Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) - USD ($) $ in Thousands	Apr. 30, 2017	Apr. 30, 2016
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	$ 62,647	$ 45,977
Marketable securities	23,787	28,861
Total	86,434	74,838
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	62,647	45,977
Marketable securities	8,984	7,374
Total	71,631	53,351
Significant Other Observable Inputs, Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable securities	14,803	21,487
Total	$ 14,803	$ 21,487