Balance Sheet Information (Details) (USD $)	3 Months Ended		6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011	Dec. 31, 2011
Total available-for-sale securities
Amortized Cost	$ 221,806,000		$ 221,806,000		$ 273,372,000
Gains in Accumulated Other Comprehensive Income	27,000		27,000		219,000
Losses in Accumulated Other Comprehensive Income	(1,000)		(1,000)
Estimated Fair Value	221,832,000		221,832,000		273,591,000
Total proceeds from sale of available-for-sale securities	55,900,000	252,000,000	99,533,000	374,226,000
Unrealized (loss) gain on available-for-sale securities	(65,000)	194,000	(193,000)	220,000
Gross realized gains on available-for-sale securities		200,000		200,000
Estimated fair value of contractual maturities of available-for-sale debt securities
Due in one year or less	30,751,000		30,751,000
Due in 1-2 years	191,081,000		191,081,000
Total available-for-sale securities	221,832,000		221,832,000		273,591,000
Government Agency Securities
Total available-for-sale securities
Amortized Cost	43,742,000		43,742,000		88,585,000
Gains in Accumulated Other Comprehensive Income	23,000		23,000		119,000
Estimated Fair Value	43,765,000		43,765,000		88,704,000
FDIC insured corporate bonds
Total available-for-sale securities
Amortized Cost	60,958,000		60,958,000		114,640,000
Gains in Accumulated Other Comprehensive Income	4,000		4,000		56,000
Estimated Fair Value	60,962,000		60,962,000		114,696,000
Treasury bills.
Total available-for-sale securities
Amortized Cost	117,106,000		117,106,000		70,147,000
Gains in Accumulated Other Comprehensive Income					44,000
Losses in Accumulated Other Comprehensive Income	(1,000)		(1,000)
Estimated Fair Value	$ 117,105,000		$ 117,105,000		$ 70,191,000