Fair Value of Financial Assets and Liabilities - Summary of Cash and Cash Equivalents (Details) - USD ($) $ in Thousands	Apr. 30, 2016	Apr. 30, 2015
Cash and Cash Equivalents [Line Items]
Total cash and cash equivalents	$ 43,963	$ 54,041
Municipal debt securities
Cash and Cash Equivalents [Line Items]
Total cash and cash equivalents	0	102
Corporate debt securities
Cash and Cash Equivalents [Line Items]
Total cash and cash equivalents	0	256
Demand deposit accounts
Cash and Cash Equivalents [Line Items]
Total cash and cash equivalents	38,692	49,977
Money market funds
Cash and Cash Equivalents [Line Items]
Total cash and cash equivalents	922	2,831
Commercial paper
Cash and Cash Equivalents [Line Items]
Total cash and cash equivalents	$ 4,349	$ 875