Fair Value of Financial Instruments - Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) - USD ($) $ in Thousands	Oct. 31, 2018	Apr. 30, 2018
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	$ 43,193	$ 46,972
Marketable securities	31,741	35,014
Total	74,934	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	43,193	46,972
Marketable securities	9,676	11,125
Total	52,869	58,097
Significant Other Observable Inputs, Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable securities	22,065	23,889
Total	$ 22,065	$ 23,889