Short-Term Investments (Details 2) (USD $) In Thousands	Jul. 31, 2011	Jul. 31, 2010
Fair value and gross unrealized losses of the Company's investments
Less Than 12 Months, Fair value	$ 27,869	$ 22,645
Less Than 12 Months, Unrealized Loss	(22)	(13)
Corporate bonds and notes [Member]
Fair value and gross unrealized losses of the Company's investments
Less Than 12 Months, Fair value	6,264	7,400
Less Than 12 Months, Unrealized Loss	(10)	(4)
U.S. government agency securities [Member]
Fair value and gross unrealized losses of the Company's investments
Less Than 12 Months, Fair value	11,576	15,245
Less Than 12 Months, Unrealized Loss	(7)	(9)
U.S. treasury bills [Member]
Fair value and gross unrealized losses of the Company's investments
Less Than 12 Months, Fair value	10,029
Less Than 12 Months, Unrealized Loss	$ (5)