Note 1 - Organization and Summary of Significant Accounting Policies - Schedule of Fair Value Measurements of Investments (Details) - USD ($) $ in Thousands	Jul. 31, 2018	Apr. 30, 2018
Securities available-for-sale	$ 18,870	$ 17,844
	22,954	22,826
Money Market Funds [Member]
Cash equivalents	4,084	4,982
Fair Value, Inputs, Level 1 [Member]
Securities available-for-sale	18,870	17,844
	22,954	22,826
Fair Value, Inputs, Level 1 [Member] | Money Market Funds [Member]
Cash equivalents	4,084	4,982
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