Quarterly report pursuant to Section 13 or 15(d)

Stock-Based Compensation - Summary of Fair Values of Stock Options Granted and Assumptions used in Black-Scholes Model (Detail)

v3.19.1
Stock-Based Compensation - Summary of Fair Values of Stock Options Granted and Assumptions used in Black-Scholes Model (Detail) - USD ($)
3 Months Ended
Mar. 31, 2019
Mar. 31, 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Estimated fair values of stock options granted $ 9,450 $ 10,390
Risk-free interest rate, minimum 2.50% 2.50%
Risk-free interest rate, maximum   2.70%
Expected volatility, minimum 78.90% 40.90%
Expected volatility, maximum   53.70%
Expected term (in years) 6 years 10 months 24 days  
Minimum [Member]    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Expected term (in years)   6 years 6 months
Maximum [Member]    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Expected term (in years)   9 years 2 months 12 days