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Short-Term Investments (Details)
$ in Thousands
3 Months Ended 9 Months Ended
Sep. 30, 2018
USD ($)
Sep. 30, 2017
USD ($)
Sep. 30, 2018
USD ($)
instrument
Sep. 30, 2017
USD ($)
Dec. 31, 2017
USD ($)
Short-Term Investments          
Amortized Cost $ 23,368   $ 23,368    
Gross Unrealized Losses (5)   (5)    
Fair Value 23,363   $ 23,363   $ 0
Number of short term investments in unrealized loss position | instrument     10    
Number of short term investments in an unrealized loss position more than 12 months | instrument     0    
Aggregate fair value of the investments in an unrealized loss position 9,400   $ 9,400    
Proceeds from Sale of Short-term Investments 0 $ 0 0 $ 0  
Corporate debt securities          
Short-Term Investments          
Amortized Cost 10,368   10,368    
Gross Unrealized Losses (5)   (5)    
Fair Value 10,363   10,363    
U.S. government-sponsored securities          
Short-Term Investments          
Amortized Cost 13,000   13,000    
Fair Value $ 13,000   $ 13,000