STOCK-BASED COMPENSATION - FAIR VALUE ASSUMPTIONS (Details)	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Stock options
Assumptions used to calculate the fair value of awards granted
Risk-free interest rate, minimum (as a percent)	1.65%	0.90%	1.04%
Risk-free interest rate, maximum (as a percent)	1.93%	1.78%	1.59%
Dividend yield	0.00%	0.00%	0.00%
Expected stock price volatility, minimum (as a percent)	51.67%	48.39%	44.78%
Expected stock price volatility, maximum (as a percent)	59.59%	50.96%	50.45%
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	4 years 3 months 18 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	4 years 3 months 22 days
ESPP
Assumptions used to calculate the fair value of awards granted
Risk-free interest rate, minimum (as a percent)	1.07%	0.49%	0.07%
Risk-free interest rate, maximum (as a percent)	1.45%	0.60%	0.41%
Dividend yield	0.00%	0.00%	0.00%
Expected option term	6 months	6 months	6 months
Expected stock price volatility, minimum (as a percent)	52.20%	48.10%	42.70%
Expected stock price volatility, maximum (as a percent)	82.00%	67.50%	58.20%

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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.

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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.

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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.

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No definition available.

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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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No definition available.

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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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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)(i)
-URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901

Reference 2: 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

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