Summary of Short-Term Investments (Detail) (USD $) In Thousands, unless otherwise specified	Oct. 31, 2013	Apr. 30, 2013
Schedule of Available-for-sale Securities [Line Items]
Cost	$ 47,337	$ 70,370
Gross Unrealized Gains	21	24
Gross Unrealized Losses	(54)	(104)
Fair Value	47,304	70,290
Certificates of Deposit
Schedule of Available-for-sale Securities [Line Items]
Cost	7,560	9,230
Gross Unrealized Gains	6	9
Gross Unrealized Losses	(3)	(4)
Fair Value	7,563	9,235
U.S. Treasury securities
Schedule of Available-for-sale Securities [Line Items]
Cost	39,777	61,140
Gross Unrealized Gains	15	15
Gross Unrealized Losses	(51)	(100)
Fair Value	$ 39,741	$ 61,055