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Marketable Securities (Summary Of Assets Measured At Fair Value On Recurring Basis Using Significant Unobservable Inputs) (Details) (USD $)
In Thousands, unless otherwise specified
 3 Months Ended 9 Months Ended
Jan. 31, 2012
Jan. 31, 2011
Jan. 31, 2012
Jan. 31, 2011
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]        
Balance, beginning of period     $ 71,363  
Realized gain included in operations     (1,954) 4,187
Sale of securities     (35,466) (23,453)
Balance, end of period 80,500   80,500  
Auction Rate Securities [Member]
        
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]        
Balance, beginning of period          8,200
Auction rate securities put option          (745)
Realized gain included in operations          745
Sale of securities          (8,200)
Balance, end of period