Annual report pursuant to Section 13 and 15(d)

Amortized Cost and Fair Values of Marketable Securities Classified as Available-For-Sale Investments (Detail)

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Amortized Cost and Fair Values of Marketable Securities Classified as Available-For-Sale Investments (Detail) (USD $)
In Thousands, unless otherwise specified
Apr. 30, 2013
Apr. 30, 2012
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost $ 43,835 $ 53,497
Gross Unrealized Gains 94 82
Gross Unrealized Losses (14) [1] (21) [1]
Estimated Fair Value 43,915 53,558
Corporate Bonds
   
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 42,033 44,498
Gross Unrealized Gains 92 81
Gross Unrealized Losses (14) [1] (16) [1]
Estimated Fair Value 42,111 44,563
U.S. Treasury And Agency Securities
   
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 1,802 3,006
Gross Unrealized Gains 2  
Estimated Fair Value 1,804 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.