Assumptions Used in Estimating Fair Value of Stock Options Granted (Detail) |
12 Months Ended | ||
|---|---|---|---|
Dec. 31, 2018 |
Dec. 31, 2017 |
Dec. 31, 2016 |
|
| Share Based Compensation Arrangement By Share Based Payment Award Fair Value Assumptions And Methodology [Abstract] | |||
| Expected term (years) | 5 years 10 months 13 days | 5 years 6 months 3 days | 5 years 11 months 15 days |
| Volatility | 81.00% | 79.00% | 86.00% |
| Risk-free interest rate | 2.54% | 1.99% | 1.39% |
| X | ||||||||||
- References No definition available.
|
| X | ||||||||||
- Definition The estimated measure of the percentage by which a share price is expected to fluctuate during a period. Volatility also may be defined as a probability-weighted measure of the dispersion of returns about the mean. The volatility of a share price is the standard deviation of the continuously compounded rates of return on the share over a specified period. That is the same as the standard deviation of the differences in the natural logarithms of the stock prices plus dividends, if any, over the period. Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
|
| X | ||||||||||
- Definition The risk-free interest rate assumption that is used in valuing an option on its own shares. Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
|
| X | ||||||||||
- Definition Expected term of share-based compensation awards, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
|