Note 6 - Stockholders' Equity, Stock Plans and Stock-based Compensation Expense - Options Outstanding and Exercisable Price Ranges (Details)	12 Months Ended
	Dec. 31, 2017 $ / shares shares
Range of exercise prices, lower limit (in dollars per share)	$ 6.88
Range of exercise prices, upper limit (in dollars per share)	$ 47.40
Options outstanding, number outstanding (in shares) | shares	839,919
Options outstanding, weighted-average remaining contractual life (Year)	4 years 40 days
Options exercisable, number outstanding (in shares) | shares	467,794
Options exercisable, weighted-average exercise price (in dollars per share)	$ 9.53
Exercise Price Range 1 [Member]
Range of exercise prices, lower limit (in dollars per share)	6.88
Range of exercise prices, upper limit (in dollars per share)	$ 7.15
Options outstanding, number outstanding (in shares) | shares	84,739
Options outstanding, weighted-average remaining contractual life (Year)	1 year 156 days
Options exercisable, number outstanding (in shares) | shares	84,739
Options exercisable, weighted-average exercise price (in dollars per share)	$ 6.92
Exercise Price Range 2 [Member]
Range of exercise prices, lower limit (in dollars per share)	7.44
Range of exercise prices, upper limit (in dollars per share)	$ 8.72
Options outstanding, number outstanding (in shares) | shares	15,978
Options outstanding, weighted-average remaining contractual life (Year)	317 days
Options exercisable, number outstanding (in shares) | shares	15,978
Options exercisable, weighted-average exercise price (in dollars per share)	$ 8.30
Exercise Price Range 3 [Member]
Range of exercise prices, lower limit (in dollars per share)	8.80
Range of exercise prices, upper limit (in dollars per share)	$ 8.80
Options outstanding, number outstanding (in shares) | shares	141,956
Options outstanding, weighted-average remaining contractual life (Year)	2 years 164 days
Options exercisable, number outstanding (in shares) | shares	141,956
Options exercisable, weighted-average exercise price (in dollars per share)	$ 8.80
Exercise Price Range 4 [Member]
Range of exercise prices, lower limit (in dollars per share)	9.28
Range of exercise prices, upper limit (in dollars per share)	$ 9.97
Options outstanding, number outstanding (in shares) | shares	95,535
Options outstanding, weighted-average remaining contractual life (Year)	3 years 87 days
Options exercisable, number outstanding (in shares) | shares	73,402
Options exercisable, weighted-average exercise price (in dollars per share)	$ 9.69
Exercise Price Range 5 [Member]
Range of exercise prices, lower limit (in dollars per share)	10.03
Range of exercise prices, upper limit (in dollars per share)	$ 10.86
Options outstanding, number outstanding (in shares) | shares	88,044
Options outstanding, weighted-average remaining contractual life (Year)	4 years 167 days
Options exercisable, number outstanding (in shares) | shares	51,517
Options exercisable, weighted-average exercise price (in dollars per share)	$ 10.38
Exercise Price Range 6 [Member]
Range of exercise prices, lower limit (in dollars per share)	10.90
Range of exercise prices, upper limit (in dollars per share)	$ 11.67
Options outstanding, number outstanding (in shares) | shares	91,021
Options outstanding, weighted-average remaining contractual life (Year)	4 years 80 days
Options exercisable, number outstanding (in shares) | shares	50,356
Options exercisable, weighted-average exercise price (in dollars per share)	$ 11.36
Exercise Price Range 7 [Member]
Range of exercise prices, lower limit (in dollars per share)	11.98
Range of exercise prices, upper limit (in dollars per share)	$ 17.90
Options outstanding, number outstanding (in shares) | shares	109,896
Options outstanding, weighted-average remaining contractual life (Year)	4 years 240 days
Options exercisable, number outstanding (in shares) | shares	49,033
Options exercisable, weighted-average exercise price (in dollars per share)	$ 13.36
Exercise Price Range 8 [Member]
Range of exercise prices, lower limit (in dollars per share)	18.55
Range of exercise prices, upper limit (in dollars per share)	$ 25.70
Options outstanding, number outstanding (in shares) | shares	88,500
Options outstanding, weighted-average remaining contractual life (Year)	5 years 317 days
Options exercisable, number outstanding (in shares) | shares	813
Options exercisable, weighted-average exercise price (in dollars per share)	$ 18.55
Exercise Price Range 9 [Member]
Range of exercise prices, lower limit (in dollars per share)	39.30
Range of exercise prices, upper limit (in dollars per share)	$ 39.30
Options outstanding, number outstanding (in shares) | shares	77,000
Options outstanding, weighted-average remaining contractual life (Year)	6 years 302 days
Options exercisable, number outstanding (in shares) | shares
Options exercisable, weighted-average exercise price (in dollars per share)
Exercise Price Range 10 [Member]
Range of exercise prices, lower limit (in dollars per share)	47.40
Range of exercise prices, upper limit (in dollars per share)	$ 47.40
Options outstanding, number outstanding (in shares) | shares	47,250
Options outstanding, weighted-average remaining contractual life (Year)	6 years 350 days
Options exercisable, number outstanding (in shares) | shares
Options exercisable, weighted-average exercise price (in dollars per share)