Fair Value Measurement - Estimated Fair Values Per Share of Underlying Redeemable Convertible Preferred Stock (Details) - $ / shares	Sep. 30, 2018	Dec. 31, 2017
Series AA
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis Valuation Techniques [Line Items]
Estimated fair values preferred stock		$ 5.75
Series B
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis Valuation Techniques [Line Items]
Estimated fair values preferred stock	$ 8.95	3.76
Series C-1
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis Valuation Techniques [Line Items]
Estimated fair values preferred stock	$ 8.95	$ 3.76