Fair Value of Financial Assets and Liabilities - Summary of Fair Value of Financial Assets and Liabilities (Detail) - USD ($) $ in Thousands	Apr. 30, 2015	Apr. 30, 2014
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	$ 54,041	$ 31,934
Short-term investments	52,730	40,700
Municipal Bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	102
Commercial Paper
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	875
Corporate Bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	256
Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	4,064	7,213
Restricted cash		604
Short-term investments	52,730	40,700
Total assets	56,794	48,517
Fair Value, Measurements, Recurring | Certificates of Deposit
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents		242
Short-term investments	4,004	7,474
Fair Value, Measurements, Recurring | Municipal Bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	102
Short-term investments	4,565
Fair Value, Measurements, Recurring | Commercial Paper
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	875
Short-term investments	6,273
Fair Value, Measurements, Recurring | US Treasury notes
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Short-term investments	23,809	33,226
Fair Value, Measurements, Recurring | U.S. Treasury Bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Short-term investments	4,999
Fair Value, Measurements, Recurring | Corporate Notes
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Short-term investments	2,708
Fair Value, Measurements, Recurring | Corporate Bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	256
Short-term investments	6,372
Fair Value, Measurements, Recurring | Money Market Funds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	2,831	6,971
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	2,831	7,213
Restricted cash		604
Short-term investments	28,808	33,226
Total assets	31,639	41,043
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1 | Certificates of Deposit
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents		242
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1 | US Treasury notes
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Short-term investments	23,809	33,226
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1 | U.S. Treasury Bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Short-term investments	4,999
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1 | Money Market Funds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	2,831	6,971
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	1,233
Short-term investments	23,922	7,474
Total assets	25,155	7,474
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | Certificates of Deposit
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Short-term investments	4,004	$ 7,474
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | Municipal Bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	102
Short-term investments	4,565
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | Commercial Paper
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	875
Short-term investments	6,273
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | Corporate Notes
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Short-term investments	2,708
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | Corporate Bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	256
Short-term investments	$ 6,372