Marketable Securities (Details) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Summary of cash, cash equivalents and marketable securities
Amortized Cost	$ 1,430,659	$ 1,321,019
Gross Unrealized Gains	193	217
Gross Unrealized Losses	(156)	(21)
Fair Value	1,430,696	1,321,215
Total cash and cash equivalents
Summary of cash, cash equivalents and marketable securities
Amortized Cost	531,247	489,407
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Value	531,247	489,407
Cash and money market funds
Summary of cash, cash equivalents and marketable securities
Amortized Cost	530,567	489,407
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Value	530,567	489,407
Government-sponsored enterprise securities
Summary of cash, cash equivalents and marketable securities
Available-for-sale Securities, Amortized Cost Basis	598,440	440,181
Available-for-sale Securities, Gross Unrealized Gains	17	49
Available-for-sale Securities, Gross Unrealized Losses	(46)	(5)
Available-for-sale Securities, Fair Value Disclosure	598,411	440,225
Available-for-sale Securities [Member]
Summary of cash, cash equivalents and marketable securities
Available-for-sale Securities, Amortized Cost Basis	899,412	831,612
Available-for-sale Securities, Gross Unrealized Gains	193	217
Available-for-sale Securities, Gross Unrealized Losses	(156)	(21)
Available-for-sale Securities, Fair Value Disclosure	899,449	831,808
U.S. Treasury securities
Summary of cash, cash equivalents and marketable securities
Amortized Cost	680
Gross Unrealized Gains	0
Gross Unrealized Losses	0
Fair Value	680
Available-for-sale Securities, Amortized Cost Basis	3,050	111,350
Available-for-sale Securities, Gross Unrealized Gains	0	2
Available-for-sale Securities, Gross Unrealized Losses	0	(2)
Available-for-sale Securities, Fair Value Disclosure	3,050	111,350
Commercial paper
Summary of cash, cash equivalents and marketable securities
Available-for-sale Securities, Amortized Cost Basis	192,702	225,294
Available-for-sale Securities, Gross Unrealized Gains	176	155
Available-for-sale Securities, Gross Unrealized Losses	0	0
Available-for-sale Securities, Fair Value Disclosure	192,878	225,449
Corporate debt securities (due within 1 year)
Summary of cash, cash equivalents and marketable securities
Available-for-sale Securities, Amortized Cost Basis	105,220	15,429
Available-for-sale Securities, Gross Unrealized Gains	0	1
Available-for-sale Securities, Gross Unrealized Losses	(110)	(1)
Available-for-sale Securities, Fair Value Disclosure	105,110	15,429
Corporate Debt Securities, Due After One Year Through Five Years [Member]
Summary of cash, cash equivalents and marketable securities
Available-for-sale Securities, Amortized Cost Basis		39,358
Available-for-sale Securities, Gross Unrealized Gains		10
Available-for-sale Securities, Gross Unrealized Losses		(13)
Available-for-sale Securities, Fair Value Disclosure		$ 39,355

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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