Stock Plan and Stock-Based Compensation - Summary of Stock-Based Compensation Expense (Detail) - USD ($) $ in Thousands |
3 Months Ended | |
|---|---|---|
Mar. 31, 2019 |
Mar. 31, 2018 |
|
| Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items] | ||
| Stock-based compensation expense | $ 2,342 | $ 1,796 |
| Research and Development [Member] | ||
| Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items] | ||
| Stock-based compensation expense | 1,112 | 649 |
| General and Administrative [Member] | ||
| Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items] | ||
| Stock-based compensation expense | $ 1,230 | $ 1,147 |