Note 6 - Stockholders' Equity, Stock Plans and Stock-based Compensation Expense - Stock-based Compensation Expense (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Allocated share-based compensation expense	$ 9,832	$ 7,157	$ 5,110
Share-based Payment Arrangement, Option [Member]
Allocated share-based compensation expense	622	838	815
Restricted Stock Units (RSUs) [Member]
Allocated share-based compensation expense	4,786	4,648	1,813
Performance Shares [Member]
Allocated share-based compensation expense	3,948	1,105	2,093
ESPP [Member]
Allocated share-based compensation expense	$ 476	$ 566	$ 389