Available-for-Sale Securities (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2014	Mar. 31, 2013	Dec. 31, 2013
Available-for-sale securities
Amortized Cost	$ 425,301		$ 502,669
Gross Unrealized Gains	181		172
Gross Unrealized Losses	(10)		(10)
Estimated Fair Value	425,472		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	5,000
Cash and cash equivalents
Available-for-sale securities
Estimated Fair Value	119,160		125,009
Short-term investments
Available-for-sale securities
Estimated Fair Value	245,648		321,615
Marketable securities
Available-for-sale securities
Estimated Fair Value	59,831		55,374
Restricted cash
Available-for-sale securities
Estimated Fair Value	833		833
U.S. government securities
Available-for-sale securities
Amortized Cost	37,042		42,104
Gross Unrealized Gains	74		55
Gross Unrealized Losses	0		(1)
Estimated Fair Value	37,116		42,158
U.S. government agencies
Available-for-sale securities
Amortized Cost	110,171		141,278
Gross Unrealized Gains	59		61
Gross Unrealized Losses	0		(8)
Estimated Fair Value	110,230		141,331
U.S. corporate notes
Available-for-sale securities
Amortized Cost	88,607		94,923
Gross Unrealized Gains	48		54
Gross Unrealized Losses	(10)		0
Estimated Fair Value	88,645		94,977
U.S. commercial paper
Available-for-sale securities
Amortized Cost	74,487		102,021
Gross Unrealized Gains	0		2
Gross Unrealized Losses	0		(1)
Estimated Fair Value	74,487		102,022
Money market funds
Available-for-sale securities
Amortized Cost	114,994		122,343
Gross Unrealized Gains	0		0
Gross Unrealized Losses	0		0
Estimated Fair Value	$ 114,994		$ 122,343