AVAILABLE-FOR-SALE SECURITIES (Details) (USD $) In Thousands, unless otherwise specified	9 Months Ended
	Sep. 30, 2013	Dec. 31, 2012
Available-for-sale securities
Amortized Cost	$ 587,970	$ 335,866
Gross Unrealized Gains	210	127
Gross Unrealized Losses	(8)	(28)
Estimated Fair Value	588,172	335,965
Maturity period for marketable securities
Maximum contractual maturity period	2 years
Average contractual maturity period	8 months
Cash and cash equivalents
Available-for-sale securities
Estimated Fair Value	164,745	86,298
Short-term investments
Available-for-sale securities
Estimated Fair Value	334,261	153,640
Marketable securities
Available-for-sale securities
Estimated Fair Value	88,333	95,194
Restricted cash
Available-for-sale securities
Estimated Fair Value	833	833
U.S. government securities
Available-for-sale securities
Amortized Cost	42,169	27,197
Gross Unrealized Gains	60	10
Gross Unrealized Losses		(2)
Estimated Fair Value	42,229	27,205
U.S. government agencies
Available-for-sale securities
Amortized Cost	161,751	115,397
Gross Unrealized Gains	107	85
Gross Unrealized Losses	(1)	(16)
Estimated Fair Value	161,857	115,466
U.S. corporate notes
Available-for-sale securities
Amortized Cost	103,773	91,544
Gross Unrealized Gains	43	32
Gross Unrealized Losses	(7)	(10)
Estimated Fair Value	103,809	91,566
U.S. commercial paper
Available-for-sale securities
Amortized Cost	133,818	23,082
Estimated Fair Value	133,818	23,082
Money market funds
Available-for-sale securities
Amortized Cost	146,459	78,646
Estimated Fair Value	$ 146,459	$ 78,646