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, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Numerator:
Net income	$ 12,071	$ 23,642	$ 15,014	$ 14,477	$ 16,573	$ 14,373	$ 11,198	$ 10,575	$ 65,203	$ 52,720	$ 35,172
Denominator:
Weighted-average shares outstanding, basic (in shares)									41,350	40,436	39,470
Effect of dilutive securities (in shares)									2,228	1,479	1,399
Weighted-average shares outstanding, diluted (in shares)	44,610	43,486	43,397	43,268	42,404	41,895	41,716	41,646	43,578	41,915	40,869
Net income per share:
Earnings per share, basic (in dollars per share)	$ 0.29	$ 0.57	$ 0.36	$ 0.35	$ 0.41	$ 0.35	$ 0.28	$ 0.26	$ 1.58	$ 1.30	$ 0.89
Earnings per share, diluted (in dollars per share)	$ 0.27	$ 0.54	$ 0.35	$ 0.33	$ 0.39	$ 0.34	$ 0.27	$ 0.25	$ 1.50	$ 1.26	$ 0.86