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STOCKHOLDERS' EQUITY - PRICING MODEL ASSUMPTIONS (Details) - USD ($) | 12 Months Ended
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Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Compensation cost related to unvested stock awards not yet recognized |  |  |
Compensation cost not yet recognized, net of estimated forfeitures (in dollars) | $ 8,008,000 |  |
Period for recognition of compensation cost on weighted average basis | 26 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) | 88.40% | 88.80% | 87.40%
Expected volatility range, maximum (as a percent) | 89.20% | 89.00% | 88.40%
Risk-free interest rate range, minimum (as a percent) | 1.98% | 1.21% | 1.68%
Risk-free interest rate range, maximum (as a percent) | 1.99% | 1.38% | 1.71%
Expected term | 5 years 6 months | 5 years 6 months | 5 years 6 months
Additional disclosures |  |  |
Weighted average estimated fair value of employee stock options granted (in dollars per share) | $ 1.58 | $ 1.83 | $ 3.06
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) | 57.70% | 64.10% | 65.40%
Expected volatility range, maximum (as a percent) | 64.10% | 68.40% | 139.20%
Risk-free interest rate range, minimum (as a percent) | 0.45% | 0.28% | 0.11%
Risk-free interest rate range, maximum (as a percent) | 0.89% | 0.45% | 0.28%
Additional disclosures |  |  |
Weighted average estimated fair value of purchase rights (in dollars per share) | $ 0.75 | $ 1.01 | $ 1.64
Employee Stock Purchase Plan | Minimum |  |  |
Assumptions used to estimate fair value of awards |  |  |
Expected term | 6 months | 6 months | 6 months
Employee Stock Purchase Plan | Maximum |  |  |
Assumptions used to estimate fair value of awards |  |  |
Expected term | 12 months | 12 months | 12 months

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- Definition Unrecognized cost of unvested share-based compensation awards. + 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 (i) -URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901 + Details
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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://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (i) -URI http://asc.fasb.org/extlink&oid=96867065&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://www.xbrl.org/2003/role/presentationRef -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=96867065&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://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 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 weighted average grant-date fair value of options granted during the reporting period as calculated by applying the disclosed option pricing methodology. + 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 (d)(1) -URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901 + Details
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