STOCKHOLDERS' EQUITY - PRICING MODEL ASSUMPTIONS (Details) - USD ($)	12 Months Ended
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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Unrecognized cost of unvested share-based compensation awards.

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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.

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Amount of cash outflow in the form of ordinary dividends to common shareholders of the parent entity.

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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).

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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.

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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.

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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.

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The maximum risk-free interest rate assumption that is used in valuing an option on its own shares.

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The minimum risk-free interest rate assumption that is used in valuing an option on its own shares.

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The weighted average grant-date fair value of options granted during the reporting period as calculated by applying the disclosed option pricing methodology.

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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.

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