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Cash, Cash Equivalents and Marketable Securities (Details) (USD $)
In Thousands, unless otherwise specified
Jul. 31, 2012
Jul. 31, 2011
Cash and Cash Equivalents and Marketable Securities [Line Items]    
Cost $ 140,112 $ 74,009
Unrealized Gains      
Unrealized Losses Less Than 12 Months      
Unrealized Losses 12 Months or Longer      
Estimated Fair Value 140,112 74,009
Cash
    
Cash and Cash Equivalents and Marketable Securities [Line Items]    
Cost 96,779 42,664
Unrealized Gains      
Unrealized Losses Less Than 12 Months      
Unrealized Losses 12 Months or Longer      
Estimated Fair Value 96,779 42,664
Money Market Funds
    
Cash and Cash Equivalents and Marketable Securities [Line Items]    
Cost 43,333 31,345
Unrealized Gains      
Unrealized Losses Less Than 12 Months      
Unrealized Losses 12 Months or Longer      
Estimated Fair Value $ 43,333 $ 31,345