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Fair Value Measurements (Details) - USD ($)
$ / shares in Units, $ in Thousands
Sep. 30, 2018
Dec. 31, 2017
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Total assets $ 149,916 $ 60,958
Corporate notes    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Investments 31,694 46,098
Asset backed securities    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Investments 12,198  
Government notes    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Investments 12,430 6,175
Level 1    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Total assets 9,850 19,963
Level 1 | Corporate notes    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Investments   5,103
Level 1 | Government notes    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Investments   6,175
Level 2    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Total assets 140,066 40,995
Level 2 | Corporate notes    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Investments 31,694 40,995
Level 2 | Asset backed securities    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Investments 12,198  
Level 2 | Government notes    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Investments 12,430  
Money market funds    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Money market funds $ 9,850 8,685
Net asset value $ 1  
Money market funds | Level 1    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Money market funds $ 9,850 $ 8,685
Commercial paper    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Investments 83,744  
Commercial paper | Level 2    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Investments $ 83,744