Note 5 - Income Taxes - Components of Income Tax Provision (Details) - USD ($) $ in Thousands	12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Mar. 31, 2016
Current taxes
Federal	$ 15,012	$ 11,095	$ 10,982
State	1,586	1,059	1,041
Total current taxes	16,598	12,154	12,023
Deferred taxes
Federal	(83)	1,781	(21)
State	(9)	170	(2)
Total deferred taxes	(92)	1,951	(23)
Total provision for income taxes	$ 16,506	$ 14,105	$ 12,000