Note 9 - Net Income Per Share - Basic and Diluted Net Income Per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended								12 Months Ended
	Mar. 31, 2021	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2021	Mar. 31, 2020	Mar. 31, 2019
Net income (numerator):
Net income	$ 6,812	$ 7,611	$ 8,412	$ 7,768	$ 7,003	$ 6,840	$ 6,665	$ 5,343	$ 30,603	$ 25,851	$ 37,740
Shares (denominator)
Weighted average number of common shares outstanding used in basic computation (in shares)									20,060	20,041	20,461
Common shares issuable upon the vesting of restricted stock (in shares)									49	4	20
Common shares issuable upon conversion of preferred shares (in shares)									10	10	10
Shares used in diluted computation (in shares)									20,119	20,055	20,491
Net income per common share
Basic (in dollars per share)									$ 1.53	$ 1.29	$ 1.84
Diluted (in dollars per share)	$ 0.34	$ 0.38	$ 0.42	$ 0.39	$ 0.35	$ 0.34	$ 0.33	$ 0.26	$ 1.52	$ 1.29	$ 1.84