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Balance Sheet Information (Details) (USD $)
12 Months Ended
Dec. 31, 2011
Dec. 31, 2010
Total available-for-sale securities    
Amortized Cost $ 273,372,000 $ 394,083,000
Gains in Accumulated Other Comprehensive Income 219,000 97,000
Estimated Fair Value 273,591,000 394,180,000
Total proceeds from sale of available-for-sale securities 707,649,000 32,971,000
Gross realized gains on available-for-sale securities 400,000  
Net unrealized gain on available-for-sale securities 43,000 97,000
Tax impact on the unrealized gains 100,000  
Estimated fair value of contractual maturities of available-for-sale debt securities    
Due in one year or less 37,088,000  
Due in 1-2 years 236,503,000  
Total available-for-sale securities 273,591,000 394,180,000
Commercial paper
   
Total available-for-sale securities    
Amortized Cost 15,889,000 128,527,000
Gains in Accumulated Other Comprehensive Income 6,000 61,000
Estimated Fair Value 15,895,000 128,588,000
FDIC insured corporate bonds
   
Total available-for-sale securities    
Amortized Cost 187,336,000 129,353,000
Gains in Accumulated Other Comprehensive Income 169,000 24,000
Estimated Fair Value 187,505,000 129,377,000
Treasury bills.
   
Total available-for-sale securities    
Amortized Cost 70,147,000 136,203,000
Gains in Accumulated Other Comprehensive Income 44,000 12,000
Estimated Fair Value $ 70,191,000 $ 136,215,000