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Marketable Securities (Summary Of Financial Assets Measured At Fair Value) (Details) (USD $)
In Thousands, unless otherwise specified
Oct. 31, 2011
Apr. 30, 2011
Total $ 182,797 $ 243,071
Cash Equivalents, Including Restricted Cash [Member]
   
Cash equivalents, including restricted cash 56,856 120,840
Mutual Funds [Member]
   
Investment owned, at fair value 78,432 71,363
Corporate Bonds [Member]
   
Investment owned, at fair value 45,090 40,444
U.S. Treasury And Agency Securities [Member]
   
Investment owned, at fair value 2,419 9,424
Commercial Paper [Member]
   
Investment owned, at fair value   1,000
Level 1 [Member]
   
Total 135,288 192,203
Level 1 [Member] | Cash Equivalents, Including Restricted Cash [Member]
   
Cash equivalents, including restricted cash 56,856 120,840
Level 1 [Member] | Mutual Funds [Member]
   
Investment owned, at fair value 78,432 71,363
Level 2 [Member]
   
Total 47,509 50,868
Level 2 [Member] | Corporate Bonds [Member]
   
Investment owned, at fair value 45,090 40,444
Level 2 [Member] | U.S. Treasury And Agency Securities [Member]
   
Investment owned, at fair value 2,419 9,424
Level 2 [Member] | Commercial Paper [Member]
   
Investment owned, at fair value   $ 1,000