Summary of Short-Term Investments (Detail) (USD $) In Thousands, unless otherwise specified	Jul. 31, 2013	Apr. 30, 2013
Schedule of Available-for-sale Securities [Line Items]
Cost	$ 58,108	$ 70,370
Gross Unrealized Gains	13	24
Gross Unrealized Losses	(68)	(104)
Fair Value	58,053	70,290
Certificates of Deposit
Schedule of Available-for-sale Securities [Line Items]
Cost	10,210	9,230
Gross Unrealized Gains	5	9
Gross Unrealized Losses	(8)	(4)
Fair Value	10,207	9,235
U.S. Treasury securities
Schedule of Available-for-sale Securities [Line Items]
Cost	47,898	61,140
Gross Unrealized Gains	8	15
Gross Unrealized Losses	(60)	(100)
Fair Value	$ 47,846	$ 61,055