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
	Mar. 31, 2019	Mar. 31, 2018
Employees
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expected term (in years)	6 years 1 month 6 days
Expected volatility, minimum	93.70%	76.10%
Expected volatility, maximum	94.80%	77.70%
Risk-free interest rate, minimum	2.50%	2.40%
Risk-free interest rate, maximum	2.60%	2.70%
Expected dividend yield	0.00%	0.00%
Non-employees
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expected volatility, minimum	91.30%	77.90%
Expected volatility, maximum	94.90%	78.10%
Risk-free interest rate, minimum	2.40%	2.70%
Risk-free interest rate, maximum	2.60%	2.80%
Expected dividend yield	0.00%	0.00%
Minimum [Member] | Employees
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Fair value of common stock	$ 8.53	$ 13.17
Expected term (in years)		5 years 9 months 18 days
Minimum [Member] | Non-employees
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Fair value of common stock	$ 6.82	$ 9.58
Expected term (in years)	8 years 2 months 12 days	9 years 2 months 12 days
Maximum | Employees
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Fair value of common stock	$ 8.90	$ 15.45
Expected term (in years)		6 years 7 months 6 days
Maximum | Non-employees
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Fair value of common stock	$ 8.61	$ 15.45
Expected term (in years)	10 years	10 years