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Maintenance Contracts (Schedule Of Deferred Maintenance Revenue) (Details) (USD $)
3 Months Ended
Jul. 31, 2012
Jul. 31, 2011
Apr. 30, 2012
Maintenance Contracts [Abstract]      
Deferred maintenance revenue beginning balance $ 8,302,000 $ 8,420,000  
Deferred maintenance revenue recognized during period (3,683,000) (3,867,000)  
Deferred maintenance revenue of new maintenance contracts 2,534,000 2,814,000  
Deferred maintenance revenue ending balance 7,153,000 7,367,000  
Long-term deferred maintenance revenue $ 200,000   $ 300,000