Marketable Securities (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2012 item
Marketable Securities
Amortized Cost	$ 72,276
Unrealized Gains	3
Unrealized Losses	(41)
Fair Value	72,238
Number of securities in unrealized loss position for less than 12 months	18
Aggregate Fair Value	51,364
Unrealized Losses	(41)
Certificate of deposit
Marketable Securities
Amortized Cost	240
Fair Value	240
Commercial paper
Marketable Securities
Amortized Cost	12,479
Unrealized Losses	(14)
Fair Value	12,465
Aggregate Fair Value	12,465
Unrealized Losses	(14)
Corporate debt securities
Marketable Securities
Amortized Cost	59,557
Unrealized Gains	3
Unrealized Losses	(27)
Fair Value	59,533
Aggregate Fair Value	38,899
Unrealized Losses	$ (27)

This item represents the aggregate fair value of investments in debt and equity securities categorized neither as held-to-maturity nor trading securities that have been in a continuous unrealized loss position for less than twelve months.

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Amount of available-for-sale debt securities at cost, net of adjustments, which include, but are not limited to, accretion, amortization, collection of cash, previous other-than-temporary impairments (OTTI) recognized in earnings (less any cumulative-effect adjustments, as defined) and fair value hedge accounting adjustments.

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- 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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- 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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- Definition

Disclosure regarding factors used to determine that the impairment of securities not categorized as either held-to-maturity securities or trading securities (hence equal to available for sale securities) where cost exceeds fair value is not an other than temporary impairment (OTTI). This item contains disclosure of the number of investment positions in the available-for-sale investments determined to be temporarily impaired.

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