Fair Value of Financial Instruments - Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) - USD ($) $ in Thousands	Jan. 31, 2019	Apr. 30, 2018
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	$ 49,190	$ 46,972
Marketable securities	29,104	35,014
Total	78,294	81,986
Quoted Prices in Active Markets for Identical Assets, Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	49,190	46,972
Marketable securities	9,810	11,125
Total	59,000	58,097
Significant Other Observable Inputs, Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable securities	19,294	23,889
Total	$ 19,294	$ 23,889