Note 7 - Shareholders' Equity - Options Range of Exercise Prices (Details) - $ / shares	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Upper range limit (in dollars per share)	$ 9.96
Options outstanding, number of outstanding options (in shares)	1,217,163
Options outstanding, weighted-average remaining contractual life (Year)	3 years 11 months 19 days
Options outstanding, weighted-average exercise price (in dollars per share)	$ 6.50
Options exercisable, number of options exercisable (in shares)	738,452
Options exercisable, weighted-average exercise price (in dollars per share)	$ 5.82	$ 6.28
Range One [Member]
Upper range limit (in dollars per share)	$ 3.94
Options outstanding, number of outstanding options (in shares)	204,244
Options outstanding, weighted-average remaining contractual life (Year)	9 months 21 days
Options outstanding, weighted-average exercise price (in dollars per share)	$ 3.80
Options exercisable, number of options exercisable (in shares)	204,244
Options exercisable, weighted-average exercise price (in dollars per share)	$ 3.80
Range Two [Member]
Upper range limit (in dollars per share)	$ 5.71
Options outstanding, number of outstanding options (in shares)	317,667
Options outstanding, weighted-average remaining contractual life (Year)	2 years 9 months 10 days
Options outstanding, weighted-average exercise price (in dollars per share)	$ 4.93
Options exercisable, number of options exercisable (in shares)	317,667
Options exercisable, weighted-average exercise price (in dollars per share)	$ 4.93
Range Three [Member]
Upper range limit (in dollars per share)	$ 7.16
Options outstanding, number of outstanding options (in shares)	267,333
Options outstanding, weighted-average remaining contractual life (Year)	6 years 1 month 17 days
Options outstanding, weighted-average exercise price (in dollars per share)	$ 6.54
Options exercisable, number of options exercisable (in shares)	13,667
Options exercisable, weighted-average exercise price (in dollars per share)	$ 6.87
Range Four [Member]
Upper range limit (in dollars per share)	$ 9.96
Options outstanding, number of outstanding options (in shares)	427,919
Options outstanding, weighted-average remaining contractual life (Year)	5 years 3 days
Options outstanding, weighted-average exercise price (in dollars per share)	$ 8.92
Options exercisable, number of options exercisable (in shares)	202,874
Options exercisable, weighted-average exercise price (in dollars per share)	$ 9.19