Summary of Short-Term Investments (Detail) (USD $) In Thousands, unless otherwise specified	Jan. 31, 2014	Apr. 30, 2013
Schedule of Available-for-sale Securities [Line Items]
Cost	$ 28,184	$ 70,370
Gross Unrealized Gains	14	24
Gross Unrealized Losses	(3)	(104)
Fair Value	28,195	70,290
Certificates of Deposit
Schedule of Available-for-sale Securities [Line Items]
Cost	5,450	9,230
Gross Unrealized Gains	4	9
Gross Unrealized Losses	(1)	(4)
Fair Value	5,453	9,235
U.S. Treasury securities
Schedule of Available-for-sale Securities [Line Items]
Cost	22,734	61,140
Gross Unrealized Gains	10	15
Gross Unrealized Losses	(2)	(100)
Fair Value	$ 22,742	$ 61,055