Fair Value of Financial Instruments Fair Value of Financial Instruments (Details 1) (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]
Amortized Cost	$ 185,980	$ 108,833
Unrealized Gains	71	0
Unrealized Losses	(13)	0
Available-for-sale Securities, Fair Value Disclosure	186,038	108,833
U.S. agency securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amortized Cost	31,120
Unrealized Gains	21
Unrealized Losses	0
Available-for-sale Securities, Fair Value Disclosure	31,141
Commercial paper
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amortized Cost	58,029
Unrealized Gains	17
Unrealized Losses	0
Available-for-sale Securities, Fair Value Disclosure	58,046
Corporate Bond Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amortized Cost	47,831
Unrealized Gains	25
Unrealized Losses	(8)
Available-for-sale Securities, Fair Value Disclosure	47,848
Foreign Government Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amortized Cost	790
Unrealized Gains	0
Unrealized Losses	(3)
Available-for-sale Securities, Fair Value Disclosure	787
Municipal debt securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amortized Cost	6,189
Unrealized Gains	8
Unrealized Losses	(2)
Available-for-sale Securities, Fair Value Disclosure	6,195
Money Market Funds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amortized Cost	42,021	105,107
Unrealized Gains	0	0
Unrealized Losses	0	0
Available-for-sale Securities, Fair Value Disclosure	42,021	105,107
Certificates of deposit
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amortized Cost		3,726
Unrealized Gains		0
Unrealized Losses		0
Available-for-sale Securities, Fair Value Disclosure		$ 3,726

This item represents the cost of debt and equity securities, which are categorized neither as held-to-maturity nor trading, net of adjustments including accretion, amortization, collection of cash, previous other-than-temporary impairments recognized in earnings (less any cumulative-effect adjustments recognized, as defined), and fair value hedge accounting adjustments, if any.

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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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- Definition

This item represents the gross unrealized gains for securities, at a point in time, which are categorized neither as held-to-maturity nor trading securities.

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