Marketable Securities (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Available-for-sale securities
Amortized Cost	$ 335,866	$ 235,734
Gross Unrealized Gains	127	36
Gross Unrealized Losses	(28)	(19)
Estimated Fair Value	335,965	235,751
Net realized gains (losses) on marketable securities
Realized gains	9		3
Realized losses		(2)
Net realized gains (losses)	9	(2)	3
Fair Value
Fair Value, In loss position for less than 12 months	52,194
Fair Value, Total	52,194
Gross Unrealized Losses
Estimated Gross Unrealized Losses, In loss position for less than 12 months	(28)
Gross Unrealized Losses, Total	(28)
Maturity period for marketable securities
Average contractual maturity period	9 months
Maximum
Maturity period for marketable securities
Maximum contractual maturity period	24 months
Cash and cash equivalents
Available-for-sale securities
Estimated Fair Value	86,298	38,721
Short-term investments
Available-for-sale securities
Estimated Fair Value	153,640	196,137
Long-term marketable securities
Available-for-sale securities
Estimated Fair Value	95,194
Restricted cash
Available-for-sale securities
Estimated Fair Value	833	893
U.S. government securities
Available-for-sale securities
Amortized Cost	27,197	66,150
Gross Unrealized Gains	10	24
Gross Unrealized Losses	(2)
Estimated Fair Value	27,205	66,174
Fair Value
Fair Value, In loss position for less than 12 months	7,002
Fair Value, Total	7,002
Gross Unrealized Losses
Estimated Gross Unrealized Losses, In loss position for less than 12 months	(2)
Gross Unrealized Losses, Total	(2)
U.S. government agencies
Available-for-sale securities
Amortized Cost	115,397	93,183
Gross Unrealized Gains	85	9
Gross Unrealized Losses	(16)	(17)
Estimated Fair Value	115,466	93,175
Fair Value
Fair Value, In loss position for less than 12 months	10,499
Fair Value, Total	10,499
Gross Unrealized Losses
Estimated Gross Unrealized Losses, In loss position for less than 12 months	(16)
Gross Unrealized Losses, Total	(16)
U.S. corporate notes
Available-for-sale securities
Amortized Cost	91,544	2,707
Gross Unrealized Gains	32
Gross Unrealized Losses	(10)	(2)
Estimated Fair Value	91,566	2,705
Fair Value
Fair Value, In loss position for less than 12 months	34,693
Fair Value, Total	34,693
Gross Unrealized Losses
Estimated Gross Unrealized Losses, In loss position for less than 12 months	(10)
Gross Unrealized Losses, Total	(10)
U.S. commercial paper
Available-for-sale securities
Amortized Cost	23,082	34,973
Gross Unrealized Gains		3
Estimated Fair Value	23,082	34,976
Money market funds
Available-for-sale securities
Amortized Cost	78,646	38,721
Estimated Fair Value	$ 78,646	$ 38,721