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Equity Incentive Plans - Option grant assumptions (Details) - Stock options | 12 Months Ended
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Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018
Assumptions used to determine fair value of options granted |  |  |
Weighted average expected term | 6 years 1 month 6 days | 5 years 9 months 18 days | 5 years 8 months 12 days
Expected volatility, low end of the range (as a percent) | 16.30% | 15.60% | 15.60%
Expected volatility, high end of the range (as a percent) | 37.10% | 28.90% | 30.70%
Weighted average volatility (as a percent) | 17.60% | 18.40% | 19.80%
Expected dividends, minimum range (as a percent) | 1.60% | 1.90% | 1.50%
Expected dividends, maximum range (as a percent) | 2.40% | 2.20% | 2.20%
Weighted average expected dividends (as a percent) | 1.80% | 2.20% | 2.00%
Risk-free rate, low end of the range (as a percent) | 0.10% | 1.30% | 1.30%
Risk-free rate, high end of the range (as a percent) | 1.80% | 2.70% | 3.20%

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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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- 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 Rate of weighted-average expected volatility for award under share-based payment arrangement. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -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=120381028&loc=d3e5070-113901 + Details
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- Definition Expected term of award under share-based payment arrangement, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -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=120381028&loc=d3e5070-113901 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -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=115993241&loc=d3e301413-122809 + Details
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