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Convertible Preferred Stock Warrants - Fair Value Measurement Inputs and Valuation Techniques (Details) - Warrants to purchase convertible preferred stock
Sep. 30, 2019
M
$ / shares
Dec. 31, 2018
Y
$ / shares
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Warrants, price (in dollars per share) $ 8.42 $ 2.27
Warrants, valuation technique us-gaap:ValuationTechniqueOptionPricingModelMember us-gaap:ValuationTechniqueOptionPricingModelMember
Measurement Input, Expected Term    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Warrants, measurement input 6 2
Measurement Input, Option Volatility    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Warrants, measurement input 0.45 0.43
Measurement Input, Risk Free Interest Rate    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Warrants, measurement input 1.8 2.3