Marketable Securities (Details) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2012	Dec. 31, 2011
Summary of cash, cash equivalents and marketable securities
Amortized Cost	$ 1,298,327	$ 969,031
Gross Unrealized Gains	235	10
Gross Unrealized Losses	(20)	(119)
Fair Value	1,298,542	968,922
Total cash and cash equivalents
Summary of cash, cash equivalents and marketable securities
Amortized Cost	436,886	475,337
Gross Unrealized Losses		(17)
Fair Value	436,886	475,320
Cash and money market funds
Summary of cash, cash equivalents and marketable securities
Amortized Cost	419,318	362,035
Fair Value	419,318	362,035
Government-sponsored enterprise securities
Summary of cash, cash equivalents and marketable securities
Amortized Cost	17,568	113,302
Gross Unrealized Losses		(17)
Fair Value	17,568	113,285
Total marketable securities
Summary of cash, cash equivalents and marketable securities
Amortized Cost	861,441	493,694
Gross Unrealized Gains	235	10
Gross Unrealized Losses	(20)	(102)
Fair Value	861,656	493,602
U.S. Treasury securities (due within 1 year)
Summary of cash, cash equivalents and marketable securities
Amortized Cost	222,473	22,105
Gross Unrealized Gains	10	2
Fair Value	222,483	22,107
Government-sponsored enterprise securities (due within 1 year)
Summary of cash, cash equivalents and marketable securities
Amortized Cost	342,717	471,589
Gross Unrealized Gains	10	8
Gross Unrealized Losses	(9)	(102)
Fair Value	342,718	471,495
Commercial paper (due within 1 year)
Summary of cash, cash equivalents and marketable securities
Amortized Cost	251,446
Gross Unrealized Gains	214
Fair Value	251,660
Corporate debt securities (due within 1 year)
Summary of cash, cash equivalents and marketable securities
Amortized Cost	44,805
Gross Unrealized Gains	1
Gross Unrealized Losses	(11)
Fair Value	$ 44,795

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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