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STOCKHOLDERS' EQUITY - PRICING MODEL ASSUMPTIONS (Details) - USD ($) | 12 Months Ended
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Dec. 31, 2015 | Dec. 31, 2014 | Dec. 31, 2013
Assumptions used to estimate fair value of awards |  |  |
Dividends paid to date | $ 0 |  |
Compensation cost related to unvested stock awards not yet recognized |  |  |
Compensation cost not yet recognized, net of estimated forfeitures (in dollars) | $ 13,321,000 |  |
Period for recognition of compensation cost on weighted average basis | 25 months |  |
Purchase Plans |  |  |
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) | 65.40% | 83.50% | 50.60%
Expected volatility range, maximum (as a percent) | 139.20% | 166.60% | 139.10%
Risk-free interest rate range, minimum (as a percent) | 0.11% | 0.06% | 0.09%
Risk-free interest rate range, maximum (as a percent) | 0.28% | 0.15% | 0.21%
Additional disclosures |  |  |
Weighted average estimated fair value of purchase rights (in dollars per share) | $ 1.64 | $ 2.10 | $ 0.75
Purchase Plans | Minimum |  |  |
Assumptions used to estimate fair value of awards |  |  |
Expected term | 6 months | 6 months | 6 months
Purchase Plans | Maximum |  |  |
Assumptions used to estimate fair value of awards |  |  |
Expected term | 12 months | 12 months | 12 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) | 87.40% | 89.80% | 74.20%
Expected volatility range, maximum (as a percent) | 88.40% | 92.20% | 79.20%
Risk-free interest rate range, minimum (as a percent) | 1.68% | 1.64% | 0.80%
Risk-free interest rate range, maximum (as a percent) | 1.71% | 1.92% | 1.97%
Expected term | 5 years 6 months | 5 years 6 months | 6 years
Additional disclosures |  |  |
Weighted average estimated fair value of employee stock options granted (in dollars per share) | $ 3.06 | $ 3.57 | $ 1.03

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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=6415400&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=6415400&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=6415400&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 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 S99 -Paragraph 1 -Subparagraph (SAB TOPIC 14.D.2) -URI http://asc.fasb.org/extlink&oid=27013229&loc=d3e301413-122809 Reference 2: 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=6415400&loc=d3e5070-113901 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Staff Accounting Bulletin (SAB) -Number Topic 14 -Section D -Subsection 2 + Details
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