Annual report pursuant to Section 13 and 15(d)

Fair Value Measurements and Short-Term Investments - Fair Value of Financial Assets and Liabilities Measured on Recurring Basis (Details)

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Fair Value Measurements and Short-Term Investments - Fair Value of Financial Assets and Liabilities Measured on Recurring Basis (Details) - USD ($)
$ in Thousands
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Redeemable Convertible Preferred Stock Warrant Liability      
Liabilities:      
Total   $ 1,708 $ 1,193
Fair Value Measurements Recurring      
Assets:      
Total $ 195,396 18,960  
Liabilities:      
Total   1,708  
Fair Value Measurements Recurring | Redeemable Convertible Preferred Stock Warrant Liability      
Liabilities:      
Total   1,708  
Fair Value Measurements Recurring | Level 1      
Assets:      
Total 116,202 6,578  
Liabilities:      
Total   0  
Fair Value Measurements Recurring | Level 1 | Redeemable Convertible Preferred Stock Warrant Liability      
Liabilities:      
Total   0  
Fair Value Measurements Recurring | Level 2      
Assets:      
Total 79,194 12,382  
Liabilities:      
Total   0  
Fair Value Measurements Recurring | Level 2 | Redeemable Convertible Preferred Stock Warrant Liability      
Liabilities:      
Total   0  
Fair Value Measurements Recurring | Level 3      
Assets:      
Total 0 0  
Liabilities:      
Total   1,708  
Fair Value Measurements Recurring | Level 3 | Redeemable Convertible Preferred Stock Warrant Liability      
Liabilities:      
Total   1,708  
Money Market Funds | Fair Value Measurements Recurring      
Assets:      
Total 116,202 6,578  
Money Market Funds | Fair Value Measurements Recurring | Level 1      
Assets:      
Total 116,202 6,578  
Money Market Funds | Fair Value Measurements Recurring | Level 2      
Assets:      
Total 0 0  
Money Market Funds | Fair Value Measurements Recurring | Level 3      
Assets:      
Total 0 0  
Commercial Paper | Fair Value Measurements Recurring      
Assets:      
Total 26,625 7,689  
Commercial Paper | Fair Value Measurements Recurring | Level 1      
Assets:      
Total 0 0  
Commercial Paper | Fair Value Measurements Recurring | Level 2      
Assets:      
Total 26,625 7,689  
Commercial Paper | Fair Value Measurements Recurring | Level 3      
Assets:      
Total 0 0  
Corporate Debt Securities | Fair Value Measurements Recurring      
Assets:      
Total 11,774 800  
Corporate Debt Securities | Fair Value Measurements Recurring | Level 1      
Assets:      
Total 0 0  
Corporate Debt Securities | Fair Value Measurements Recurring | Level 2      
Assets:      
Total 11,774 800  
Corporate Debt Securities | Fair Value Measurements Recurring | Level 3      
Assets:      
Total 0 0  
Asset-backed Securities | Fair Value Measurements Recurring      
Assets:      
Total 16,899    
Asset-backed Securities | Fair Value Measurements Recurring | Level 1      
Assets:      
Total 0    
Asset-backed Securities | Fair Value Measurements Recurring | Level 2      
Assets:      
Total 16,899    
Asset-backed Securities | Fair Value Measurements Recurring | Level 3      
Assets:      
Total 0    
U.S. Government Agency Securities | Fair Value Measurements Recurring      
Assets:      
Total 23,896 3,893  
U.S. Government Agency Securities | Fair Value Measurements Recurring | Level 1      
Assets:      
Total 0 0  
U.S. Government Agency Securities | Fair Value Measurements Recurring | Level 2      
Assets:      
Total 23,896 3,893  
U.S. Government Agency Securities | Fair Value Measurements Recurring | Level 3      
Assets:      
Total $ 0 $ 0