Stock-Based Compensation - Schedule of Assumptions to Estimate Fair Value of Stock Options for Employees and Non-Employees Using Black-Scholes Option Pricing Model (Details) - Options Issued and Outstanding - $ / shares	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Employees
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Fair value of common stock		$ 0.79		$ 0.79
Expected term (in years)		6 years 1 month 6 days		6 years 1 month 6 days
Expected volatility, minimum		74.40%		74.40%
Expected volatility, maximum			77.70%	77.70%
Risk-free interest rate, minimum		1.90%		1.90%
Risk-free interest rate, maximum			2.80%	2.80%
Expected dividend yield	0.00%	0.00%	0.00%	0.00%
Non-employees
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Fair value of common stock		$ 1.00	$ 15.45
Expected term (in years)		10 years	10 years	10 years
Expected dividend yield		0.00%	0.00%	0.00%
Expected volatility		76.90%	77.90%
Risk-free interest rate		2.30%	2.70%	2.30%
Minimum | Employees
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Fair value of common stock	$ 9.03	$ 0.79	$ 8.57
Expected term (in years)	5 years 10 months 24 days		5 years 9 months 18 days
Expected volatility, minimum	89.80%		75.90%
Risk-free interest rate, minimum	2.80%		2.40%
Minimum | Non-employees
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Fair value of common stock				$ 0.79
Expected volatility				74.60%
Maximum | Employees
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Fair value of common stock	$ 13.94		$ 15.45
Expected term (in years)	6 years 1 month 6 days		6 years 7 months 6 days
Expected volatility, minimum	90.60%		90.60%
Risk-free interest rate, minimum	2.90%		2.90%
Maximum | Non-employees
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Fair value of common stock				$ 1.00
Expected volatility				76.90%