FINANCIAL INSTRUMENTS (Details 2) (USD $) In Thousands, unless otherwise specified				0 Months Ended				6 Months Ended
	Oct. 31, 2012	Oct. 31, 2011	Oct. 31, 2010	Apr. 23, 2007 Treasury lock agreement based on U.S. Treasury security Cash flow hedges.	Oct. 31, 2012 Treasury lock agreement based on U.S. Treasury security Cash flow hedges.	Oct. 31, 2011 Treasury lock agreement based on U.S. Treasury security Cash flow hedges.	Apr. 30, 2007 Treasury lock agreement based on U.S. Treasury security Cash flow hedges. agreement	Apr. 30, 2007 Treasury lock agreement based on U.S. Treasury security, one and two Cash flow hedges.	Apr. 30, 2007 Treasury lock agreement based on U.S. Treasury security, three Cash flow hedges.
Treasury lock agreements
Number of treasury lock agreements							3
Amortization period								30 years
Principal balance								$ 30,000	$ 40,000
Net settlement of unrecognized loss portion recorded in accumulated other comprehensive loss				182
Amount of unrecognized loss portion in accumulated other comprehensive loss	$ (210)	$ (122)	$ (2,793)		$ 149	$ 155