Marketable Securities (Details) (USD $)	6 Months Ended
Jun. 30, 2011	Dec. 31, 2010
Marketable Securities
Maximum maturity period of available for sale securities (in years)	3
Cash equivalents and marketable securities
Fair Value	$ 348,397,000	$ 494,905,000
Amortized Cost	348,436,000	494,977,000
Gross Unrealized Gains	25,000	37,000
Gross Unrealized Losses	(64,000)	(109,000)
Cash, fair value	10,957,000	17,104,000
Cash	10,957,000	17,104,000
Cash, cash equivalents and marketable securities
Fair Value	359,354,000	512,009,000
Amortized Cost	359,393,000	512,081,000
Gross Unrealized Gains	25,000	37,000
Gross Unrealized Losses	(64,000)	(109,000)
Fair value of Marketable debt securities which have insignificant unrealized losses and will mature within one year	91,400,000
Contractual maturity:
Due within one year	348,397,000	494,905,000
Unrealized Loss, net
Due within one year	(39,000)	(72,000)
Money market funds
Cash equivalents and marketable securities
Fair Value	168,847,000	132,364,000
Amortized Cost	168,847,000	132,364,000
Weighted Rate of Return (as a percent)	0.01%	0.04%
U.S. government bonds and notes
Cash equivalents and marketable securities
Fair Value	65,592,000	266,817,000
Amortized Cost	65,569,000	266,840,000
Gross Unrealized Gains	23,000	29,000
Gross Unrealized Losses		(52,000)
Weighted Rate of Return (as a percent)	0.22%	0.26%
Corporate notes, bonds and commercial paper
Cash equivalents and marketable securities
Fair Value	113,958,000	95,724,000
Amortized Cost	114,020,000	95,773,000
Gross Unrealized Gains	2,000	8,000
Gross Unrealized Losses	(64,000)	(57,000)
Weighted Rate of Return (as a percent)	0.40%	0.39%
Cash equivalents
Cash equivalents and marketable securities
Fair Value	168,847,000	198,158,000
Short term marketable securities
Cash equivalents and marketable securities
Fair Value	$ 179,550,000	$ 296,747,000

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This element represents the aggregate of cash, cash equivalents and marketable securities reported on the balance sheet at period end measured at fair value by the entity.

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Represents the maximum maturity period of securities invested by the entity.

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-Name FASB Staff Position (FSP)
-Number FAS115-1/124-1
-Paragraph 7
-Footnote 2
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This item represents the fair value of debt securities which are expected to mature within one year of the balance sheet date and which are categorized neither as held-to-maturity nor trading securities.

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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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-Publisher FASB
-Name Statement of Financial Accounting Standard (FAS)
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Unrestricted cash available for day-to-day operating needs, for an entity that has cash equivalents, but does not aggregate cash equivalents with cash on the balance sheet.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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-SubTopic 10
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-Name Statement of Financial Accounting Standard (FAS)
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-URI http://asc.fasb.org/extlink&oid=6506951

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