Note 6 - Stockholders' Equity, Stock Plans and Stock-based Compensation Expense - Options Outstanding and Exercisable Price Ranges (Details)	12 Months Ended
	Dec. 31, 2018 $ / shares shares
Exercise Price Range 1 [Member]
Range of exercise prices, lower limit (in dollars per share) | $ / shares	$ 6.88
Range of exercise prices, upper limit (in dollars per share) | $ / shares	$ 7.44
Options outstanding, number outstanding (in shares)	28,832
Options outstanding, weighted-average remaining contractual life (Year)	211 days
Options exercisable, number outstanding (in shares)	28,832
Exercise Price Range 2 [Member]
Options outstanding, number outstanding (in shares)	97,268
Options outstanding, weighted-average remaining contractual life (Year)	1 year 160 days
Options exercisable, number outstanding (in shares)	97,268
Range of exercise prices (in dollars per share) | $ / shares	$ 8.80
Exercise Price Range 3 [Member]
Range of exercise prices, lower limit (in dollars per share) | $ / shares	9.65
Range of exercise prices, upper limit (in dollars per share) | $ / shares	$ 10.79
Options outstanding, number outstanding (in shares)	54,564
Options outstanding, weighted-average remaining contractual life (Year)	3 years 3 days
Options exercisable, number outstanding (in shares)	49,544
Exercise Price Range 4 [Member]
Range of exercise prices, lower limit (in dollars per share) | $ / shares	$ 10.80
Range of exercise prices, upper limit (in dollars per share) | $ / shares	$ 11.24
Options outstanding, number outstanding (in shares)	57,521
Options outstanding, weighted-average remaining contractual life (Year)	3 years 65 days
Options exercisable, number outstanding (in shares)	47,293
Exercise Price Range 5 [Member]
Range of exercise prices, lower limit (in dollars per share) | $ / shares	$ 11.67
Range of exercise prices, upper limit (in dollars per share) | $ / shares	$ 17.90
Options outstanding, number outstanding (in shares)	72,583
Options outstanding, weighted-average remaining contractual life (Year)	2 years 160 days
Options exercisable, number outstanding (in shares)	48,855
Exercise Price Range 6 [Member]
Range of exercise prices, lower limit (in dollars per share) | $ / shares	$ 18.55
Range of exercise prices, upper limit (in dollars per share) | $ / shares	$ 25.70
Options outstanding, number outstanding (in shares)	71,250
Options outstanding, weighted-average remaining contractual life (Year)	5 years 76 days
Options exercisable, number outstanding (in shares)	28,512
Exercise Price Range 7 [Member]
Options outstanding, number outstanding (in shares)	72,000
Options outstanding, weighted-average remaining contractual life (Year)	5 years 302 days
Options exercisable, number outstanding (in shares)	23,230
Range of exercise prices (in dollars per share) | $ / shares	$ 39.30
Exercise Price Range 8 [Member]
Options outstanding, number outstanding (in shares)	6,510
Options outstanding, weighted-average remaining contractual life (Year)	6 years 164 days
Options exercisable, number outstanding (in shares)
Range of exercise prices (in dollars per share) | $ / shares	$ 43.40
Exercise Price Range 9 [Member]
Options outstanding, number outstanding (in shares)	38,927
Options outstanding, weighted-average remaining contractual life (Year)	5 years 313 days
Options exercisable, number outstanding (in shares)	10,564
Range of exercise prices (in dollars per share) | $ / shares	$ 47.40
Exercise Price Range 10 [Member]
Options outstanding, number outstanding (in shares)	8,250
Options outstanding, weighted-average remaining contractual life (Year)	5 years 109 days
Options exercisable, number outstanding (in shares)	1,250
Range of exercise prices (in dollars per share) | $ / shares	$ 53.90