.966
Upper Range Limit (in dollars per share)	$ 1.2727
Number of Options (in shares) | shares	88,000
Weighted- Average Remaining Contractual Life (Year)	6 years 328 days
Weighted-Average Exercise Price (in dollars per share)	$ 0.97
Range Two [Member]
Lower Range Limit (in dollars per share)	1.288
Upper Range Limit (in dollars per share)	$ 3
Number of Options (in shares) | shares	132,000
Weighted- Average Remaining Contractual Life (Year)	4 years 219 days
Weighted-Average Exercise Price (in dollars per share)	$ 2.26
Range Three [Member]
Lower Range Limit (in dollars per share)	3.011
Upper Range Limit (in dollars per share)	$ 4.5454
Number of Options (in shares) | shares	120,000
Weighted- Average Remaining Contractual Life (Year)	2 years
Weighted-Average Exercise Price (in dollars per share)	$ 3.72
Range Four [Member]
Lower Range Limit (in dollars per share)	4.555
Upper Range Limit (in dollars per share)	$ 8.1616
Number of Options (in shares) | shares	144,500
Weighted- Average Remaining Contractual Life (Year)	2 years 146 days
Weighted-Average Exercise Price (in dollars per share)	$ 6.24