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Fair Value of Financial Instruments - Maturity (Details)
$ in Thousands
12 Months Ended
Dec. 31, 2015
USD ($)
item
Dec. 31, 2014
USD ($)
item
Estimated fair values of available-for-sale securities with unrealized losses    
Number of maturity groups | item 2 2
Investments by rolling maturity, Cost    
Due within 1 year $ 14,614 $ 20,442
Due in 1-2 years   2,252
Total 14,614 22,694
Investments by rolling maturity, Unrealized Gains    
Due within 1 year   3
Total   3
Investments by rolling maturity, Unrealized Losses    
Due within 1 year (16) (6)
Due in 1-2 years   (7)
Total (16) (13)
Investments by rolling maturity, Fair Value    
Due within 1 year 14,598 20,439
Due in 1-2 years   2,245
Total 14,598 22,684
Short-term investments.    
Estimated fair values of available-for-sale securities with unrealized losses    
Cost 8,171 13,006
Unrealized Losses (16) (6)
Fair Value 8,155 13,000
Investments by rolling maturity, Unrealized Gains    
Total   3
Investments by rolling maturity, Unrealized Losses    
Total (16) (6)
Short-term investments. | Corporate notes    
Estimated fair values of available-for-sale securities with unrealized losses    
Cost 8,171 13,006
Unrealized Losses (16) (6)
Fair Value 8,155 13,000
Investments by rolling maturity, Unrealized Gains    
Total   2
Investments by rolling maturity, Unrealized Losses    
Total $ (16) (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   2,252
Unrealized Losses   (7)
Fair Value   2,245
Investments by rolling maturity, Unrealized Losses    
Total   (7)
Long-term investments | Corporate notes    
Estimated fair values of available-for-sale securities with unrealized losses    
Cost   1,252
Unrealized Losses   (5)
Fair Value   1,247
Investments by rolling maturity, Unrealized Losses    
Total   (5)
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)