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STOCKHOLDERS' EQUITY - PRICING MODEL ASSUMPTIONS (Details) - USD ($) | 12 Months Ended
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Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018
Compensation cost related to unvested stock awards not yet recognized |  |  |
Compensation cost not yet recognized, net of estimated forfeitures (in dollars) | $ 10,690,000 |  |
Period for recognition of compensation cost on weighted average basis | 27 months |  |
Stock Options |  |  |
Assumptions used to estimate fair value of awards |  |  |
Dividend yield (as a percent) | 0.00% | 0.00% | 0.00%
Expected volatility range, minimum (as a percent) | 78.10% | 79.20% | 82.10%
Expected volatility range, maximum (as a percent) | 79.30% | 98.00% | 99.00%
Risk-free interest rate range, minimum (as a percent) | 0.31% | 1.50% | 2.55%
Risk-free interest rate range, maximum (as a percent) | 1.62% | 2.56% | 3.11%
Expected term range | 5 years 3 months |  |
Additional disclosures |  |  |
Weighted average estimated fair value of employee stock options granted (in dollars per share) | $ 0.88 | $ 0.94 | $ 1.52
Stock Options | Minimum |  |  |
Assumptions used to estimate fair value of awards |  |  |
Expected term range |  | 5 years 3 months | 5 years 3 months
Stock Options | Maximum |  |  |
Assumptions used to estimate fair value of awards |  |  |
Expected term range |  | 6 years 5 months 8 days | 6 years 7 months 13 days
Employee Stock Purchase Plan |  |  |
Assumptions used to estimate fair value of awards |  |  |
Dividend yield (as a percent) | 0.00% | 0.00% | 0.00%
Expected volatility range, minimum (as a percent) | 47.80% | 64.60% | 43.70%
Expected volatility range, maximum (as a percent) | 81.80% | 165.30% | 47.50%
Risk-free interest rate range, minimum (as a percent) | 0.16% | 1.94% | 1.53%
Risk-free interest rate range, maximum (as a percent) | 1.57% | 2.63% | 1.76%
Additional disclosures |  |  |
Weighted average estimated fair value of purchase rights (in dollars per share) | $ 0.62 | $ 0.66 | $ 0.56
Employee Stock Purchase Plan | Minimum |  |  |
Assumptions used to estimate fair value of awards |  |  |
Expected term range | 6 months | 6 months | 6 months
Employee Stock Purchase Plan | Maximum |  |  |
Assumptions used to estimate fair value of awards |  |  |
Expected term range | 12 months | 12 months | 12 months

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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 The weighted average fair value at grant date for nonvested equity-based awards issued during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan). + 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 (c)(2)(iii)(1) -URI http://asc.fasb.org/extlink&oid=120381028&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=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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