Balance Sheet Information (Details) (USD $)	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011	Dec. 31, 2011
Total available-for-sale securities
Amortized Cost	$ 185,629,000		$ 185,629,000		$ 273,372,000
Gains in Accumulated Other Comprehensive Income	64,000		64,000		233,000
Losses in Accumulated Other Comprehensive Income	(1,000)		(1,000)		(14,000)
Estimated Fair Value	185,692,000		185,692,000		273,591,000
Short-term investments , unrealized loss position fair value
Less than 12 months, fair value	18,519,000		18,519,000		33,554,000
Total, fair value	18,519,000		18,519,000		33,554,000
Short-term investments, gross unrealized loss
Less than 12 months, gross unrealized loss	(1,000)		(1,000)		(14,000)
Total, gross unrealized loss	(1,000)		(1,000)		(14,000)
Total proceeds from sale of available-for-sale securities	76,800,000	292,900,000	176,303,000	667,216,000
Unrealized (loss) gain on available-for-sale securities		(100,000)	(200,000)	100,000
Gross realized gains on available-for-sale securities		200,000		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	107,269,000		107,269,000
Due in 1-2 years	78,423,000		78,423,000
Total available-for-sale securities	185,692,000		185,692,000		273,591,000
Treasury bills
Total available-for-sale securities
Amortized Cost	117,872,000		117,872,000		70,147,000
Gains in Accumulated Other Comprehensive Income	54,000		54,000		46,000
Losses in Accumulated Other Comprehensive Income					(1,000)
Estimated Fair Value	117,926,000		117,926,000		70,192,000
Short-term investments , unrealized loss position fair value
Less than 12 months, fair value					5,024,000
Total, fair value					5,024,000
Short-term investments, gross unrealized loss
Less than 12 months, gross unrealized loss					(1,000)
Total, gross unrealized loss					(1,000)
Government agency securities
Total available-for-sale securities
Amortized Cost	39,559,000		39,559,000		88,585,000
Gains in Accumulated Other Comprehensive Income	4,000		4,000		62,000
Losses in Accumulated Other Comprehensive Income	(1,000)		(1,000)		(6,000)
Estimated Fair Value	39,562,000		39,562,000		88,641,000
Short-term investments , unrealized loss position fair value
Less than 12 months, fair value	18,519,000		18,519,000		20,497,000
Total, fair value	18,519,000		18,519,000		20,497,000
Short-term investments, gross unrealized loss
Less than 12 months, gross unrealized loss	(1,000)		(1,000)		(6,000)
Total, gross unrealized loss	(1,000)		(1,000)		(6,000)
FDIC guaranteed corporate debt
Total available-for-sale securities
Amortized Cost	28,198,000		28,198,000		114,640,000
Gains in Accumulated Other Comprehensive Income	6,000		6,000		125,000
Losses in Accumulated Other Comprehensive Income					(7,000)
Estimated Fair Value	28,204,000		28,204,000		114,758,000
Short-term investments , unrealized loss position fair value
Less than 12 months, fair value					8,033,000
Total, fair value					8,033,000
Short-term investments, gross unrealized loss
Less than 12 months, gross unrealized loss					(7,000)
Total, gross unrealized loss					$ (7,000)