Fair Value of Financial Assets and Liabilities - Summary of Cash and Cash Equivalents (Details) - USD ($) $ in Thousands	Jan. 31, 2016	Apr. 30, 2015
Cash and Cash Equivalents [Line Items]
Total cash and cash equivalents	$ 57,944	$ 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	53,041	49,977
Money market funds
Cash and Cash Equivalents [Line Items]
Total cash and cash equivalents	462	2,831
Commercial paper
Cash and Cash Equivalents [Line Items]
Total cash and cash equivalents	1,940	875
US government agencies debt securities
Cash and Cash Equivalents [Line Items]
Total cash and cash equivalents	$ 2,501	$ 0