Balance Sheet Information (Details) (USD $)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Total available-for-sale securities
Amortized Cost	$ 192,158,000	$ 273,373,000
Gains in Accumulated Other Comprehensive Income	76,000	232,000
Losses in Accumulated Other Comprehensive Income		(14,000)
Estimated Fair Value	192,234,000	273,591,000
Short-term investments , unrealized loss position fair value
Short-term investments which had been in an unrealized loss position for less than 12 months		33,500,000
Short-term investments, gross unrealized loss
Aggregate unrealized fair value loss which had been in an unrealized loss position for less than 12 months		100,000
Tax impact on the unrealized gains		100,000
Total proceeds from sale of available-for-sale securities	244,929,000	707,649,000	32,971,000
Net unrealized gain (loss) on available-for-sale securities	(100,000)	200,000
Gross realized gains on available-for-sale securities		400,000
Securities in continuous unrealized loss position, minimum period		12 months
Estimated fair value of contractual maturities of available-for-sale debt securities
Due in one year or less	120,621,000
Due in 1-2 years	71,613,000
Total available-for-sale securities	192,234,000	273,591,000
Maximum
Short-term investments, gross unrealized loss
Aggregate unrealized fair value loss which had been in an unrealized loss position for less than 12 months		200,000
Tax impact on the unrealized gains	100,000
Treasury bills.
Total available-for-sale securities
Amortized Cost	184,102,000	70,147,000
Gains in Accumulated Other Comprehensive Income	76,000	46,000
Losses in Accumulated Other Comprehensive Income		(1,000)
Estimated Fair Value	184,178,000	70,192,000
Government agency securities
Total available-for-sale securities
Amortized Cost	8,056,000	88,585,000
Gains in Accumulated Other Comprehensive Income		62,000
Losses in Accumulated Other Comprehensive Income		(6,000)
Estimated Fair Value	8,056,000	88,641,000
FDIC guaranteed corporate debt
Total available-for-sale securities
Amortized Cost		114,641,000
Gains in Accumulated Other Comprehensive Income		124,000
Losses in Accumulated Other Comprehensive Income		(7,000)
Estimated Fair Value		$ 114,758,000