Note 4 - Stock-based Compensation - Summarizes Information about Stock Options (Details)	12 Months Ended
	Dec. 31, 2019 $ / shares shares
Options Outstanding (in shares)	2,052,976
Weighted Average Contractual Life (Year)	7 years 255 days
Options Exercisable (in shares)	858,667
Price Range One [Member]
Exercise Price or Range of Exercise Price, Lower Limit (in dollars per share) | $ / shares	$ 1.85
Exercise Price or Range of Exercise Price, Upper Limit (in dollars per share) | $ / shares	$ 5.89
Options Outstanding (in shares)	530,657
Weighted Average Contractual Life (Year)	5 years 200 days
Options Exercisable (in shares)	411,949
Price Range Two [Member]
Exercise Price or Range of Exercise Price, Lower Limit (in dollars per share) | $ / shares	$ 7.80
Exercise Price or Range of Exercise Price, Upper Limit (in dollars per share) | $ / shares	$ 9.01
Options Outstanding (in shares)	32,250
Weighted Average Contractual Life (Year)	9 years 135 days
Options Exercisable (in shares)	425
Price Range Three [Member]
Exercise Price or Range of Exercise Price, Lower Limit (in dollars per share) | $ / shares	$ 9.31
Exercise Price or Range of Exercise Price, Upper Limit (in dollars per share) | $ / shares	$ 9.31
Options Outstanding (in shares)	637,229
Weighted Average Contractual Life (Year)	7 years 288 days
Options Exercisable (in shares)	312,197
Price Range Four [Member]
Exercise Price or Range of Exercise Price, Lower Limit (in dollars per share) | $ / shares	$ 10.04
Exercise Price or Range of Exercise Price, Upper Limit (in dollars per share) | $ / shares	$ 21.36
Options Outstanding (in shares)	852,840
Weighted Average Contractual Life (Year)	8 years 328 days
Options Exercisable (in shares)	134,096