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Equity Incentive Plans - Option grant assumptions (Details) - Stock options | 12 Months Ended
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Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Assumptions used to determine fair value of options granted |  |  |
Weighted average expected term | 6 years 1 month 6 days | 5 years | 6 years 6 months
Expected volatility, low end of the range (as a percent) | 15.70% | 16.00% | 16.00%
Expected volatility, high end of the range (as a percent) | 32.70% | 34.30% | 37.80%
Weighted average volatility (as a percent) | 21.00% | 24.30% | 24.70%
Expected dividends, minimum range (as a percent) | 1.40% | 1.90% | 1.60%
Expected dividends, maximum range (as a percent) | 1.90% | 2.10% | 2.10%
Weighted average expected dividends (as a percent) | 1.90% | 2.10% | 1.70%
Risk-free rate, low end of the range (as a percent) | 0.50% | 0.20% | 0.00%
Risk-free rate, high end of the range (as a percent) | 2.50% | 2.40% | 2.40%

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- Definition The estimated Maximum dividend rate (a percentage of the share price) to be paid (expected dividends) to holders of the underlying shares over the option's term. + References No definition available. + Details
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- Definition The estimated minimum dividend rate (a percentage of the share price) to be paid (expected dividends) to holders of the underlying shares over the option's term. + References No definition available. + Details
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- Definition The estimated dividend rate (a percentage of the share price) to be paid (Weighted Average expected dividends) to holders of the underlying shares over the option's term. + References No definition available. + Details
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- References No definition available. + Details
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- Definition The estimated measure of the maximum 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. + References No definition available. + Details
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- Definition The estimated measure of the minimum 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. + References No definition available. + Details
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- Definition The maximum risk-free interest rate assumption that is used in valuing an option on its own shares. + References No definition available. + Details
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- Definition The minimum risk-free interest rate assumption that is used in valuing an option on its own shares. + References No definition available. + Details
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- Definition Weighted average expected volatility rate of share-based compensation awards. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (f)(2)(ii) -URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901 + Details
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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. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (f)(2)(i) -URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SAB Topic 14.D.2) -URI http://asc.fasb.org/extlink&oid=27013229&loc=d3e301413-122809 + Details
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