Note 3 - Net Income Per Share - Basic and Diluted Net Income Per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended		6 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Net income	$ 10,752	$ 8,760	$ 23,334	$ 18,036
Weighted average number of common shares outstanding used in basic computation (in shares)	20,463	20,349	20,436	20,321
Common shares issuable upon vesting of restricted stock (in shares)	47	78	39	111
Common shares issuable upon conversion of preferred shares (in shares)	10	10	10	10
Shares used in diluted computation (in shares)	20,520	20,437	20,485	20,442
Basic (in dollars per share)	$ 0.53	$ 0.43	$ 1.14	$ 0.89
Diluted (in dollars per share)	$ 0.52	$ 0.43	$ 1.14	$ 0.88