Summary of Significant Accounting Policies - Summary of Weighted Average Assumptions Used to Compute Stock-Based Compensation Costs for Stock Options Granted (Detail) - Options to purchase common stock	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Risk-free interest rate	2.85%	2.01%
Expected dividend yield	0.00%	0.00%
Weighted average expected life	6 years 29 days	6 years 7 days
Volatility	78.54%	78.52%