Stock-based Compensation - Schedule of Assumptions Used to Estimate Fair Value on Grant Date Using Black Scholes Option Pricing Model (Detail) - ESPP [Member] |
12 Months Ended | |
---|---|---|
Dec. 31, 2018 |
Dec. 31, 2017 |
|
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] | ||
Expected term (in years) | 6 months | 6 months |
Dividend | 0.00% | 0.00% |
Minimum [Member] | ||
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] | ||
Volatility | 48.20% | 48.20% |
Risk-free interest rate | 1.30% | 0.50% |
Dividend | 0.00% | 0.00% |
Maximum [Member] | ||
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] | ||
Volatility | 61.20% | 77.80% |
Risk-free interest rate | 2.05% | 1.30% |
Dividend | 0.00% | 0.00% |