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Fair Value of Financial Instruments (Details 2)
$ in Thousands
9 Months Ended
Sep. 30, 2015
USD ($)
item
Dec. 31, 2014
USD ($)
Estimated fair values of available-for-sale securities with unrealized losses    
Number of maturity groups | item 2  
Investments by rolling maturity, Cost    
Due within 1 year $ 21,599 $ 20,442
Due in 1-2 years 2,270 2,252
Total 23,869 22,694
Investments by rolling maturity, Unrealized Gains    
Due within 1 year 2 3
Total 2 3
Investments by rolling maturity, Unrealized Losses    
Due within 1 year (4) (6)
Due in 1-2 years (1) (7)
Total (5) (13)
Investments by rolling maturity, Fair Value    
Due within 1 year 21,597 20,439
Due in 1-2 years 2,269 2,245
Total 23,866 22,684
Short-term investments.    
Estimated fair values of available-for-sale securities with unrealized losses    
Cost 10,824 13,006
Unrealized Losses (4) (6)
Fair Value 10,820 13,000
Investments by rolling maturity, Unrealized Gains    
Total 2 3
Investments by rolling maturity, Unrealized Losses    
Total (4) (6)
Short-term investments. | U.S. government-sponsored enterprise bonds    
Estimated fair values of available-for-sale securities with unrealized losses    
Cost 3,363  
Unrealized Losses (1)  
Fair Value 3,362  
Investments by rolling maturity, Unrealized Gains    
Total 2  
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 7,461 13,006
Unrealized Losses (3) (6)
Fair Value 7,458 13,000
Investments by rolling maturity, Unrealized Gains    
Total   2
Investments by rolling maturity, Unrealized Losses    
Total (3) (6)
Short-term investments. | Certificates of deposit    
Investments by rolling maturity, Unrealized Gains    
Total   1
Long-term investments    
Estimated fair values of available-for-sale securities with unrealized losses    
Cost 2,270 2,252
Unrealized Losses (1) (7)
Fair Value 2,269 2,245
Investments by rolling maturity, Unrealized Losses    
Total (1) (7)
Long-term investments | U.S. government-sponsored enterprise bonds    
Estimated fair values of available-for-sale securities with unrealized losses    
Cost   1,000
Unrealized Losses   (2)
Fair Value   998
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 2,270 1,252
Unrealized Losses (1) (5)
Fair Value 2,269 1,247
Investments by rolling maturity, Unrealized Losses    
Total $ (1) $ (5)