Note 6 - 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, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2018	Mar. 31, 2017	Mar. 31, 2016
Net income (numerator):
Net income	$ 10,183	$ 9,064	$ 8,760	$ 9,276	$ 7,503	$ 4,823	$ 4,899	$ 6,594	$ 37,283	$ 23,819	$ 20,567
Shares (denominator)
Weighted average number of common shares outstanding used in basic computation (in shares)									20,346	20,232	20,124
Common shares issuable upon the vesting of restricted stock (in shares)									77	136	120
Common shares issuable upon conversion of preferred shares (in shares)									10	10	10
Shares used in diluted computation (in shares)									20,433	20,378	20,254
Net income per common share
Basic (in dollars per share)									$ 1.83	$ 1.18	$ 1.02
Diluted (in dollars per share)	$ 0.50	$ 0.44	$ 0.43	$ 0.45	$ 0.37	$ 0.24	$ 0.24	$ 0.32	$ 1.82	$ 1.17	$ 1.02