SHARE-BASED COMPENSATION - Fair value of the options at the respective grant (Details) - 2015 Plan - Non-employee options	12 Months Ended
Dec. 31, 2020 $ / shares	Dec. 31, 2019 $ / shares	Dec. 31, 2018 $ / shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected volatility, minimum	37.00%	37.00%	36.00%
Expected volatility, maximum	41.00%	40.00%	38.00%
Risk-free interest rate, minimum	1.70%	2.40%	3.50%
Risk-free interest rate, maximum	2.30%	3.60%	3.70%
Exercise multiples	2.2	2.2	2.2
Expected dividend yield	0.00%	0.00%	0.00%
Minimum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Fair value of underlying ordinary shares	$ 4.08	$ 2.26	$ 2.01
Fair value of share option	3.32	1.59	1.35
Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Fair value of underlying ordinary shares	5.79	3.87	2.26
Fair value of share option	$ 5.03	$ 3.14	$ 1.59

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- Definition

Represents share based compensation arrangement by share based payment award fair value assumptions weighted average fair value.

+ References

No definition available.

+ Details

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- Definition

Agreed-upon price for the exchange of the underlying asset relating to the share-based payment award.

+ References

No definition available.

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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://fasb.org/us-gaap/role/ref/legacyRef
-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=120381028&loc=d3e5070-113901

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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.

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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.

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- Definition

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

+ References

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.

+ References

No definition available.

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- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

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