Note 5 - Income Taxes - Reconciliation of Income Tax Provision (Details) - USD ($)	12 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Mar. 31, 2017
Income taxes at U.S. statutory rates	$ 10,315,000	$ 16,943,000	$ 13,274,000
State income taxes, net of federal tax benefit	1,233,000	1,078,000	858,000
Restricted stock windfall adjustment	(176,000)	(1,086,000)
Reduction of deferred tax liability due to rate reduction		(430,000)
Other	9,000	1,000	(27,000)
Provision for income taxes	$ 11,381,000	$ 16,506,000	$ 14,105,000