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		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Employees and Directors
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expected term (in years)		6 years 7 months 6 days
Expected volatility, minimum	94.50%	77.40%	93.70%	75.90%
Expected volatility, maximum	104.80%	77.70%	104.80%	90.60%
Risk-free interest rate, minimum	1.90%	2.60%	1.90%	2.40%
Risk-free interest rate, maximum	2.40%	2.80%	2.60%	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				$ 15.45
Expected term (in years)				10 years
Expected volatility, minimum	93.40%		90.00%
Expected volatility, maximum	94.20%		94.90%
Risk-free interest rate, minimum	1.90%		1.90%
Risk-free interest rate, maximum	2.10%		2.60%
Expected dividend yield	0.00%		0.00%	0.00%
Expected volatility				77.90%
Risk-free interest rate				2.70%
Minimum [Member] | Employees and Directors
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Fair value of common stock	$ 6.97	$ 8.57	$ 6.97	$ 8.57
Expected term (in years)	5 years 6 months		5 years 6 months	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	$ 8.90		$ 6.82
Expected term (in years)	8 years		8 years
Maximum | Employees and Directors
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Fair value of common stock	$ 8.08	$ 9.91	$ 8.90	$ 15.45
Expected term (in years)	6 years 1 month 6 days		6 years 1 month 6 days	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	$ 10.20		$ 10.20
Expected term (in years)	9 years 8 months 12 days		10 years