Amortized Cost and Fair Values of Marketable Securities Classified as Available-For-Sale Investments (Detail) (USD $) In Thousands, unless otherwise specified	Jan. 31, 2013		Apr. 30, 2012
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost			$ 53,497
Gross Unrealized Gains			82
Gross Unrealized Losses			(21)	[1]
Estimated Fair Value			53,558
Corporate Bonds
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	41,259		44,498
Gross Unrealized Gains	89		81
Gross Unrealized Losses	(44)	[1]	(16)	[1]
Estimated Fair Value	41,304		44,563
U.S. Treasury And Agency Securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost			3,006
Estimated Fair Value			3,006
Commercial Paper
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost			5,993
Gross Unrealized Gains			1
Gross Unrealized Losses			(5)	[1]
Estimated Fair Value			$ 5,989

[1] There are no marketable securities that have been in a continuous unrealized loss position for 12 months or more.