Summary of Fair Value of Financial Assets and Liabilities (Detail) (USD $) In Thousands, unless otherwise specified	Jan. 31, 2013	Apr. 30, 2012
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Short-term investments	$ 81,646	$ 50,834
Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	9,000	29,006
Restricted cash	604	500
Short-term investments	81,646	50,834
Total assets	91,250	80,340
Contingent consideration	4,270
Total liabilities	4,270
Fair Value, Measurements, Recurring | U.S. Treasury securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	8,997	7,499
Short-term investments	70,060	28,099
Fair Value, Measurements, Recurring | Corporate bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents		21,219
Short-term investments		18,239
Fair Value, Measurements, Recurring | Certificates of deposit
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Short-term investments	11,586	4,496
Fair Value, Measurements, Recurring | Money market funds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	3	288
Quoted prices in active markets for identical assets (Level 1) | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	9,000	7,787
Restricted cash	604	500
Short-term investments	70,060	28,099
Total assets	79,664	36,386
Quoted prices in active markets for identical assets (Level 1) | Fair Value, Measurements, Recurring | U.S. Treasury securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	8,997	7,499
Short-term investments	70,060	28,099
Quoted prices in active markets for identical assets (Level 1) | Fair Value, Measurements, Recurring | Money market funds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	3	288
Significant other observable inputs (Level 2) | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents		21,219
Short-term investments	11,586	22,735
Total assets	11,586	43,954
Significant other observable inputs (Level 2) | Fair Value, Measurements, Recurring | Corporate bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents		21,219
Short-term investments		18,239
Significant other observable inputs (Level 2) | Fair Value, Measurements, Recurring | Certificates of deposit
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Short-term investments	11,586	4,496
Significant unobservable inputs (Level 3) | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Contingent consideration	4,270
Total liabilities	$ 4,270