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, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Net income	$ 27,638	$ 31,568	$ 24,206	$ 21,856	$ 12,071	$ 23,642	$ 15,014	$ 14,477	$ 105,268	$ 65,203	$ 52,720
Weighted-average shares outstanding, basic (in shares)									42,247	41,350	40,436
Effect of dilutive securities (in shares)									2,355	2,228	1,479
Weighted-average shares outstanding, diluted (in shares)	45,058	44,669	44,400	44,282	44,610	43,486	43,397	43,268	44,602	43,578	41,915
Earnings per share, basic (in dollars per share)	$ 0.65	$ 0.75	$ 0.57	$ 0.52	$ 0.29	$ 0.57	$ 0.36	$ 0.35	$ 2.49	$ 1.58	$ 1.30
Earnings per share, diluted (in dollars per share)	$ 0.61	$ 0.71	$ 0.55	$ 0.49	$ 0.27	$ 0.54	$ 0.35	$ 0.33	$ 2.36	$ 1.50	$ 1.26