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, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Numerator:
Net income	$ 32,436	$ 29,528	$ 20,693	$ 26,181	$ 27,638	$ 31,568	$ 24,206	$ 21,856	$ 108,839	$ 105,268	$ 65,203
Denominator:
Weighted-average outstanding shares - basic (in shares)									43,165	42,247	41,350
Effect of dilutive securities (in shares)									2,598	2,355	2,228
Weighted-average outstanding shares - diluted (in shares)	46,503	45,833	45,483	45,232	45,058	44,669	44,400	44,282	45,763	44,602	43,578
Net income per share:
Earnings per share, basic (in dollars per share)	$ 0.75	$ 0.68	$ 0.48	$ 0.61	$ 0.65	$ 0.75	$ 0.57	$ 0.52	$ 2.52	$ 2.49	$ 1.58
Earnings per share, diluted (in dollars per share)	$ 0.70	$ 0.64	$ 0.45	$ 0.58	$ 0.61	$ 0.71	$ 0.55	$ 0.49	$ 2.38	$ 2.36	$ 1.50