Available-for-Sale Securities (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Available-for-sale securities
Amortized Cost	$ 502,669	$ 335,866
Gross Unrealized Gains	172	127
Gross Unrealized Losses	(10)	(28)
Estimated Fair Value	502,831	335,965
Maturity period for marketable securities
Maximum contractual maturity period	2 years
Average contractual maturity period	7 months
Amount of available-for-sale securities sold	22,600	49,729	17,321
Cash and cash equivalents
Available-for-sale securities
Estimated Fair Value	125,009	86,298
Short-term investments
Available-for-sale securities
Estimated Fair Value	321,615	153,640
Marketable securities
Available-for-sale securities
Estimated Fair Value	55,374	95,194
Restricted cash
Available-for-sale securities
Estimated Fair Value	833	833
U.S. government securities
Available-for-sale securities
Amortized Cost	42,104	27,197
Gross Unrealized Gains	55	10
Gross Unrealized Losses	(1)	(2)
Estimated Fair Value	42,158	27,205
U.S. government agencies
Available-for-sale securities
Amortized Cost	141,278	115,397
Gross Unrealized Gains	61	85
Gross Unrealized Losses	(8)	(16)
Estimated Fair Value	141,331	115,466
U.S. corporate notes
Available-for-sale securities
Amortized Cost	94,923	91,544
Gross Unrealized Gains	54	32
Gross Unrealized Losses	0	(10)
Estimated Fair Value	94,977	91,566
U.S. commercial paper
Available-for-sale securities
Amortized Cost	102,021	23,082
Gross Unrealized Gains	2	0
Gross Unrealized Losses	(1)	0
Estimated Fair Value	102,022	23,082
Money market funds
Available-for-sale securities
Amortized Cost	122,343	78,646
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Estimated Fair Value	$ 122,343	$ 78,646