AVAILABLE-FOR-SALE SECURITIES (Details) (USD $) In Thousands, unless otherwise specified	6 Months Ended
Jun. 30, 2013	Dec. 31, 2012
Available-for-sale securities
Amortized Cost	$ 526,792	$ 335,866
Gross Unrealized Gains	78	127
Gross Unrealized Losses	(93)	(28)
Estimated Fair Value	526,777	335,965
Maturity period for marketable securities
Maximum contractual maturity period	2 years
Average contractual maturity period	9 months
Cash and cash equivalents
Available-for-sale securities
Estimated Fair Value	191,862	86,298
Short-term investments
Available-for-sale securities
Estimated Fair Value	243,768	153,640
Long-term marketable securities
Available-for-sale securities
Estimated Fair Value	90,314	95,194
Restricted cash
Available-for-sale securities
Estimated Fair Value	833	833
U.S. government securities
Available-for-sale securities
Amortized Cost	27,101	27,197
Gross Unrealized Gains	15	10
Gross Unrealized Losses		(2)
Estimated Fair Value	27,116	27,205
U.S. government agencies
Available-for-sale securities
Amortized Cost	157,932	115,397
Gross Unrealized Gains	48	85
Gross Unrealized Losses	(51)	(16)
Estimated Fair Value	157,929	115,466
U.S. corporate notes
Available-for-sale securities
Amortized Cost	104,098	91,544
Gross Unrealized Gains	15	32
Gross Unrealized Losses	(42)	(10)
Estimated Fair Value	104,071	91,566
U.S. commercial paper
Available-for-sale securities
Amortized Cost	44,967	23,082
Estimated Fair Value	44,967	23,082
Money market funds
Available-for-sale securities
Amortized Cost	192,694	78,646
Estimated Fair Value	$ 192,694	$ 78,646

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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