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Fair Value Measurements - Summary of Change in Derivative Liability (Details) - Derivative Liability
$ in Thousands
12 Months Ended
Dec. 31, 2018
USD ($)
Fair Value Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]  
Fair Value at beginning of period $ 1,886
Bifurcated derivative liability 113
Change in fair value 206
Conversion of promissory notes to Series B redeemable convertible preferred stock $ (2,205)