Note 6 - Stockholders' Equity, Stock Plans and Stock-based Compensation Expense - Restricted Stock Units Activity (Details) - Restricted Stock Units (RSUs) [Member] - USD ($) $ / shares in Units, $ in Thousands		12 Months Ended
		Dec. 31, 2017		Dec. 31, 2016		Dec. 31, 2015		Dec. 31, 2014
Outstanding, number of shares (in shares)		445,317		371,630		434,321
Outstanding, weighted-average grant date fair value (in dollars per share)		$ 11.15		$ 12.39		$ 9.31
Outstanding, aggregate intrinsic value	[1]	$ 23,155		$ 7,726		$ 4,753		$ 4,639
Granted, number of shares (in shares)		412,208		480,191		203,104
Granted, weighted-average grant date fair value (in dollars per share)		$ 28.74		$ 10.80		$ 14.81
Vested, number of shares (in shares)	[2]	(224,799)		(172,990)		(222,220)
Vested, weighted-average grant date fair value (in dollars per share)	[2]	$ 10.91		$ 12.56		$ 11.79
Vested, aggregate fair value	[2],[4]	$ 5,168	[3]	$ 1,906	[5]	$ 3,285	[6]
Forfeited, number of shares (in shares)		(122,139)		(233,514)		(43,575)
Forfeited, weighted-average grant date fair value (in dollars per share)		$ 13.56		$ 11.36		$ 9.09
Outstanding, number of shares (in shares)		510,587		445,317		371,630
Outstanding, weighted-average grant date fair value (in dollars per share)		$ 24.88		$ 11.15		$ 12.39

[1] Based on the closing stock price of the Company's stock of $45.35 on December 31, 2017, $17.35 on December 31, 2016, $12.79 on December 31, 2015 and $10.68 on December 30, 2014.

[2] The number of restricted stock units vested includes shares that the Company withheld on behalf of the employees to satisfy the statutory tax withholding requirements.

[3] On the grant date, the fair value for these vested awards was $2.5 million.

[4] Represents the value of the Company's stock on the date that the restricted stock units and performance stock units vest.

[5] On the grant date, the fair value for these vested awards was $2.2 million.

[6] On the grant date, the fair value for these vested awards was $2.6 million.