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, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Balances, shares available for grant (in shares)		1,494,866	721,657	1,263,425
Balances, stock options outstanding (in shares)		839,919	1,116,472	2,148,797
Balances, weighted-average exercise price (in dollars per share)		$ 16.46	$ 9.56	$ 9.31
Balances, options outstanding weighted-average remaining contractual life (Year)		3 years 189 days	3 years 361 days	3 years 255 days	3 years 146 days
Balances, aggregate intrinsic value	[1]	$ 2,000	$ 24,400	$ 8,700	$ 7,900
Options granted, shares available for grant (in shares)		(21,010)	(278,250)	(162,000)
Options granted, stock options outstanding (in shares)		21,010	278,250	162,000
Options granted, weighted-average exercise price (in dollars per share)		$ 50.65	$ 31	$ 11.55
Options exercised, stock options outstanding (in shares)		(271,902)	(488,398)	(1,051,138)
Options exercised, weighted-average exercise price (in dollars per share)		$ 9.99	$ 8.96	$ 8.89
Options cancelled (expired or forfeited), shares available for grant (in shares)		81,322	66,405	143,187
Options cancelled (expired or forfeited), stock options outstanding (in shares)		(81,322)	(66,405)	(143,187)
Options cancelled (expired or forfeited), weighted-average exercise price (in dollars per share)		$ 2,155	$ 16.54	$ 12.93
Stock awards granted, shares available for grant (in shares)		(562,070)	(873,881)	(1,018,005)
Stock awards cancelled (expired or forfeited), shares available for grant (in shares)		148,197	258,935	495,050
Additional shares reserved, shares available for grant (in shares)	[2]		1,600,000
Balances, shares available for grant (in shares)		1,141,305	1,494,866	721,657	1,263,425
Balances, stock options outstanding (in shares)		507,705	839,919	1,116,472	2,148,797
Balances, weighted-average exercise price (in dollars per share)		$ 20.52	$ 16.46	$ 9.56	$ 9.31
Exercisable, stock options outstanding (in shares)		335,348
Exercisable, weighted-average exercise price (in dollars per share)		$ 14.68
Exercisable, options outstanding weighted-average remaining contractual life (Year)		2 years 266 days
Exercisable, aggregate intrinsic value	[1]	$ 1,870
Vested and expected to vest (in shares)		485,469
Vested and expected to vest, weighted-average exercise price (in dollars per share)		$ 19.88
Vested and expected to vest, weighted-average remaining contractual life (Year)		3 years 153 days
Vested and expected to vest, aggregate intrinsic value	[1]	$ 19,860

[1] Based on the closing stock price of $17.02 of the Company’s stock on December 31, 2018, $45.35 on December 30, 2017, $17.35 on December 31 2016 and $12.79 on December 31, 2015.

[2] Approved by the board of directors and stockholders in 2017..