Stock-Based Payments - Weighted-Average Assumptions to Estimate Fair Value of Stock Options (Details) - Stock Option - $ / shares |
12 Months Ended | ||
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Dec. 31, 2018 |
Dec. 31, 2017 |
Dec. 31, 2016 |
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| Share Based Compensation Arrangement By Share Based Payment Award [Line Items] | |||
| Weighted-average risk-free interest rate | 2.56% | 2.07% | 1.36% |
| Expected option term (in years) | 6 years 15 days | 6 years 18 days | 5 years 11 months 23 days |
| Volatility | 90.26% | 87.83% | 85.39% |
| Weighted-average grant date fair value of options granted (in dollars per share) | $ 7.80 | $ 8.75 | $ 8.58 |
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- 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
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- 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
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- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. No definition available.
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- Definition The weighted average grant-date fair value of options granted during the reporting period as calculated by applying the disclosed option pricing methodology. Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
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- 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
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- Details
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