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Fair Value of Financial Instruments (Details 2) (USD $)
In Thousands, unless otherwise specified
6 Months Ended
Jun. 30, 2013
Item
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 $ 31,817 $ 30,790
Due in 1-2 years 19,006 7,380
Total 50,823 38,170
Investments by rolling maturity, Unrealized Gains    
Due within 1 year 9 14
Due in 1-2 years 6 3
Total 15 17
Investments by rolling maturity, Unrealized Losses    
Due within 1 year (13) (6)
Due in 1-2 years (61)  
Total (74) (6)
Investments by rolling maturity, Fair Value    
Due within 1 year 31,813 30,798
Due in 1-2 years 18,951 7,383
Total 50,764 38,181
Short-term investments
   
Estimated fair values of available-for-sale securities with unrealized losses    
Cost 12,554 8,214
Unrealized Losses (13) (6)
Fair Value 12,541 8,208
Investments by rolling maturity, Unrealized Gains    
Total 9 14
Investments by rolling maturity, Unrealized Losses    
Total (13) (6)
Short-term investments | U.S. government debt securities
   
Estimated fair values of available-for-sale securities with unrealized losses    
Cost 3,804 2,546
Unrealized Losses (2) (2)
Fair Value 3,802 2,544
Investments by rolling maturity, Unrealized Gains    
Total 6 6
Investments by rolling maturity, Unrealized Losses    
Total (2) (2)
Short-term investments | Corporate notes
   
Estimated fair values of available-for-sale securities with unrealized losses    
Cost 5,870 5,668
Unrealized Losses (6) (4)
Fair Value 5,864 5,664
Investments by rolling maturity, Unrealized Gains    
Total 2 8
Investments by rolling maturity, Unrealized Losses    
Total (6) (4)
Short-term investments | Certificates of deposit
   
Estimated fair values of available-for-sale securities with unrealized losses    
Cost 2,880  
Unrealized Losses (5)  
Fair Value 2,875  
Investments by rolling maturity, Unrealized Gains    
Total 1  
Investments by rolling maturity, Unrealized Losses    
Total (5)  
Long-term investments
   
Estimated fair values of available-for-sale securities with unrealized losses    
Cost 15,530  
Unrealized Losses (61)  
Fair Value 15,469  
Investments by rolling maturity, Unrealized Gains    
Total 6 3
Investments by rolling maturity, Unrealized Losses    
Total (61)  
Long-term investments | U.S. government debt securities
   
Estimated fair values of available-for-sale securities with unrealized losses    
Cost 6,020  
Unrealized Losses (10)  
Fair Value 6,010  
Investments by rolling maturity, Unrealized Gains    
Total 5 2
Investments by rolling maturity, Unrealized Losses    
Total (10)  
Long-term investments | Corporate notes
   
Estimated fair values of available-for-sale securities with unrealized losses    
Cost 8,310  
Unrealized Losses (46)  
Fair Value 8,264  
Investments by rolling maturity, Unrealized Gains    
Total   1
Investments by rolling maturity, Unrealized Losses    
Total (46)  
Long-term investments | Certificates of deposit
   
Estimated fair values of available-for-sale securities with unrealized losses    
Cost 1,200  
Unrealized Losses (5)  
Fair Value 1,195  
Investments by rolling maturity, Unrealized Gains    
Total 1  
Investments by rolling maturity, Unrealized Losses    
Total $ (5)