Note 5 - Income Taxes - Reconciliation of Income Tax Provision (Details) - USD ($) $ in Thousands	12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Mar. 31, 2016
Income taxes at U.S. statutory rates	$ 16,943	$ 13,274	$ 11,399
State income taxes, net of federal tax benefit	1,078	858	675
Restricted stock windfall adjustment	(1,086)
Reduction of deferred tax liability due to rate reduction		(430)
Permanent differences	1	1	(23)
Other		(28)	(51)
Total provision for income taxes	$ 16,506	$ 14,105	$ 12,000