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Balance Sheet Information (Details) (USD $)	3 Months Ended		6 Months Ended
Balance Sheet Information (Details) (USD $)	Jun. 30, 2011	Jun. 30, 2010	Jun. 30, 2011	Jun. 30, 2010	Dec. 31, 2010
Total available-for-sale securities
Amortized Cost	$ 380,190,000		$ 380,190,000		$ 394,083,000
Gains in Accumulated Other Comprehensive Income	316,000		316,000		97,000
Estimated Fair Value	380,506,000		380,506,000		394,180,000
Total proceeds from sale of available-for-sale securities	252,000,000		374,226,000
Net unrealized gain on available-for-sale securities	200,000		200,000
Total proceeds from available-for-sale securities matured		120,000,000		160,141,000
Estimated fair value of contractual maturities of available-for-sale debt securities
Due in one year or less	146,462,000		146,462,000
Due in 1-2 years	234,044,000		234,044,000
Total investments in debt securities	380,506,000		380,506,000		394,180,000
Gross realized gains	200,000		200,000
Commercial paper
Total available-for-sale securities
Amortized Cost	102,850,000		102,850,000		128,527,000
Gains in Accumulated Other Comprehensive Income	26,000		26,000		61,000
Estimated Fair Value	102,876,000		102,876,000		128,588,000
Estimated fair value of contractual maturities of available-for-sale debt securities
Total investments in debt securities	102,876,000		102,876,000		128,588,000
FDIC insured corporate bonds
Total available-for-sale securities
Amortized Cost	127,856,000		127,856,000		129,353,000
Gains in Accumulated Other Comprehensive Income	217,000		217,000		24,000
Estimated Fair Value	128,073,000		128,073,000		129,377,000
Estimated fair value of contractual maturities of available-for-sale debt securities
Total investments in debt securities	128,073,000		128,073,000		129,377,000
Treasury bills
Total available-for-sale securities
Amortized Cost	149,484,000		149,484,000		136,203,000
Gains in Accumulated Other Comprehensive Income	73,000		73,000		12,000
Estimated Fair Value	149,557,000		149,557,000		136,215,000
Estimated fair value of contractual maturities of available-for-sale debt securities
Total investments in debt securities	$ 149,557,000		$ 149,557,000		$ 136,215,000