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Fair Value of Financial Instruments (Details 2) (USD $)
In Thousands, unless otherwise specified
9 Months Ended
Sep. 30, 2013
Group
Dec. 31, 2012
Estimated fair values of available-for-sale securities with unrealized losses    
Number of maturity groups 2  
Investments by rolling maturity, Cost    
Due within 1 year $ 33,401 $ 30,790
Due in 1-2 years 17,687 7,380
Total 51,088 38,170
Investments by rolling maturity, Unrealized Gains    
Due within 1 year 15 14
Due in 1-2 years 5 3
Total 20 17
Investments by rolling maturity, Unrealized Losses    
Due within 1 year (7) (6)
Due in 1-2 years (21)  
Total (28) (6)
Investments by rolling maturity, Fair Value    
Due within 1 year 33,409 30,798
Due in 1-2 years 17,671 7,383
Total 51,080 38,181
Short-term investments
   
Estimated fair values of available-for-sale securities with unrealized losses    
Cost 8,631 8,214
Unrealized Losses (7) (6)
Fair Value 8,624 8,208
Investments by rolling maturity, Unrealized Gains    
Total 15 14
Investments by rolling maturity, Unrealized Losses    
Total (7) (6)
Short-term investments | U.S. government debt securities
   
Estimated fair values of available-for-sale securities with unrealized losses    
Cost 2,041 2,546
Unrealized Losses (3) (2)
Fair Value 2,038 2,544
Investments by rolling maturity, Unrealized Gains    
Total 12 6
Investments by rolling maturity, Unrealized Losses    
Total (3) (2)
Short-term investments | Corporate notes
   
Estimated fair values of available-for-sale securities with unrealized losses    
Cost 3,270 5,668
Unrealized Losses (1) (4)
Fair Value 3,269 5,664
Investments by rolling maturity, Unrealized Gains    
Total 2 8
Investments by rolling maturity, Unrealized Losses    
Total (1) (4)
Short-term investments | Certificates of deposit
   
Estimated fair values of available-for-sale securities with unrealized losses    
Cost 3,320  
Unrealized Losses (3)  
Fair Value 3,317  
Investments by rolling maturity, Unrealized Gains    
Total 1  
Investments by rolling maturity, Unrealized Losses    
Total (3)  
Long-term investments
   
Estimated fair values of available-for-sale securities with unrealized losses    
Cost 14,724  
Unrealized Losses (21)  
Fair Value 14,703  
Investments by rolling maturity, Unrealized Gains    
Total 5 3
Investments by rolling maturity, Unrealized Losses    
Total (21)  
Long-term investments | U.S. government debt securities
   
Estimated fair values of available-for-sale securities with unrealized losses    
Cost 3,758  
Unrealized Losses (1)  
Fair Value 3,757  
Investments by rolling maturity, Unrealized Gains    
Total 3 2
Investments by rolling maturity, Unrealized Losses    
Total (1)  
Long-term investments | Corporate notes
   
Estimated fair values of available-for-sale securities with unrealized losses    
Cost 9,286  
Unrealized Losses (13)  
Fair Value 9,273  
Investments by rolling maturity, Unrealized Gains    
Total 1 1
Investments by rolling maturity, Unrealized Losses    
Total (13)  
Long-term investments | Certificates of deposit
   
Estimated fair values of available-for-sale securities with unrealized losses    
Cost 1,680  
Unrealized Losses (7)  
Fair Value 1,673  
Investments by rolling maturity, Unrealized Gains    
Total 1  
Investments by rolling maturity, Unrealized Losses    
Total $ (7)