Organization and Summary of Significant Accounting Policies - Levels of fair value measurements of investments (Details) - USD ($) $ in Thousands	Jan. 31, 2016	Apr. 30, 2015
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	$ 9,641	$ 5,272
Securities available-for-sale	4,550	9,632
Investments, fair value disclosure	14,191	14,904
Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	9,641	5,272
Securities available-for-sale	4,550	9,632
Investments, fair value disclosure	$ 14,191	$ 14,904
Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents
Securities available-for-sale
Investments, fair value disclosure
Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents
Securities available-for-sale
Investments, fair value disclosure