Fair Value Measurement and Financial Instruments - Summary of Fair Value Inputs and Assumptions (Details)	6 Months Ended	12 Months Ended
	Jun. 30, 2018 $ / shares	Dec. 31, 2017 $ / shares
Estimated Fair Value of Convertible Preferred Stock | Series E
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis Valuation Techniques [Line Items]
Fair value assumption	0.92	0.33
Estimated Fair Value of Convertible Preferred Stock | Series F
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis Valuation Techniques [Line Items]
Fair value assumption	0.92	0.38
Exercise Price | Series E
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis Valuation Techniques [Line Items]
Fair value assumption	0.6746	0.6746
Exercise Price | Series F
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis Valuation Techniques [Line Items]
Fair value assumption	0.3356	0.3356
Remaining Term (in years) | Series E
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis Valuation Techniques [Line Items]
Remaining term (in years)	4 years 6 months	5 years
Remaining Term (in years) | Series F | Minimum
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis Valuation Techniques [Line Items]
Remaining term (in years)	2 years 7 months 6 days	3 years 1 month 6 days
Remaining Term (in years) | Series F | Maximum
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis Valuation Techniques [Line Items]
Remaining term (in years)	6 years 6 months	7 years
Risk-free Interest Rate | Series E
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis Valuation Techniques [Line Items]
Fair value assumption	2.7	2.2
Risk-free Interest Rate | Series F | Minimum
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis Valuation Techniques [Line Items]
Fair value assumption	2.6	2.0
Risk-free Interest Rate | Series F | Maximum
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis Valuation Techniques [Line Items]
Fair value assumption	2.8	2.3
Expected Volatility | Series E
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis Valuation Techniques [Line Items]
Fair value assumption	43	43
Expected Volatility | Series F
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis Valuation Techniques [Line Items]
Fair value assumption	43
Expected Volatility | Series F | Minimum
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis Valuation Techniques [Line Items]
Fair value assumption		43
Expected Volatility | Series F | Maximum
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis Valuation Techniques [Line Items]
Fair value assumption		44
Dividend Yield | Series E
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis Valuation Techniques [Line Items]
Fair value assumption	0	0
Dividend Yield | Series F
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis Valuation Techniques [Line Items]
Fair value assumption	0	0