Maintenance Contracts (Schedule Of Deferred Maintenance Revenue) (Details) (USD $)	3 Months Ended		6 Months Ended
	Oct. 31, 2012	Oct. 31, 2011	Oct. 31, 2012	Oct. 31, 2011	Apr. 30, 2012
Maintenance Contracts [Abstract]
Deferred maintenance revenue beginning balance	$ 7,153,000	$ 7,367,000	$ 8,302,000	$ 8,420,000
Deferred maintenance revenue recognized during period	(4,346,000)	(3,903,000)	(8,028,000)	(7,770,000)
Deferred maintenance revenue of new maintenance contracts	3,795,000	3,338,000	6,328,000	6,152,000
Deferred maintenance revenue ending balance	6,602,000	6,802,000	6,602,000	6,802,000
Long-term deferred maintenance revenue	$ 400,000		$ 400,000		$ 300,000