Short-Term Investments (Details) (USD $) In Thousands	Jul. 31, 2011	Jul. 31, 2010
Summary of Short-Term Investments
Cost Basis	$ 152,970	$ 123,975
Gross Unrealized Gains	237	205
Gross Unrealized Losses	(22)	(13)
Total Short-term investments, Fair value	153,185	124,167
Corporate bonds and notes [Member]
Summary of Short-Term Investments
Cost Basis	41,912	23,802
Gross Unrealized Gains	118	69
Gross Unrealized Losses	(10)	(4)
Total Short-term investments, Fair value	42,020	23,867
U.S. government agency securities [Member]
Summary of Short-Term Investments
Cost Basis	40,824	80,683
Gross Unrealized Gains	33	78
Gross Unrealized Losses	(7)	(9)
Total Short-term investments, Fair value	40,850	80,752
U.S. treasury bills [Member]
Summary of Short-Term Investments
Cost Basis	57,026	12,816
Gross Unrealized Gains	72	57
Gross Unrealized Losses	(5)	0
Total Short-term investments, Fair value	57,093	12,873
Commercial paper [Member]
Summary of Short-Term Investments
Cost Basis	5,590	4,495
Gross Unrealized Gains	2	0
Gross Unrealized Losses	0	0
Total Short-term investments, Fair value	5,592	4,495
Certificates of deposit [Member]
Summary of Short-Term Investments
Cost Basis	7,618	2,179
Gross Unrealized Gains	12	1
Gross Unrealized Losses	0	0
Total Short-term investments, Fair value	$ 7,630	$ 2,180