Note 6 - Stockholders' Equity, Stock Plans and Stock-based Compensation Expense - Activity Under the 2004 Equity Incentive Plan (Details) - USD ($) $ / shares in Units, $ in Thousands		12 Months Ended
		Dec. 31, 2019		Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Balances, shares available for grant (in shares)		1,141,305		1,494,866	721,657
Balances, stock options outstanding (in shares)		507,705		839,919	1,116,472
Balances, weighted-average exercise price (in dollars per share)		$ 20.52		$ 16.46	$ 9.56
Balances, options outstanding weighted-average remaining contractual life (Year)		3 years 69 days		3 years 189 days	3 years 361 days	3 years 255 days
Balances, aggregate intrinsic value	[1]	$ 3,040		$ 2,000	$ 24,400	$ 8,700
Options granted, shares available for grant (in shares)				(21,010)	(278,250)
Options granted, stock options outstanding (in shares)				21,010	278,250
Options granted, weighted-average exercise price (in dollars per share)				$ 50.65	$ 31
Options exercised, stock options outstanding (in shares)		(160,798)		(271,902)	(488,398)
Options exercised, weighted-average exercise price (in dollars per share)		$ 10.03		$ 9.99	$ 8.96
Options cancelled (expired or forfeited), shares available for grant (in shares)		51,208		81,322	66,405
Options cancelled (expired or forfeited), stock options outstanding (in shares)		(51,208)		(81,322)	(66,405)
Options cancelled (expired or forfeited), weighted-average exercise price (in dollars per share)		$ 24.61		$ 2,155	$ 16.54
Stock awards granted, shares available for grant (in shares)		(1,538,128)		(562,070)	(873,881)
Stock awards cancelled (expired or forfeited), shares available for grant (in shares)		407,320		148,197	258.935
Additional shares reserved, shares available for grant (in shares)		700,000	[2]		1,600,000
Balances, shares available for grant (in shares)		761,705		1,141,305	1,494,866	721,657
Balances, stock options outstanding (in shares)		295,699		507,705	839,919	1,116,472
Balances, weighted-average exercise price (in dollars per share)		$ 25.52		$ 20.52	$ 16.46	$ 9.56
Exercisable, stock options outstanding (in shares)		222,400
Exercisable, weighted-average exercise price (in dollars per share)		$ 22.16
Exercisable, options outstanding weighted-average remaining contractual life (Year)		2 years 302 days
Exercisable, aggregate intrinsic value	[1]	$ 3,040
Vested and expected to vest (in shares)		285,462
Vested and expected to vest, weighted-average exercise price (in dollars per share)		$ 25.08
Vested and expected to vest, weighted-average remaining contractual life (Year)		3 years
Vested and expected to vest, aggregate intrinsic value	[1]	$ 3,060

[1] Based on the closing stock price of $35.81 of the Company's stock on December 31, 2019, $17.02 on December 31, 2018, $45.35 on December 31 2017 and $17.35 on December 31, 2016.

[2] Approved by the board of directors and stockholders in 2019.