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Fair Value Measurements - Redeemable Convertible Preferred Stock Warrants (Details) - Redeemable convertible preferred stock warrant liability - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]      
Fair value, beginning balance $ 1,104 $ 1,130 $ 1,016
Change in fair value 6,056 118 114
Conversion to common stock warrant in connection with the initial public offering (7,160) (144)  
Fair value, ending balance $ 0 $ 1,104 $ 1,130