Note 1 - Organization and Summary of Significant Accounting Policies - Schedule of Fair Value Measurements of Investments (Details) - USD ($) $ in Thousands	Jul. 31, 2017	Apr. 30, 2017
	$ 23,927	$ 22,642
Securities available-for-sale	17,710	16,576
Money Market Funds [Member]
Cash equivalents	6,217	6,066
Fair Value, Inputs, Level 1 [Member]
	23,927	22,642
Securities available-for-sale	17,710	16,576
Fair Value, Inputs, Level 1 [Member] | Money Market Funds [Member]
Cash equivalents	6,217	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