SHARE-BASED COMPENSATION - Employee share options activity (Details) - 2010 Plan $ / shares in Units, ¥ in Thousands, $ in Thousands	12 Months Ended
Dec. 31, 2019 USD ($) $ / shares shares	Dec. 31, 2018 $ / shares shares	Dec. 31, 2019 CNY (¥) shares
Number of options
Outstanding, beginning balance \| shares	1,479,214
Exercised (in shares) \| shares	(33,869)
Outstanding, ending balance \| shares	1,445,345	1,479,214
Share-based Compensation Arrangement by Share-based Payment Award, Options, Vested and Expected to Vest, Outstanding, Number \| shares	1,445,345		1,445,345
Share-based Compensation Arrangement by Share-based Payment Award, Options, Vested and Expected to Vest, Outstanding, Weighted Average Exercise Price \| $ / shares	$ 0.51
Weighted average exercise price
Outstanding, beginning balance \| $ / shares	0.51
Exercised \| $ / shares	0.55
Outstanding, ending balance \| $ / shares	$ 0.51	$ 0.51
Weighted average remaining contractual term
Outstanding, December 31, 2019	1 year 3 months 18 days	2 years 2 months 12 days
Vested and expected to vest at December 31, 2018	1 year 3 months 18 days
Exercisable as of December 31, 2019	1 year 3 months 18 days
Aggregate intrinsic value
Outstanding, December 31, 2019	$ 1,005		¥ 6,997
SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsExercisableIntrinsicValue1 \| $	$ 1,005

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Amount by which the current fair value of the underlying stock exceeds the exercise price of options outstanding.

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Number of options outstanding, including both vested and non-vested options.

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A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

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Weighted average price at which grantees can acquire the shares reserved for issuance under the stock option plan.

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Number of fully vested and expected to vest options outstanding that can be converted into shares under option plan. Includes, but is not limited to, unvested options for which requisite service period has not been rendered but that are expected to vest based on achievement of performance condition, if forfeitures are recognized when they occur.

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Weighted-average exercise price, at which grantee can acquire shares reserved for issuance, for fully vested and expected to vest options outstanding. Includes, but is not limited to, unvested options for which requisite service period has not been rendered but that are expected to vest based on achievement of performance condition, if forfeitures are recognized when they occur.

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Weighted average price at which option holders acquired shares when converting their stock options into shares.

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Amount of difference between fair value of the underlying shares reserved for issuance and exercise price of vested portions of options outstanding and currently exercisable.

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Weighted average remaining contractual term for vested portions of options outstanding and currently exercisable or convertible, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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Weighted average remaining contractual term for option awards outstanding, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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Weighted average remaining contractual term for fully vested and expected to vest options outstanding, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days. Includes, but is not limited to, unvested options for which requisite service period has not been rendered but that are expected to vest based on achievement of performance condition, if forfeitures are recognized when they occur.

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Number of share options (or share units) exercised during the current period.

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Share-based Compensation Arrangement by Share-based Payment Award, Options, Aggregate Intrinsic Value [Abstract]

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