Summary of Short-Term Investments (Detail) (USD $) In Thousands, unless otherwise specified	Jan. 31, 2015	Apr. 30, 2014
Schedule of Available-for-sale Securities [Line Items]
Cost	$ 59,040	$ 40,692
Gross Unrealized Gains	26	19
Gross Unrealized Losses	(58)	(11)
Fair Value	59,008us-gaap_AvailableForSaleSecurities	40,700us-gaap_AvailableForSaleSecurities
Certificates of Deposit
Schedule of Available-for-sale Securities [Line Items]
Cost	5,680	7,466
Gross Unrealized Gains	6	17
Gross Unrealized Losses	(3)	(9)
Fair Value	5,683	7,474
Municipal Bonds
Schedule of Available-for-sale Securities [Line Items]
Cost	4,714
Gross Unrealized Gains	10
Gross Unrealized Losses	(26)
Fair Value	4,698
Commercial Paper
Schedule of Available-for-sale Securities [Line Items]
Cost	5,839
Gross Unrealized Gains	1
Gross Unrealized Losses	(2)
Fair Value	5,838
US Treasury notes
Schedule of Available-for-sale Securities [Line Items]
Cost	28,823	33,226
Gross Unrealized Gains	4	2
Gross Unrealized Losses	(17)	(2)
Fair Value	28,810	33,226
U.S. Treasury Bonds
Schedule of Available-for-sale Securities [Line Items]
Cost	5,001
Gross Unrealized Losses	(3)
Fair Value	4,998
Corporate Notes
Schedule of Available-for-sale Securities [Line Items]
Cost	2,709
Gross Unrealized Gains	4
Fair Value	2,713
Corporate Bonds
Schedule of Available-for-sale Securities [Line Items]
Cost	6,274
Gross Unrealized Gains	1
Gross Unrealized Losses	(7)
Fair Value	$ 6,268

This item represents the cost of debt securities, which are categorized neither as held-to-maturity nor trading, net of adjustments including accretion, amortization, collection of cash, previous other-than-temporary impairments recognized in earnings (less any cumulative-effect adjustments recognized, as defined), and fair value hedge accounting adjustments, if any.

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