Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) - USD ($) $ in Thousands	Jul. 31, 2017	Apr. 30, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	$ 61,496	$ 62,647
Marketable securities	24,331	23,787
Total	85,827	86,434
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	61,496	62,647
Marketable securities	10,122	8,984
Total	71,618	71,631
Significant Other Observable Inputs, Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable securities	14,209	14,803
Total	$ 14,209	$ 14,803