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SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - STOCK OPTIONS AND EMPLOYEE STOCK PURCHASE PLAN (Details) | 6 Months Ended
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Jun. 30, 2019 | Jun. 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) | 80.20% | 82.10%
Expected volatility range, maximum (as a percent) | 98.00% | 83.20%
Risk-free interest rate range, minimum (as a percent) | 1.79% | 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) | 133.30% | 43.70%
Expected volatility range, maximum (as a percent) | 165.30% | 47.50%
Risk-free interest rate range, minimum (as a percent) | 2.56% | 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 which an employee's right to exercise an award is no longer contingent on satisfaction of either a service condition, market condition or a performance condition, 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 (a)(1) -URI http://asc.fasb.org/extlink&oid=109197908&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=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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