Note 1 - Organization and Summary of Significant Accounting Policies - Schedule of Fair Value Measurements of Investments (Details) - USD ($) $ in Thousands	Oct. 31, 2019	Apr. 30, 2019
Securities available-for-sale	$ 21,690	$ 21,828
	25,645	27,445
Money Market Funds [Member]
Cash equivalents	3,955	5,617
Fair Value, Inputs, Level 1 [Member]
Securities available-for-sale	21,690	21,828
	25,645	27,445
Fair Value, Inputs, Level 1 [Member] | Money Market Funds [Member]
Cash equivalents	3,955	5,617
Fair Value, Inputs, Level 2 [Member]
Securities available-for-sale
Fair Value, Inputs, Level 2 [Member] | Money Market Funds [Member]
Cash equivalents
Fair Value, Inputs, Level 3 [Member]
Securities available-for-sale
Fair Value, Inputs, Level 3 [Member] | Money Market Funds [Member]
Cash equivalents