Note 6 - Stockholders' Equity, Stock Plans and Stock-based Compensation Expense - Options Outstanding and Exercisable Price Ranges (Details)	12 Months Ended
	Dec. 31, 2019 $ / shares shares
Exercise Price Range 1 [Member]
Range of exercise prices, upper limit (in dollars per share) | $ / shares	$ 8.80
Options outstanding, number outstanding (in shares)	48,864
Options outstanding, weighted-average remaining contractual life (Year)	175 days
Options exercisable, number outstanding (in shares)	48,864
Exercise Price Range 2 [Member]
Options outstanding, number outstanding (in shares)	33,743
Options outstanding, weighted-average remaining contractual life (Year)	1 year 357 days
Options exercisable, number outstanding (in shares)	32,806
Range of exercise prices (in dollars per share) | $ / shares	$ 10.79
Range of exercise prices, lower limit (in dollars per share) | $ / shares	$ 9.65
Exercise Price Range 3 [Member]
Options outstanding, number outstanding (in shares)	36,146
Options outstanding, weighted-average remaining contractual life (Year)	3 years 87 days
Options exercisable, number outstanding (in shares)	33,752
Range of exercise prices (in dollars per share) | $ / shares	$ 14.04
Range of exercise prices, lower limit (in dollars per share) | $ / shares	$ 10.80
Exercise Price Range 4 [Member]
Options outstanding, number outstanding (in shares)	10,896
Options outstanding, weighted-average remaining contractual life (Year)	3 years 270 days
Options exercisable, number outstanding (in shares)	8,584
Range of exercise prices (in dollars per share) | $ / shares	$ 18.55
Range of exercise prices, lower limit (in dollars per share) | $ / shares	15.32
Exercise Price Range 5 [Member]
Range of exercise prices, upper limit (in dollars per share) | $ / shares	$ 19.55
Options outstanding, number outstanding (in shares)	12,000
Options outstanding, weighted-average remaining contractual life (Year)	4 years 116 days
Options exercisable, number outstanding (in shares)	8,292
Exercise Price Range 6 [Member]
Range of exercise prices, upper limit (in dollars per share) | $ / shares	$ 27.50
Options outstanding, number outstanding (in shares)	38,625
Options outstanding, weighted-average remaining contractual life (Year)	4 years 215 days
Options exercisable, number outstanding (in shares)	23,594
Exercise Price Range 7 [Member]
Range of exercise prices, upper limit (in dollars per share) | $ / shares	$ 39.30
Options outstanding, number outstanding (in shares)	72,000
Options outstanding, weighted-average remaining contractual life (Year)	4 years 153 days
Options exercisable, number outstanding (in shares)	41,230
Exercise Price Range 8 [Member]
Range of exercise prices, upper limit (in dollars per share) | $ / shares	$ 43.40
Options outstanding, number outstanding (in shares)	6,510
Options outstanding, weighted-average remaining contractual life (Year)	91 days
Options exercisable, number outstanding (in shares)	2,849
Exercise Price Range 9 [Member]
Range of exercise prices, upper limit (in dollars per share) | $ / shares	$ 47.40
Options outstanding, number outstanding (in shares)	29,915
Options outstanding, weighted-average remaining contractual life (Year)	3 years 262 days
Options exercisable, number outstanding (in shares)	19,116
Exercise Price Range 10 [Member]
Range of exercise prices, upper limit (in dollars per share) | $ / shares	$ 53.90
Options outstanding, number outstanding (in shares)	7,000
Options outstanding, weighted-average remaining contractual life (Year)	5 years 73 days
Options exercisable, number outstanding (in shares)	3,313
Exercise Price Range 11 [Member]
Options outstanding, number outstanding (in shares)	295,669
Options outstanding, weighted-average remaining contractual life (Year)	2 years 302 days
Options exercisable, number outstanding (in shares)	222,400
Range of exercise prices (in dollars per share) | $ / shares	$ 53.90
Range of exercise prices, lower limit (in dollars per share) | $ / shares	$ 8.80