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Stock-Based Compensation - FV of employee stock options (Details) - USD ($) $ in Millions |  | 3 Months Ended
--- | --- | ---
Mar. 31, 2021 | Mar. 31, 2021 | Mar. 31, 2020
Employee stock options |  |  |
Weighted average assumptions for estimated fair value of employee stock options |  |  |
Expected volatility (as a percent) |  | 55.60% | 53.90%
Risk-free interest rate (as a percent) |  | 1.02% | 1.71%
Expected term (years) |  | 6 years 2 months 12 days | 6 years 3 months 18 days
Compensation expense |  |  |
Unamortized compensation expense related to unvested options | $ 71.4 | $ 71.4 |
Period to recognize unamortized compensation expense | 3 years |  |
ESPP |  |  |
Weighted average assumptions for estimated fair value of employee stock options |  |  |
Expected volatility, low end of range (as a percent) |  | 50.80% |
Expected volatility, high end of range (as a percent) |  | 66.40% |
Expected volatility (as a percent) |  |  | 50.80%
Risk-free interest rate, low end of range (as a percent) |  | 0.09% | 1.56%
Risk-free interest rate, high end of range (as a percent) |  | 1.65% | 1.65%
Compensation expense |  |  |
Unamortized compensation expense related to unvested options | $ 0.7 | $ 0.7 |
Period to recognize unamortized compensation expense | 8 months 12 days |  |
ESPP | Minimum |  |  |
Weighted average assumptions for estimated fair value of employee stock options |  |  |
Expected term (years) |  | 6 months | 6 months
ESPP | Maximum |  |  |
Weighted average assumptions for estimated fair value of employee stock options |  |  |
Expected term (years) |  | 2 years | 2 years

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- Definition Weighted-average period over which cost not yet recognized is expected to be recognized for 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 (i) -URI http://asc.fasb.org/extlink&oid=120381028&loc=d3e5070-113901 + Details
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- Definition Amount of cost to be recognized for option 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 (i) -URI http://asc.fasb.org/extlink&oid=120381028&loc=d3e5070-113901 + Details
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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. + 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 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 risk-free interest rate assumption that is used in valuing an option on its own shares. + 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)(iv) -URI http://asc.fasb.org/extlink&oid=120381028&loc=d3e5070-113901 + 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 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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