Fair Value of Financial Assets and Liabilities - Summary of Cash and Cash Equivalents (Details) - USD ($) $ in Thousands	Jul. 31, 2015	Apr. 30, 2015
Cash and Cash Equivalents [Line Items]
Total cash and cash equivalents	$ 53,450	$ 54,041
Corporate bonds
Cash and Cash Equivalents [Line Items]
Total cash and cash equivalents	0	256
Municipal bonds
Cash and Cash Equivalents [Line Items]
Total cash and cash equivalents	1,597	102
Demand deposit accounts
Cash and Cash Equivalents [Line Items]
Total cash and cash equivalents	40,873	49,977
Money market funds
Cash and Cash Equivalents [Line Items]
Total cash and cash equivalents	4,210	2,831
Commercial paper
Cash and Cash Equivalents [Line Items]
Total cash and cash equivalents	$ 6,770	$ 875