Note 5 - Net Income Per Share - Net Income Per Share Basic and Diluted (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Numerator:
Net income	$ 23,642	$ 14,373	$ 53,132	$ 36,146
Denominator:
Weighted-average shares outstanding, basic (in shares)	41,458	40,590	41,276	40,335
Effect of dilutive securities (in shares)	2,028	1,305	2,108	1,417
Weighted-average shares outstanding, diluted (in shares)	43,486	41,895	43,384	41,752
Net income per share:
Earnings per share, basic (in dollars per share)	$ 0.57	$ 0.35	$ 1.29	$ 0.90
Earnings per share, diluted (in dollars per share)	$ 0.54	$ 0.34	$ 1.22	$ 0.87