Stock Option and Purchase Plans - Summary of Weighted-Average Assumptions (Detail) |
12 Months Ended | ||
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Dec. 31, 2017 |
Dec. 31, 2016 |
Dec. 31, 2015 |
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| Share-based Compensation Arrangement by Share-based Payment Award | |||
| Dividend Yield | 0.00% | 0.00% | 0.00% |
| Minimum | |||
| Share-based Compensation Arrangement by Share-based Payment Award | |||
| Risk-free Interest Rate | 2.05% | 1.08% | 1.36% |
| Volatility Factor | 0.40% | 0.40% | 0.40% |
| Expected Life in Years | 5 years | 4 years | 4 years |
| Maximum | |||
| Share-based Compensation Arrangement by Share-based Payment Award | |||
| Risk-free Interest Rate | 2.36% | 2.34% | 2.10% |
| Volatility Factor | 0.41% | 0.45% | 0.51% |
| Expected Life in Years | 8 years | 8 years | 8 years |
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- Definition The estimated dividend rate (a percentage of the share price) to be paid (expected dividends) to holders of the underlying shares over the option's term. Reference 1: http://www.xbrl.org/2003/role/presentationRef
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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://www.xbrl.org/2003/role/presentationRef
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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://www.xbrl.org/2003/role/presentationRef
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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 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://www.xbrl.org/2003/role/presentationRef
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