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SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - STOCK OPTIONS AND EMPLOYEE STOCK PURCHASE PLAN (Details) | 9 Months Ended
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Sep. 30, 2019 | Sep. 30, 2018
Stock Options |  |
Stock-Based Compensation |  |
Vesting period of awards | 4 years |
Assumptions used to estimate fair value of awards |  |
Dividend yield (as a percent) | 0.00% | 0.00%
Expected volatility range, minimum (as a percent) | 79.30% | 82.10%
Expected volatility range, maximum (as a percent) | 98.00% | 83.20%
Risk-free interest rate range, minimum (as a percent) | 1.50% | 2.55%
Risk-free interest rate range, maximum (as a percent) | 2.56% | 2.94%
Expected term range |  | 5 years 3 months
Stock Options | Minimum |  |
Assumptions used to estimate fair value of awards |  |
Expected term range | 5 years 3 months |
Stock Options | Maximum |  |
Assumptions used to estimate fair value of awards |  |
Expected term range | 6 years 5 months 8 days |
Employee Stock Purchase Plan |  |
Assumptions used to estimate fair value of awards |  |
Dividend yield (as a percent) | 0.00% | 0.00%
Expected volatility range, minimum (as a percent) | 64.60% | 43.70%
Expected volatility range, maximum (as a percent) | 165.30% | 47.50%
Risk-free interest rate range, minimum (as a percent) | 1.94% | 1.53%
Risk-free interest rate range, maximum (as a percent) | 2.63% | 1.76%
Employee Stock Purchase Plan | Minimum |  |
Assumptions used to estimate fair value of awards |  |
Expected term range | 6 months | 6 months
Employee Stock Purchase Plan | Maximum |  |
Assumptions used to estimate fair value of awards |  |
Expected term range | 12 months | 12 months

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- Definition Period over which grantee's right to exercise award under share-based payment arrangement is no longer contingent on satisfaction of service or performance condition, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days. Includes, but is not limited to, combination of market, performance or service condition. + 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 (a)(1) -URI http://asc.fasb.org/extlink&oid=116856206&loc=d3e5070-113901 + 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://fasb.org/us-gaap/role/ref/legacyRef -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=116856206&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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