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, 2021	Jun. 30, 2021	Mar. 31, 2021	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Sep. 30, 2021	Sep. 30, 2020
Net income	$ 4,835	$ 12,915	$ 3,712	$ 14,697	$ 12,607	$ 13,984	$ 21,462	$ 41,288
Basic - weighted-average shares outstanding: (in shares)	27,713,229			26,736,709			27,638,611	26,724,715
Employee stock options and other (in shares)	16,876			149,724			69,173	136,932
Diluted - weighted-average shares outstanding: (in shares)	27,730,105			26,886,433			27,707,784	26,861,647
Basic (in dollars per share)	$ 0.17			$ 0.55			$ 0.78	$ 1.54
Diluted (in dollars per share)	$ 0.17			$ 0.55			$ 0.77	$ 1.54