Note 2 - Net Income Per Share - Reconciliation of the Numerators and Denominators of the Basic and Diluted Net Income per Share Computations (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended		6 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Net income (numerator):
Net income	$ 8,760	$ 4,899	$ 18,036	$ 11,493
Shares (denominator):
Weighted average number of common shares outstanding used in basic computation (in shares)	20,349	20,234	20,321	20,208
Common shares issuable upon vesting of restricted stock (in shares)	78	133	111	137
Common shares issuable upon conversion of preferred shares (in shares)	10	10	10	10
Shares used in diluted computation (in shares)	20,437	20,377	20,442	20,355
Net income per common share
Basic (in dollars per share)	$ 0.43	$ 0.24	$ 0.89	$ 0.57
Diluted (in dollars per share)	$ 0.43	$ 0.24	$ 0.88	$ 0.56