Summary of Short-Term Investments (Detail) (USD $) In Thousands, unless otherwise specified	Jul. 31, 2014	Apr. 30, 2014
Schedule of Available-for-sale Securities [Line Items]
Cost	$ 67,862	$ 40,692
Gross Unrealized Gains	46	19
Gross Unrealized Losses	(10)	(11)
Fair Value	67,898	40,700
Certificates of Deposit
Schedule of Available-for-sale Securities [Line Items]
Cost	7,230	7,466
Gross Unrealized Gains	1	17
Gross Unrealized Losses	(7)	(9)
Fair Value	7,224	7,474
Municipal Bonds
Schedule of Available-for-sale Securities [Line Items]
Cost	3,377
Gross Unrealized Gains	31
Gross Unrealized Losses	(1)
Fair Value	3,407
Commercial Paper
Schedule of Available-for-sale Securities [Line Items]
Cost	5,997
Gross Unrealized Losses	(1)
Fair Value	5,996
US Treasury securities
Schedule of Available-for-sale Securities [Line Items]
Cost	51,258	33,226
Gross Unrealized Gains	14	2
Gross Unrealized Losses	(1)	(2)
Fair Value	$ 51,271	$ 33,226