Description of Business, Basis of Presentation and Summary of Significant Accounting Policies - Schedule of Assumptions Used to Determine Fair Value of Preferred Stock Warrant Liability (Detail)	12 Months Ended
	Dec. 31, 2018 $ / shares
Accounting Policies [Abstract]
Fair value of underlying preferred stock	$ 0.29
Risk-free interest rate, minimum	2.40%
Risk-free interest rate, maximum	2.70%
Expected volatility, minimum	75.30%
Expected volatility, maximum	76.40%
Expected term, minimum (in years)	3 years 8 months 12 days
Expected term, maximum (in years)	4 years