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Stockholders' Equity - Option and ESPP Valuation Assumptions (Details) | 1 Months Ended | 12 Months Ended
--- | --- | ---
Nov. 30, 2017 | May 31, 2017 | Nov. 30, 2016 | May 31, 2016 | Nov. 30, 2015 | May 31, 2015 | Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Employee Stock Options |  |  |  |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |  |  |  |
Expected term (years) |  |  |  |  |  |  | 5 years 10 months 24 days | 5 years 8 months 6 days | 6 years 29 days
Volatility |  |  |  |  |  |  | 49.00% | 46.00% | 49.00%
Risk-free interest rate |  |  |  |  |  |  | 2.10% | 1.40% | 1.60%
Dividend yield |  |  |  |  |  |  | 0.00% | 0.00% | 0.00%
ESPP |  |  |  |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |  |  |  |
Expected term (years) | 6 months | 6 months | 6 months | 6 months | 6 months | 6 months |  |  |
Volatility | 36.00% | 43.00% | 42.00% | 58.00% | 54.00% | 43.00% |  |  |
Risk-free interest rate | 1.40% | 1.00% | 0.60% | 0.40% | 0.30% | 0.10% |  |  |
Dividend yield | 0.00% | 0.00% | 0.00% | 0.00% | 0.00% | 0.00% |  |  |

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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. + References No definition available. + Details
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- Definition The estimated 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 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)(iii) -URI http://asc.fasb.org/extlink&oid=96867065&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://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 The risk-free interest rate assumption that is used in valuing an option on its own shares. + 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)(iv) -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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