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, 2018		Dec. 31, 2017		Dec. 31, 2016		Dec. 31, 2015
Outstanding, number of shares (in shares)		474,291		510,587		445,317
Outstanding, weighted-average grant date fair value (in dollars per share)		$ 38.44		$ 24.88		$ 11.15
Outstanding, aggregate intrinsic value	[1]	$ 37,442		$ 8,072		$ 23,155		$ 7,726
Granted, number of shares (in shares)		963,814		265,124		412,208
Granted, weighted-average grant date fair value (in dollars per share)		$ 18.68		$ 44.57		$ 28.74
Vested, number of shares (in shares)	[2]	(172,281)		(231,515)		(224,799)
Vested, weighted-average grant date fair value (in dollars per share)	[2]	$ 33.66		$ 21.10		$ 10.91
Vested, aggregate fair value	[2],[4]	$ 6,169	[3]	$ 9,483	[5]	$ 5,168	[6]
Forfeited, number of shares (in shares)		(161,022)		(69,905)		(122,139)
Forfeited, weighted-average grant date fair value (in dollars per share)		$ 37.91		$ 20.01		$ 13.56
Outstanding, number of shares (in shares)		1,104,802		474,291		510,587
Outstanding, weighted-average grant date fair value (in dollars per share)		$ 22.10		$ 38.44		$ 24.88

[1] Based on the closing stock price of the Company's stock of $35.81 on December 31, 2019, $17.02 on December 31, 2018, $45.35 on December 31, 2017 and $17.35 on December 30, 2016.

[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 $5.9 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 $4.9 million.

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