Balance Sheet Information (Details) (USD $)	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012	Dec. 31, 2012
Total available-for-sale securities
Amortized Cost	$ 362,703,000		$ 192,158,000
Gains in Accumulated Other Comprehensive Income	71,000		76,000
Losses in Accumulated Other Comprehensive Income	(27,000)
Estimated Fair Value	362,747,000		192,234,000
Short-term investments , unrealized loss position fair value
Total, Estimated Fair Value	28,491,000
Short-term investments, gross unrealized loss
Total, Unrealized Losses	(27,000)
Total proceeds from sale of available-for-sale securities	101,236,000	43,569,000
Gross realized gains on available-for-sale securities	100,000
Estimated fair value of contractual maturities of available-for-sale debt securities
Due in one year or less	288,752,000
Due in 1-2 years	32,811,000
Due in 2-3 years	41,184,000
Total available-for-sale securities	362,747,000		192,234,000
Treasury bills
Total available-for-sale securities
Amortized Cost	271,168,000		184,102,000
Gains in Accumulated Other Comprehensive Income	33,000		76,000
Estimated Fair Value	271,201,000		184,178,000
Government agency securities
Total available-for-sale securities
Amortized Cost	8,055,000		8,056,000
Estimated Fair Value	8,055,000		8,056,000
Corporate bonds
Total available-for-sale securities
Amortized Cost	63,488,000
Gains in Accumulated Other Comprehensive Income	38,000
Losses in Accumulated Other Comprehensive Income	(27,000)
Estimated Fair Value	63,499,000
Short-term investments , unrealized loss position fair value
Total, Estimated Fair Value	28,491,000
Short-term investments, gross unrealized loss
Total, Unrealized Losses	(27,000)
Commercial paper
Total available-for-sale securities
Amortized Cost	19,992,000
Estimated Fair Value	$ 19,992,000