Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) (USD $) In Thousands, unless otherwise specified	Jan. 31, 2015	Apr. 30, 2014
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	$ 38,990	$ 51,561
Marketable securities	29,733	23,505
Total	68,723	75,066
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	38,990	51,561
Marketable securities	8,434	8,338
Total	47,424	59,899
Significant Other Observable Inputs, Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable securities	21,299	15,167
Total	$ 21,299	$ 15,167