Note 11 - Net Income Per Share - Net Income Per Share Basic and Diluted (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Numerator:
Net income	$ 16,573	$ 14,373	$ 11,198	$ 10,575	$ 10,110	$ 11,201	$ 7,899	$ 5,963	$ 52,720	$ 35,172	$ 35,495
Denominator:
Weighted-average shares outstanding, basic (in shares)									40,436	39,470	38,686
Effect of dilutive securities (in shares)									1,479	1,399	1,107
Weighted-average shares outstanding, diluted (in shares)	42,404	41,895	41,716	41,646	41,445	40,689	40,745	40,596	41,915	40,869	39,793
Net income per share:
Earnings per share, basic (in dollars per share)	$ 0.41	$ 0.35	$ 0.28	$ 0.26	$ 0.26	$ 0.28	$ 0.20	$ 0.15	$ 1.30	$ 0.89	$ 0.92
Earnings per share, diluted (in dollars per share)	$ 0.39	$ 0.34	$ 0.27	$ 0.25	$ 0.24	$ 0.28	$ 0.19	$ 0.15	$ 1.26	$ 0.86	$ 0.89