STOCK-BASED COMPENSATION - Fair value of options (Details)	12 Months Ended
Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Dividend yield	0.00%	0.00%	0.00%
Minimum
Expected option life (years)	5 years 6 months	5 years 5 months 23 days	5 years 3 months 29 days
Risk-free interest rate	1.91%	2.64%	1.93%
Expected stock price volatility	63.10%	55.10%	50.30%
Maximum
Expected option life (years)	6 years 3 months	6 years 3 months	7 years
Risk-free interest rate	2.58%	2.93%	2.33%
Expected stock price volatility	66.70%	60.60%	57.40%

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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- 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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- Definition

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

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- Definition

Expected term of award under share-based payment arrangement, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days.

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Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
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-Name Accounting Standards Codification
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-URI http://asc.fasb.org/extlink&oid=115993241&loc=d3e301413-122809

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