Fair Value of Financial Instruments Fair Value of Financial Instruments (Details 3) (USD $) In Thousands, unless otherwise specified	Apr. 30, 2013	Jul. 31, 2012
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available For Sale Securities, Debt Maturities Current Fiscal Year	$ 95,396
Available For Sale Securities, Debt Maturities Next Fiscal Year	60,600
Available For Sale Securities, Debt Maturities Greater Than Two Years	30,042
Total	186,038	108,833
U.S. agency securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available For Sale Securities, Debt Maturities Current Fiscal Year	3,608
Available For Sale Securities, Debt Maturities Next Fiscal Year	8,093
Available For Sale Securities, Debt Maturities Greater Than Two Years	19,440
Total	31,141
Commercial paper
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available For Sale Securities, Debt Maturities Current Fiscal Year	46,266
Available For Sale Securities, Debt Maturities Next Fiscal Year	11,780
Available For Sale Securities, Debt Maturities Greater Than Two Years	0
Total	58,046
Corporate Bond Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available For Sale Securities, Debt Maturities Current Fiscal Year	3,501
Available For Sale Securities, Debt Maturities Next Fiscal Year	38,054
Available For Sale Securities, Debt Maturities Greater Than Two Years	6,293
Total	47,848
Foreign Government Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available For Sale Securities, Debt Maturities Current Fiscal Year	0
Available For Sale Securities, Debt Maturities Next Fiscal Year	787
Available For Sale Securities, Debt Maturities Greater Than Two Years	0
Total	787
Money Market Funds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available For Sale Securities, Debt Maturities Current Fiscal Year	42,021
Available For Sale Securities, Debt Maturities Next Fiscal Year	0
Available For Sale Securities, Debt Maturities Greater Than Two Years	0
Total	42,021	105,107
Municipal debt securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available For Sale Securities, Debt Maturities Current Fiscal Year	0
Available For Sale Securities, Debt Maturities Next Fiscal Year	1,886
Available For Sale Securities, Debt Maturities Greater Than Two Years	4,309
Total	$ 6,195

- Definition

This element represents the portion of the balance sheet assertion valued at fair value by the entity whether such amount is presented as a separate caption or as a parenthetical disclosure. Additionally, this element may be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements. The element may be used in both the balance sheet and disclosure in the same submission. This item represents Available-for-sale Securities which consist of all investments in certain debt and equity securities neither classified as trading or held-to-maturity securities. A debt security represents a creditor relationship with an enterprise. Debt securities include, among other items, US Treasury securities, US government securities, municipal securities, corporate bonds, convertible debt, commercial paper, and all securitized debt instruments. An equity security represents an ownership interest in an enterprise or the right to acquire or dispose of an ownership interest in an enterprise at fixed or determinable prices. Equity securities include, among other things, common stock, certain preferred stock, warrant rights, call options, and put options, but do not include convertible debt. An entity may opt to provide the reader with additional narrative text to better understand the nature of investments in debt and equity securities which are categorized as Available-for-sale.

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