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Stock-Based Compensation - FV of employee stock options (Details) - USD ($) $ in Millions | 12 Months Ended
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Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016
Employee stock options |  |  |
Weighted average assumptions for estimated fair value of employee stock options |  |  |
Common stock fair value per share minimum | 21.80 | 19.61 | 11.50
Common stock fair value per share maximum | 43.16 | 25.67 | 26.76
Expected volatility, low end of range (as a percent) | 70.97% | 77.42% | 75.77%
Expected volatility, high end of range (as a percent) | 73.10% | 96.73% | 90.83%
Risk-free interest rate, low end of range (as a percent) | 2.29% | 0.96% | 1.03%
Risk-free interest rate, high end of range (as a percent) | 3.10% | 2.37% | 2.18%
Compensation expense |  |  |
Unamortized compensation expense related to unvested options | $ 42.8 | $ 30.7 |
Period to recognize unamortized compensation expense | 2 years 8 months 16 days |  |
Employee stock options | Minimum |  |  |
Weighted average assumptions for estimated fair value of employee stock options |  |  |
Expected term (years) | 5 years 2 months 23 days | 5 years 2 months 23 days | 5 years 2 months 23 days
Employee stock options | Maximum |  |  |
Weighted average assumptions for estimated fair value of employee stock options |  |  |
Expected term (years) | 6 years 29 days | 6 years 29 days | 6 years 29 days
ESPP |  |  |
Weighted average assumptions for estimated fair value of employee stock options |  |  |
Expected volatility, low end of range (as a percent) | 57.00% | 67.80% | 67.80%
Expected volatility, high end of range (as a percent) | 71.40% | 79.80% | 79.80%
Risk-free interest rate, low end of range (as a percent) | 1.47% | 0.47% | 0.47%
Risk-free interest rate, high end of range (as a percent) | 2.70% | 1.80% | 0.93%
Compensation expense |  |  |
Unamortized compensation expense related to unvested options | $ 0.8 | $ 1.1 |
Period to recognize unamortized compensation expense | 11 months 6 days |  |
ESPP | Minimum |  |  |
Weighted average assumptions for estimated fair value of employee stock options |  |  |
Expected term (years) | 6 months | 6 months | 6 months
ESPP | Maximum |  |  |
Weighted average assumptions for estimated fair value of employee stock options |  |  |
Expected term (years) | 2 years | 2 years | 2 years

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- Definition Weighted average period over which unrecognized compensation is expected to be recognized for equity-based compensation plans, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the 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=109197908&loc=d3e5070-113901 + Details
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- Definition Unrecognized cost of unvested options awarded to employees as compensation. + 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=109197908&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 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 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://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=109197908&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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- Definition The estimated fair value of the entity's common stock at the date of grant of stock options, used as an input in calculating the grant date fair value of the options maximum. + References No definition available. + Details
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- Definition The minimum estimated fair value of the entity's common stock at the date of grant of stock options, used as an input in calculating the grant date fair value of the options. + References No definition available. + Details
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