STOCK-BASED COMPENSATION - Schedule of Fair Value Assumptions (Details)	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Stock options
Assumptions used to calculate the fair value of awards granted
Risk-free interest rate	2.17%
Risk-free interest rate, minimum		1.65%	0.90%
Risk-free interest rate, maximum		1.93%	1.78%
Expected stock price volatility	61.94%
Expected stock price volatility, minimum		51.67%	48.39%
Expected stock price volatility, maximum		59.59%	50.96%
Stock options \| Minimum
Assumptions used to calculate the fair value of awards granted
Expected option term		4 years 2 months 27 days	4 years 2 months 23 days
Stock options \| Maximum
Assumptions used to calculate the fair value of awards granted
Expected option term		4 years 3 months 18 days	4 years 3 months 22 days
ESPP
Assumptions used to calculate the fair value of awards granted
Risk-free interest rate, minimum	2.05%	1.07%	0.49%
Risk-free interest rate, maximum	2.50%	1.45%	0.60%
Expected option term	6 months	6 months	6 months
Expected stock price volatility, minimum	56.10%	52.20%	48.10%
Expected stock price volatility, maximum	58.60%	82.00%	67.50%

No definition available.

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- Definition

The estimated measure of the 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 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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No definition available.

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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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No definition available.

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- Definition

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

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- Definition

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

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- Definition

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

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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.

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