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Marketable Securities (Summary Of Assets Measured At Fair Value On Recurring Basis Using Significant Unobservable Inputs) (Details) (USD $)
In Thousands
3 Months Ended
Jul. 31, 2011
Jul. 31, 2010
Sale of securities $ (18,879) $ (14,092)
Auction Rate Securities [Member]
   
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   $ 8,200