Note 4 - Net Income Per Common Share - Computation of Basic and Diluted Net Income Per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Net income	$ 63,655	$ 76,588	$ 51,778
Basic - weighted-average shares outstanding: (in shares)	26,866,985	26,982,614	26,647,610
Employee stock options and other (in shares)	182,438	296,202	197,461
Diluted - weighted-average shares outstanding: (in shares)	27,049,423	27,278,816	26,845,071
Basic (in dollars per share)	$ 2.37	$ 2.84	$ 1.94
Diluted (in dollars per share)	$ 2.35	$ 2.81	$ 1.93