Note 1 - Organization and Summary of Significant Accounting Policies - Schedule of Fair Value Measurements of Investments (Details) - USD ($) $ in Thousands	Jan. 31, 2018	Apr. 30, 2017
Securities available-for-sale	$ 18,404	$ 16,576
	22,120	22,642
Money Market Funds [Member]
Cash equivalents	3,716	6,066
Fair Value, Inputs, Level 1 [Member]
Securities available-for-sale	18,404	16,576
	22,120	22,642
Fair Value, Inputs, Level 1 [Member] | Money Market Funds [Member]
Cash equivalents	3,716	6,066
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