Note 3 - Net Income Per Common Share - Computation of Basic and Diluted Net Income Per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Net income	$ 15,511	$ 18,051
Basic - weighted-average shares outstanding: (in shares)	26,963,366	26,879,388
Effect of dilutive securities:
Employee stock options and other (in shares)	213,673	317,711
Diluted - weighted-average shares outstanding: (in shares)	27,177,039	27,197,099
Basic (in dollars per share)	$ 0.58	$ 0.67
Diluted (in dollars per share)	$ 0.57	$ 0.66