Quarterly report pursuant to Section 13 or 15(d)

Marketable Securities (Summary Of Financial Assets Measured At Fair Value) (Details)

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Marketable Securities (Summary Of Financial Assets Measured At Fair Value) (Details) (USD $)
In Thousands, unless otherwise specified
Jan. 31, 2012
Apr. 30, 2011
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Total $ 205,838 $ 243,071
Cash Equivalents, Including Restricted Cash [Member]
   
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Cash equivalents, including restricted cash 71,630 120,840
Mutual Funds [Member]
   
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Investment owned, at fair value 80,500 71,363
Corporate Bonds [Member]
   
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Investment owned, at fair value 45,725 40,444
U.S. Treasury And Agency Securities [Member]
   
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Investment owned, at fair value 2,002 9,424
Commercial Paper [Member]
   
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Investment owned, at fair value 5,981 1,000
Level 1 [Member]
   
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Total 152,130 192,203
Level 1 [Member] | Cash Equivalents, Including Restricted Cash [Member]
   
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Cash equivalents, including restricted cash 71,630 120,840
Level 1 [Member] | Mutual Funds [Member]
   
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Investment owned, at fair value 80,500 71,363
Level 2 [Member]
   
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Total 53,708 50,868
Level 2 [Member] | Corporate Bonds [Member]
   
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Investment owned, at fair value 45,725 40,444
Level 2 [Member] | U.S. Treasury And Agency Securities [Member]
   
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Investment owned, at fair value 2,002 9,424
Level 2 [Member] | Commercial Paper [Member]
   
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Investment owned, at fair value 5,981 1,000
Level 3 [Member]
   
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Total      
Level 3 [Member] | Cash Equivalents, Including Restricted Cash [Member]
   
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Cash equivalents, including restricted cash      
Level 3 [Member] | Mutual Funds [Member]
   
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Investment owned, at fair value      
Level 3 [Member] | Corporate Bonds [Member]
   
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Investment owned, at fair value      
Level 3 [Member] | U.S. Treasury And Agency Securities [Member]
   
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Investment owned, at fair value      
Level 3 [Member] | Commercial Paper [Member]
   
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Investment owned, at fair value