Summary of Cash and Cash Equivalents (Detail) (USD $) In Thousands, unless otherwise specified	Jan. 31, 2013	Apr. 30, 2012
Cash and Cash Equivalents [Line Items]
Total cash and cash equivalents	$ 23,845	$ 74,367
U.S. Treasury securities
Cash and Cash Equivalents [Line Items]
Total cash and cash equivalents	8,997	7,499
Corporate bonds
Cash and Cash Equivalents [Line Items]
Total cash and cash equivalents		21,219
Demand deposit accounts
Cash and Cash Equivalents [Line Items]
Total cash and cash equivalents	14,845	45,361
Money market funds
Cash and Cash Equivalents [Line Items]
Total cash and cash equivalents	$ 3	$ 288