Note 3 - Fair Value of Financial Instruments (Details Textual)	12 Months Ended
	Mar. 31, 2019
Available for Sale Debt Securitites, Percent of Total Assets	89.00%
Number of Other-than-temporarily Impaired Available-for-sale Securities	0
Number of Other-than-temporarily Impaired Marketable Securities	0
Minimum [Member]
Available for Sale Debt Securities, Remaining Maturity	35 days
Maximum [Member]
Available for Sale Debt Securities, Remaining Maturity	4 years 330 days