Fair Value Measurements (Details 2) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Marketable securities classified as available-for-sale
Amortized Cost	$ 81,045	$ 67,635
Unrealized Gains	38	24
Unrealized Losses	(23)	(54)
Estimated Fair Value	81,060	67,605
U.S. Treasury securities
Marketable securities classified as available-for-sale
Amortized Cost	1,275
Unrealized Gains	1
Estimated Fair Value	1,276
Debt securities of U.S. government-sponsored entities
Marketable securities classified as available-for-sale
Amortized Cost	761	19,351
Unrealized Gains		2
Unrealized Losses		(2)
Estimated Fair Value	761	19,351
Commercial paper
Marketable securities classified as available-for-sale
Amortized Cost	28,854	19,232
Unrealized Gains	35	17
Estimated Fair Value	28,889	19,249
Corporate debt securities
Marketable securities classified as available-for-sale
Amortized Cost	50,155	29,052
Unrealized Gains	2	5
Unrealized Losses	(23)	(52)
Estimated Fair Value	$ 50,134	$ 29,005

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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