Stock Plans (Details 2) - USD ($) |
12 Months Ended | |||||||
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Dec. 31, 2017 |
Dec. 31, 2016 |
Dec. 31, 2015 |
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| Share-based Goods and Nonemployee Services Transaction [Line Items] | ||||||||
| Weighted-average risk-free interest rate | 2.13% | 1.53% | 1.88% | |||||
| Expected dividend payments | $ 0 | $ 0 | $ 0 | |||||
| Expected holding period (years) | [1] | 5 years 10 months 24 days | 6 years 6 months 11 days | 6 years 5 months 23 days | ||||
| Weighted-average volatility factor | [2] | 0.90% | 0.92% | 1.16% | ||||
| Estimated forfeiture rates for options granted | 27.00% | 29.00% | 30.00% | |||||
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- Definition Estimated forfeiture rates for options granted to management. No definition available.
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- Definition The estimated amount of dividends to be paid to holders of the underlying shares (expected dividends) over the option's term. Dividends are taken into account because payment of dividends to shareholders reduces the fair value of the underlying shares, and option holders generally do not receive dividends. 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 Weighted average expected volatility rate of share-based compensation awards. 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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