Note 7 - 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, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2019	Mar. 31, 2018	Mar. 31, 2017
Net income (numerator):
Net income	$ 6,620	$ 7,787	$ 10,752	$ 12,582	$ 10,183	$ 9,064	$ 8,760	$ 9,276	$ 37,740	$ 37,283	$ 23,819
Shares (denominator)
Weighted average number of common shares outstanding used in basic computation (in shares)									20,461	20,346	20,232
Common shares issuable upon the vesting of restricted stock (in shares)									20	77	136
Common shares issuable upon conversion of preferred shares (in shares)									10	10	10
Shares used in diluted computation (in shares)									20,491	20,433	20,378
Net income per common share
Basic (in dollars per share)									$ 1.84	$ 1.83	$ 1.18
Diluted (in dollars per share)	$ 0.32	$ 0.38	$ 0.52	$ 0.62	$ 0.50	$ 0.44	$ 0.43	$ 0.45	$ 1.84	$ 1.82	$ 1.17