SHARE BASED COMPENSATION - Fair value of options granted (Details) - Share options - $ / shares	12 Months Ended
Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Share-based Compensation Arrangement by Share-based Payment Award
Fair value per ordinary share at grant date						$ 0.008
Exercise price (in dollars per share)				$ 0.00001	$ 0.00001	$ 0.00001
Expected volatility			39.00%	39.00%	39.00%	40.00%
Contractual life	10 years	10 years	10 years	10 years	10 years	10 years
Expected dividend	0.00%	0.00%		0.00%	0.00%	0.00%
Minimum
Share-based Compensation Arrangement by Share-based Payment Award
Fair value per ordinary share at grant date	$ 0.274	$ 0.147	$ 0.034	$ 0.019	$ 0.008
Exercise price (in dollars per share)	$ 0.00001	$ 0.0001	$ 0.00001
Expected volatility	37.00%	35.00%
Risk free interest rate	3.00%	3.10%	3.00%	2.30%	2.50%	3.00%
Maximum
Share-based Compensation Arrangement by Share-based Payment Award
Fair value per ordinary share at grant date	$ 0.484	$ 0.405	$ 0.059	$ 0.030	$ 0.016
Exercise price (in dollars per share)	$ 0.274	$ 0.200	$ 0.040
Expected volatility	39.00%	38.00%
Risk free interest rate	3.40%	3.80%	3.20%	3.00%	3.10%	3.10%

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

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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 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

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.

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- Definition

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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- 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.

+ 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)(i)
-URI http://asc.fasb.org/extlink&oid=120381028&loc=d3e5070-113901

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-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=115993241&loc=d3e301413-122809

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