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						9 Months Ended
	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Sep. 30, 2019	Sep. 30, 2018
Net income	$ 16,792	$ 16,166	$ 15,511	$ 20,937	$ 18,314	$ 18,051	$ 48,469	$ 57,302
Basic - weighted-average shares outstanding: (in shares)	26,846,030			27,038,585			26,894,420	26,963,205
Employee stock options and other (in shares)	159,311			299,301			178,453	305,106
Diluted - weighted-average shares outstanding: (in shares)	27,005,341			27,337,886			27,072,873	27,268,311
Basic (in dollars per share)	$ 0.63			$ 0.77			$ 1.80	$ 2.13
Diluted (in dollars per share)	$ 0.62			$ 0.77			$ 1.79	$ 2.10