Fair Value Measurements - Summary of Fair Value of Convertible Preferred Stock Warrant Liability Estimated Using Black-Scholes Option Pricing Model (Details)	Jun. 30, 2019	Dec. 31, 2018
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Fair value liability valuation technique [Extensible List]	us-gaap:ValuationTechniqueOptionPricingModelMember	us-gaap:ValuationTechniqueOptionPricingModelMember
Fair Value Estimate | Convertible Preferred Stock Warrant Liability
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Fair value liability, measurement input	48.30	8.27
Expected Term | Convertible Preferred Stock Warrant Liability
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Expected term (in years)	1 year 9 months 21 days	2 years 3 months 21 days
Risk Free Interest Rate | Convertible Preferred Stock Warrant Liability
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Fair value liability, measurement input	0.018	0.025
Expected Volatility | Convertible Preferred Stock Warrant Liability
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Fair value liability, measurement input	0.611	0.553