Note 6 - Net Income Per Share - Net Income Per Share Basic and Diluted (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Net income	$ 24,206	$ 15,014	$ 46,062	$ 29,490
Weighted-average shares outstanding, basic (in shares)	42,237	41,323	42,079	41,185
Effect of dilutive securities (in shares)	2,163	2,074	2,262	2,147
Weighted-average shares outstanding, diluted (in shares)	44,400	43,397	44,341	43,332
Earnings per share, basic (in dollars per share)	$ 0.57	$ 0.36	$ 1.09	$ 0.72
Earnings per share, diluted (in dollars per share)	$ 0.55	$ 0.35	$ 1.04	$ 0.68