Available-for-Sale Securities (Details) (USD $) In Thousands, unless otherwise specified	6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Dec. 31, 2013
Available-for-sale securities
Amortized Cost	$ 342,894		$ 502,669
Gross Unrealized Gains	3,722		172
Gross Unrealized Losses	(16)		(10)
Estimated Fair Value	346,600		502,831
Maturity period for marketable securities
Maximum contractual maturity period	2 years
Average contractual maturity period	7 months
Amount of available-for-sale securities sold	5,000	17,600
Cash and cash equivalents
Available-for-sale securities
Estimated Fair Value	184,994		125,009
Short-term investments
Available-for-sale securities
Estimated Fair Value	126,424		321,615
Marketable securities
Available-for-sale securities
Estimated Fair Value	34,349		55,374
Restricted cash
Available-for-sale securities
Estimated Fair Value	833		833
U.S. government securities
Available-for-sale securities
Amortized Cost	23,509		42,104
Gross Unrealized Gains	44		55
Gross Unrealized Losses	0		(1)
Estimated Fair Value	23,553		42,158
U.S. government agencies
Available-for-sale securities
Amortized Cost	54,712		141,278
Gross Unrealized Gains	15		61
Gross Unrealized Losses	(5)		(8)
Estimated Fair Value	54,722		141,331
U.S. corporate notes
Available-for-sale securities
Amortized Cost	33,632		94,923
Gross Unrealized Gains	7		54
Gross Unrealized Losses	(11)		0
Estimated Fair Value	33,628		94,977
U.S. commercial paper
Available-for-sale securities
Amortized Cost	34,942		102,021
Gross Unrealized Gains	0		2
Gross Unrealized Losses	0		(1)
Estimated Fair Value	34,942		102,022
Equity securities
Available-for-sale securities
Amortized Cost	10,269
Gross Unrealized Gains	3,656
Gross Unrealized Losses	0
Estimated Fair Value	13,925		0
Holding period for equity securities	6 months
Money market funds
Available-for-sale securities
Amortized Cost	185,830		122,343
Gross Unrealized Gains	0		0
Gross Unrealized Losses	0		0
Estimated Fair Value	$ 185,830		$ 122,343