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Financial Instruments (Details)
$ in Thousands
9 Months Ended
Sep. 30, 2019
USD ($)
Dec. 31, 2018
USD ($)
Sep. 30, 2018
USD ($)
Financial Instruments      
Transfers between fair value measurement levels $ 0   $ 0
Minimum | Reverse repurchase agreements | Government Securities and Obligations      
Financial Instruments      
Percentage of investment amount to collateralized deposits value 102    
Recurring      
Financial Instruments      
Total assets $ 38,500 $ 33,180  
Recurring | Level 1      
Financial Instruments      
Total assets 6,144 5,956  
Recurring | Level 2      
Financial Instruments      
Total assets 32,356 27,224  
Recurring | Money market funds      
Financial Instruments      
Cash and cash equivalents 6,144 5,956  
Recurring | Money market funds | Level 1      
Financial Instruments      
Cash and cash equivalents 6,144 5,956  
Recurring | Corporate debt securities      
Financial Instruments      
Cash and cash equivalents 7,506 1,948  
Short-term investments 1,850 7,276  
Recurring | Corporate debt securities | Level 2      
Financial Instruments      
Cash and cash equivalents 7,506 1,948  
Short-term investments 1,850 7,276  
Recurring | Reverse repurchase agreements      
Financial Instruments      
Cash and cash equivalents 2,000 3,000  
Short-term investments 21,000 15,000  
Recurring | Reverse repurchase agreements | Level 2      
Financial Instruments      
Cash and cash equivalents 2,000 3,000  
Short-term investments $ 21,000 $ 15,000