Fair Value of Financial Assets and Liabilities - Summary of Fair Value of Financial Assets and Liabilities (Details) - USD ($) $ in Thousands	Oct. 31, 2016	Apr. 30, 2016
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents:	$ 44,427	$ 43,963
Short-term investments:	41,704	50,682
Fair value, measurements, recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents:	9,771	5,271
Short-term investments:	41,704	50,682
Total assets	51,475	55,953
Fair value, measurements, recurring | Fair value, inputs, level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents:	9,421	922
Short-term investments:	30,253	33,600
Total assets	39,674	34,522
Fair value, measurements, recurring | Fair value, inputs, level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents:	350	4,349
Short-term investments:	11,451	17,082
Total assets	11,801	21,431
Fair value, measurements, recurring | Fair value, inputs, level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents:	0	0
Short-term investments:	0	0
Total assets	0	0
Certificates of deposit | Fair value, measurements, recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Short-term investments:	2,470	7,089
Certificates of deposit | Fair value, measurements, recurring | Fair value, inputs, level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Short-term investments:	0	0
Certificates of deposit | Fair value, measurements, recurring | Fair value, inputs, level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Short-term investments:	2,470	7,089
Certificates of deposit | Fair value, measurements, recurring | Fair value, inputs, level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Short-term investments:	0	0
Municipal debt securities | Fair value, measurements, recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Short-term investments:	546	0
Municipal debt securities | Fair value, measurements, recurring | Fair value, inputs, level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Short-term investments:	0	0
Municipal debt securities | Fair value, measurements, recurring | Fair value, inputs, level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Short-term investments:	546	0
Municipal debt securities | Fair value, measurements, recurring | Fair value, inputs, level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Short-term investments:	0	0
Commercial paper | Fair value, measurements, recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents:	350	4,349
Short-term investments:	848	4,043
Commercial paper | Fair value, measurements, recurring | Fair value, inputs, level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents:	0	0
Short-term investments:	0	0
Commercial paper | Fair value, measurements, recurring | Fair value, inputs, level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents:	350	4,349
Short-term investments:	848	4,043
Commercial paper | Fair value, measurements, recurring | Fair value, inputs, level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents:	0	0
Short-term investments:	0	0
U.S. Treasury securities | Fair value, measurements, recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Short-term investments:	8,709	8,764
U.S. Treasury securities | Fair value, measurements, recurring | Fair value, inputs, level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Short-term investments:	8,709	8,764
U.S. Treasury securities | Fair value, measurements, recurring | Fair value, inputs, level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Short-term investments:	0	0
U.S. Treasury securities | Fair value, measurements, recurring | Fair value, inputs, level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Short-term investments:	0	0
US government agencies debt securities | Fair value, measurements, recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Short-term investments:	21,544	24,836
US government agencies debt securities | Fair value, measurements, recurring | Fair value, inputs, level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Short-term investments:	21,544	24,836
US government agencies debt securities | Fair value, measurements, recurring | Fair value, inputs, level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Short-term investments:	0	0
US government agencies debt securities | Fair value, measurements, recurring | Fair value, inputs, level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Short-term investments:	0	0
Corporate debt securities | Fair value, measurements, recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Short-term investments:	7,587	5,950
Corporate debt securities | Fair value, measurements, recurring | Fair value, inputs, level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Short-term investments:	0	0
Corporate debt securities | Fair value, measurements, recurring | Fair value, inputs, level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Short-term investments:	7,587	5,950
Corporate debt securities | Fair value, measurements, recurring | Fair value, inputs, level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Short-term investments:	0	0
Money market funds | Fair value, measurements, recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents:	9,421	922
Money market funds | Fair value, measurements, recurring | Fair value, inputs, level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents:	9,421	922
Money market funds | Fair value, measurements, recurring | Fair value, inputs, level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents:	0	0
Money market funds | Fair value, measurements, recurring | Fair value, inputs, level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents:	$ 0	$ 0