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Fair Value of Financial Instruments (Details 2) (USD $)
In Thousands, unless otherwise specified
 6 Months Ended
Jun. 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 $ 27,975 $ 32,183
Due in 1-2 years 9,115 13,922
Total 37,090 46,105
Investments by rolling maturity, Unrealized Gains    
Due within 1 year 20 14
Due in 1-2 years 1 9
Total 21 23
Investments by rolling maturity, Unrealized Losses    
Due within 1 year (1) (5)
Due in 1-2 years (10) (5)
Total (11) (10)
Investments by rolling maturity, Fair Value    
Due within 1 year 27,994 32,192
Due in 1-2 years 9,106 13,926
Total 37,100 46,118
Short-term investments
    
Estimated fair values of available-for-sale securities with unrealized losses    
Cost 1,432 12,213
Unrealized Losses (1) (5)
Fair Value 1,431 12,208
Investments by rolling maturity, Unrealized Gains    
Total 20 14
Investments by rolling maturity, Unrealized Losses    
Total (1) (5)
Short-term investments | U.S. government debt securities
    
Estimated fair values of available-for-sale securities with unrealized losses    
Cost   5,289
Unrealized Losses   (1)
Fair Value   5,288
Investments by rolling maturity, Unrealized Gains    
Total 5 11
Investments by rolling maturity, Unrealized Losses    
Total   (1)
Short-term investments | Corporate notes
    
Estimated fair values of available-for-sale securities with unrealized losses    
Cost 1,432 3,844
Unrealized Losses (1) (2)
Fair Value 1,431 3,842
Investments by rolling maturity, Unrealized Gains    
Total 14 2
Investments by rolling maturity, Unrealized Losses    
Total (1) (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 7,870 3,694
Unrealized Losses (10) (5)
Fair Value 7,860 3,689
Investments by rolling maturity, Unrealized Gains    
Total 1 9
Investments by rolling maturity, Unrealized Losses    
Total (10) (5)
Long-term investments | U.S. government debt securities
    
Estimated fair values of available-for-sale securities with unrealized losses    
Cost 2,800 1,253
Unrealized Losses (2)  
Fair Value 2,798 1,253
Investments by rolling maturity, Unrealized Losses    
Total (2)  
Long-term investments | Corporate notes
    
Estimated fair values of available-for-sale securities with unrealized losses    
Cost 5,070 1,001
Unrealized Losses (8) (1)
Fair Value 5,062 1,000
Investments by rolling maturity, Unrealized Gains    
Total 1 9
Investments by rolling maturity, Unrealized Losses    
Total (8) (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)