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Marketable Securities (Summary Of Amortized Cost And Fair Values Of Available-For-Sale Investments) (Details) (USD $)
In Thousands, unless otherwise specified
Oct. 31, 2011
Apr. 30, 2011
Amortized Cost $ 47,601 $ 50,796
Gross Unrealized Gains 74 117
Gross Unrealized Losses (166) [1] (45) [1]
Estimated Fair Value 47,509 50,868
Corporate Bonds [Member]
   
Amortized Cost 45,186 40,369
Gross Unrealized Gains 70 107
Gross Unrealized Losses (166) [1] (32) [1]
Estimated Fair Value 45,090 40,444
U.S. Treasury And Agency Securities [Member]
   
Amortized Cost 2,415 9,427
Gross Unrealized Gains 4 10
Gross Unrealized Losses   (13) [1]
Estimated Fair Value 2,419 9,424
Commercial Paper [Member]
   
Amortized Cost   1,000
Estimated Fair Value   $ 1,000
[1] There are no marketable securities that have been in a continuous unrealized loss position for 12 months or more.