Marketable Securities (Details) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2012	Dec. 31, 2011
Summary of cash, cash equivalents and marketable securities
Fair Value	$ 1,223,334	$ 968,922
Amortized Cost	1,223,188	969,031
Gross Unrealized Gains	209	10
Gross Unrealized Losses	(63)	(119)
Total cash and cash equivalents
Summary of cash, cash equivalents and marketable securities
Amortized Cost	454,074	475,337
Gross Unrealized Losses	(13)	(17)
Fair Value	454,061	475,320
Cash and money market funds
Summary of cash, cash equivalents and marketable securities
Amortized Cost	272,840	362,035
Fair Value	272,840	362,035
U.S. Treasury securities
Summary of cash, cash equivalents and marketable securities
Amortized Cost	181,234
Gross Unrealized Losses	(13)
Fair Value	181,221
Government-sponsored enterprise securities
Summary of cash, cash equivalents and marketable securities
Amortized Cost		113,302
Gross Unrealized Losses		(17)
Fair Value		113,285
Total marketable securities
Summary of cash, cash equivalents and marketable securities
Amortized Cost	769,114	493,694
Gross Unrealized Gains	209	10
Gross Unrealized Losses	(50)	(102)
Fair Value	769,273	493,602
U.S. Treasury securities (due within 1 year)
Summary of cash, cash equivalents and marketable securities
Amortized Cost	114,060	22,105
Gross Unrealized Gains	2	2
Gross Unrealized Losses	(7)
Fair Value	114,055	22,107
Government-sponsored enterprise securities (due within 1 year)
Summary of cash, cash equivalents and marketable securities
Amortized Cost	371,170	471,589
Gross Unrealized Gains	15	8
Gross Unrealized Losses	(27)	(102)
Fair Value	371,158	471,495
Commercial paper (due within 1 year)
Summary of cash, cash equivalents and marketable securities
Amortized Cost	229,243
Gross Unrealized Gains	192
Fair Value	229,435
Corporate debt securities (due within 1 year)
Summary of cash, cash equivalents and marketable securities
Amortized Cost	54,641
Gross Unrealized Losses	(16)
Fair Value	$ 54,625

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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