Marketable Securities (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
Mar. 31, 2013	Dec. 31, 2012
Marketable Securities
Weighted average maturity of current marketable securities	9 months
Summary of Available-for-sale marketable securities
Estimated Fair Value, Current	$ 294,157	$ 331,192
Estimated Fair Value, Non-current	8,178	8,323
Adjusted Cost	302,293	339,440
Unrealized Gains	266	334
Unrealized Losses	(224)	(259)
Estimated Fair Value, available-for-sale marketable securities	302,335	339,515
Agency bond investments
Summary of Available-for-sale marketable securities
Adjusted Cost, Current	100,320	107,673
Unrealized Gains, Current	82	65
Estimated Fair Value, Current	100,402	107,738
Adjusted Cost	100,320	107,673
Unrealized Gains	82	65
Estimated Fair Value, agency bond investments and corporate investments	100,402	107,738
Corporate debt investments
Summary of Available-for-sale marketable securities
Adjusted Cost, Current	193,598	223,192
Unrealized Gains, Current	184	269
Unrealized Losses, Current	(27)	(7)
Estimated Fair Value, Current	193,755	223,454
Adjusted Cost, Non-current	8,375	8,575
Unrealized Losses, Non-current	(197)	(252)
Estimated Fair Value, Non-current	8,178	8,323
Adjusted Cost	201,973	231,767
Unrealized Gains	184	269
Unrealized Losses	(224)	(259)
Estimated Fair Value, agency bond investments and corporate investments	$ 201,933	$ 231,777

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Total of debt securities categorized neither as held-to-maturity nor trading which are intended be sold or mature within one year from the balance sheet date or the normal operating cycle, whichever is longer.

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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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