Note 6 - Stockholders' Equity, Stock Plans and Stock-based Compensation Expense - Stock-based Compensation Expense by Department (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Allocated share-based compensation expense	$ 5,110	$ 3,713	$ 4,084
Cost of Sales [Member]
Allocated share-based compensation expense	660	341	447
Selling and Marketing Expense [Member] | Employee [Member]
Allocated share-based compensation expense	1,629	1,179	1,054
Selling and Marketing Expense [Member] | Non-employees [Member]
Allocated share-based compensation expense	13
Research and Development Expense [Member]
Allocated share-based compensation expense	936	596	662
General and Administrative Expense [Member]
Allocated share-based compensation expense	$ 1,872	$ 1,597	$ 1,921