Fair Value Measurement - Weighted-Average Assumptions Used In Determining Fair Value of Preferred Stock Warrant Liability Valued Using Black-Scholes Option Pricing Model (Details) - Weighted-Average Assumptions - Preferred Stock Warrant Liability	9 Months Ended	12 Months Ended
	Sep. 30, 2018	Dec. 31, 2017
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis Valuation Techniques [Line Items]
Contractual term (in years)	5 years 8 months 12 days	5 years 10 months 24 days
Expected Volatility
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis Valuation Techniques [Line Items]
Expected volatility	0.687	0.680
Risk-Free Interest Rate
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis Valuation Techniques [Line Items]
Expected volatility	0.030	0.022