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, 2020	Dec. 31, 2019	Dec. 31, 2018
Net income	$ 50,867	$ 63,655	$ 76,588
Basic - weighted-average shares outstanding: (in shares)	26,737,425	26,866,985	26,982,614
Employee stock options and other (in shares)	158,932	182,438	296,202
Diluted - weighted-average shares outstanding: (in shares)	26,896,357	27,049,423	27,278,816
Basic (in dollars per share)	$ 1.90	$ 2.37	$ 2.84
Diluted (in dollars per share)	$ 1.89	$ 2.35	$ 2.81