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Fair Value of Financial Instruments (Details 2) (USD $)
In Thousands, unless otherwise specified
 3 Months Ended
Mar. 31, 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 $ 32,720 $ 32,183
Due in 1-2 years 10,839 13,922
Total 43,559 46,105
Investments by rolling maturity, Unrealized Gains    
Due within 1 year 21 14
Due in 1-2 years 4 9
Total 25 23
Investments by rolling maturity, Unrealized Losses    
Due within 1 year (2) (5)
Due in 1-2 years (19) (5)
Total (21) (10)
Investments by rolling maturity, Fair Value    
Due within 1 year 32,739 32,192
Due in 1-2 years 10,824 13,926
Total 43,563 46,118
Short-term investments
    
Estimated fair values of available-for-sale securities with unrealized losses    
Cost 4,612 12,213
Unrealized Losses (2) (5)
Fair Value 4,610 12,208
Investments by rolling maturity, Unrealized Gains    
Total 21 14
Investments by rolling maturity, Unrealized Losses    
Total (2) (5)
Short-term investments | U.S. government debt securities
    
Estimated fair values of available-for-sale securities with unrealized losses    
Cost 2,012 5,289
Unrealized Losses (1) (1)
Fair Value 2,011 5,288
Investments by rolling maturity, Unrealized Gains    
Total 7 11
Investments by rolling maturity, Unrealized Losses    
Total (1) (1)
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 13 2
Investments by rolling maturity, Unrealized Losses    
Total   (2)
Short-term investments | Certificates of deposit
    
Estimated fair values of available-for-sale securities with unrealized losses    
Cost 2,600 3,080
Unrealized Losses (1) (2)
Fair Value 2,599 3,078
Investments by rolling maturity, Unrealized Gains    
Total 1 1
Investments by rolling maturity, Unrealized Losses    
Total (1) (2)
Long-term investments
    
Estimated fair values of available-for-sale securities with unrealized losses    
Cost 8,828 3,694
Unrealized Losses (19) (5)
Fair Value 8,809 3,689
Investments by rolling maturity, Unrealized Gains    
Total 4 9
Investments by rolling maturity, Unrealized Losses    
Total (19) (5)
Long-term investments | U.S. government debt securities
    
Estimated fair values of available-for-sale securities with unrealized losses    
Cost 2,801 1,253
Unrealized Losses (6)  
Fair Value 2,795 1,253
Investments by rolling maturity, Unrealized Losses    
Total (6)  
Long-term investments | Corporate notes
    
Estimated fair values of available-for-sale securities with unrealized losses    
Cost 4,827 1,001
Unrealized Losses (11) (1)
Fair Value 4,816 1,000
Investments by rolling maturity, Unrealized Gains    
Total 4 9
Investments by rolling maturity, Unrealized Losses    
Total (11) (1)
Long-term investments | Certificates of deposit
    
Estimated fair values of available-for-sale securities with unrealized losses    
Cost 1,200 1,440
Unrealized Losses (2) (4)
Fair Value 1,198 1,436
Investments by rolling maturity, Unrealized Losses    
Total $ (2) $ (4)