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Fair Value of Financial Instruments - Outstanding (Details) - USD ($)
$ in Thousands
Mar. 31, 2016
Dec. 31, 2015
Mar. 31, 2015
Dec. 31, 2014
Estimated fair values of financial instruments        
Cash and cash equivalents, cost $ 10,503 $ 5,640 $ 7,020 $ 3,110
Cash and cash equivalents, fair value 10,503 5,640    
Short-term investments.        
Estimated fair values of financial instruments        
Cost 10,382 14,614    
Unrealized Losses   (16)    
Fair Value 10,382 14,598    
Short-term investments. | U.S. government-sponsored enterprise bonds        
Estimated fair values of financial instruments        
Cost 3,461 6,243    
Fair Value 3,461 6,243    
Short-term investments. | Municipal Bonds        
Estimated fair values of financial instruments        
Cost   200    
Fair Value   200    
Short-term investments. | Corporate notes        
Estimated fair values of financial instruments        
Cost 6,921 8,171    
Unrealized Losses   (16)    
Fair Value $ 6,921 $ 8,155