Fair Value of Financial Assets and Liabilities (Details) (USD $) In Thousands, unless otherwise specified	Jul. 31, 2012	Apr. 30, 2012
Summary of cash and cash equivalents
Total cash and cash equivalents	$ 68,145	$ 74,367
Demand deposit accounts [Member]
Summary of cash and cash equivalents
Total cash and cash equivalents	62,698	45,361
Money market funds [Member]
Summary of cash and cash equivalents
Total cash and cash equivalents	447	288
U.S. Treasury bills [Member]
Summary of cash and cash equivalents
Total cash and cash equivalents	5,000	7,499
Corporate bonds (U.S. Gov. guaranteed) [Member]
Summary of cash and cash equivalents
Total cash and cash equivalents		$ 21,219