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Fair Value of Financial Instruments (Details 2) (USD $)
In Thousands, unless otherwise specified
 9 Months Ended
Sep. 30, 2014
item
Dec. 31, 2013
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 $ 21,909 $ 32,183
Due in 1-2 years 7,070 13,922
Total 28,979 46,105
Investments by rolling maturity, Unrealized Gains    
Due within 1 year 13 14
Due in 1-2 years   9
Total 13 23
Investments by rolling maturity, Unrealized Losses    
Due within 1 year (5) (5)
Due in 1-2 years (15) (5)
Total (20) (10)
Investments by rolling maturity, Fair Value    
Due within 1 year 21,917 32,192
Due in 1-2 years 7,055 13,926
Total 28,972 46,118
Short-term investments.
    
Estimated fair values of available-for-sale securities with unrealized losses    
Cost 2,188 12,213
Unrealized Losses (5) (5)
Fair Value 2,183 12,208
Investments by rolling maturity, Unrealized Gains    
Total 13 14
Investments by rolling maturity, Unrealized Losses    
Total (5) (5)
Short-term investments. | U.S. government-sponsored enterprise bonds
    
Estimated fair values of available-for-sale securities with unrealized losses    
Cost 2,188  
Unrealized Losses (5)  
Fair Value 2,183  
Investments by rolling maturity, Unrealized Gains    
Total 1  
Investments by rolling maturity, Unrealized Losses    
Total   (1)
Short-term investments. | U S government-sponsored enterprise and municipal bonds
    
Estimated fair values of available-for-sale securities with unrealized losses    
Cost   5,289
Unrealized Losses   (1)
Fair Value   5,288
Short-term investments. | Corporate notes
    
Estimated fair values of available-for-sale securities with unrealized losses    
Cost   3,844
Unrealized Losses   (2)
Fair Value   3,842
Investments by rolling maturity, Unrealized Gains    
Total 10 2
Investments by rolling maturity, Unrealized Losses    
Total (5) (2)
Short-term investments. | Certificates of deposit
    
Estimated fair values of available-for-sale securities with unrealized losses    
Cost   3,080
Unrealized Losses   (2)
Fair Value   3,078
Investments by rolling maturity, Unrealized Gains    
Total 1 1
Investments by rolling maturity, Unrealized Losses    
Total   (2)
Long-term investments
    
Estimated fair values of available-for-sale securities with unrealized losses    
Cost 7,070 3,694
Unrealized Losses (15) (5)
Fair Value 7,055 3,689
Investments by rolling maturity, Unrealized Gains    
Total   9
Investments by rolling maturity, Unrealized Losses    
Total (15) (5)
Long-term investments | U.S. government-sponsored enterprise bonds
    
Estimated fair values of available-for-sale securities with unrealized losses    
Cost 1,000 1,253
Unrealized Losses (5)  
Fair Value 995 1,253
Investments by rolling maturity, Unrealized Losses    
Total (5)  
Long-term investments | Corporate notes
    
Estimated fair values of available-for-sale securities with unrealized losses    
Cost 6,070 1,001
Unrealized Losses (10) (1)
Fair Value 6,060 1,000
Investments by rolling maturity, Unrealized Gains    
Total   9
Investments by rolling maturity, Unrealized Losses    
Total (10) (1)
Long-term investments | Certificates of deposit
    
Estimated fair values of available-for-sale securities with unrealized losses    
Cost   1,440
Unrealized Losses   (4)
Fair Value   1,436
Investments by rolling maturity, Unrealized Losses    
Total   $ (4)