Available-for-Sale Securities and Fair Value Measurements (Details) (USD $) In Thousands, unless otherwise specified	9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Dec. 31, 2013
Available-for-sale securities
Amortized Cost	$ 316,940		$ 502,669
Gross Unrealized Gains	57		172
Gross Unrealized Losses	(254)		(10)
Estimated Fair Value	316,743		502,831
Maturity period for marketable securities
Maximum contractual maturity period	2 years
Average contractual maturity period	8 months
Amount of marketable securities sold	5,000	22,600
Cash and cash equivalents
Available-for-sale securities
Estimated Fair Value	119,022		125,009
Short-term marketable securities
Available-for-sale securities
Estimated Fair Value	143,536		321,615
Marketable securities
Available-for-sale securities
Estimated Fair Value	47,752		55,374
Restricted cash
Available-for-sale securities
Estimated Fair Value	6,433		833
U.S. government securities
Available-for-sale securities
Amortized Cost	33,526		42,104
Gross Unrealized Gains	39		55
Gross Unrealized Losses	(1)		(1)
Estimated Fair Value	33,564		42,158
U.S. government agency securities
Available-for-sale securities
Amortized Cost	43,611		141,278
Gross Unrealized Gains	9		61
Gross Unrealized Losses	(6)		(8)
Estimated Fair Value	43,614		141,331
U.S. corporate notes
Available-for-sale securities
Amortized Cost	59,644		94,923
Gross Unrealized Gains	9		54
Gross Unrealized Losses	(46)		0
Estimated Fair Value	59,607		94,977
U.S. commercial paper
Available-for-sale securities
Amortized Cost	44,434		102,021
Gross Unrealized Gains	0		2
Gross Unrealized Losses	0		(1)
Estimated Fair Value	44,434		102,022
Equity securities
Available-for-sale securities
Amortized Cost	10,269
Gross Unrealized Gains	0
Gross Unrealized Losses	(201)
Estimated Fair Value	10,068
Holding period for equity securities	6 months
Money market funds
Available-for-sale securities
Amortized Cost	125,456		122,343
Gross Unrealized Gains	0		0
Gross Unrealized Losses	0		0
Estimated Fair Value	$ 125,456		$ 122,343