STOCK-BASED COMPENSATION - Stock option activity (Details) - $ / shares	12 Months Ended
Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Weighted average grant date fair value
Shares issued			1,416,645
Employees
Weighted average grant date fair value
Shares issued	30,820
Time-Based Stock Options \| Employees
Number of shares
Outstanding at beginning of period (in shares)	2,281,308	2,243,588
Granted (in shares)	221,941	472,253
Forfeited (in shares)	(173,788)	(434,533)
Outstanding at end of period (in shares)	2,329,461	2,281,308	2,243,588
Exercisable at end of period (in shares)	1,054,059
Vested or expected to vest, at end of period (in shares)	2,329,461
Weighted average exercise price per share
Outstanding at beginning of period (in dollars per share)	$ 5.84	$ 5.29
Granted (in dollars per share)	17.99	7.93
Forfeited (in dollars per share)	14.65	5.29
Outstanding at end of period (in dollars per share)	6.36	5.84	$ 5.29
Exercisable at end of period (in dollars per share)	5.64
Vested or expected to vest, at end of period (in dollars per share)	6.36
Weighted average grant date fair value
Outstanding at beginning of period (in dollars per share)	2.54	2.39
Granted (in dollars per share)	7.02	3.14
Forfeited (in dollars per share)	5.70	2.39
Outstanding at end of period (in dollars per share)	2.74	$ 2.54	$ 2.39
Exercisable at end of period (in dollars per share)	2.50
Vested or expected to vest, at end of period (in dollars per share)	$ 2.74
Weighted average remaining contractual term, outstanding (in years)	7 years 7 months 6 days	9 years	9 years 9 months 18 days
Performance-Based Stock Options \| Employees
Number of shares
Outstanding at beginning of period (in shares)	3,502,947	3,583,923
Granted (in shares)	346,984	746,836
Forfeited (in shares)	(272,679)	(827,812)
Outstanding at end of period (in shares)	3,577,252	3,502,947	3,583,923
Weighted average exercise price per share
Outstanding at beginning of period (in dollars per share)	$ 5.86	$ 5.29
Granted (in dollars per share)	17.99	7.96
Forfeited (in dollars per share)	14.65	5.29
Outstanding at end of period (in dollars per share)	6.38	5.86	$ 5.29
Weighted average grant date fair value
Outstanding at beginning of period (in dollars per share)	1.61	1.50
Granted (in dollars per share)	4.36	2.01
Forfeited (in dollars per share)	3.59	1.50
Outstanding at end of period (in dollars per share)	$ 1.72	$ 1.61	$ 1.50
Weighted average remaining contractual term, outstanding (in years)	7 years 7 months 6 days	9 years	9 years 9 months 18 days

The weighted average grant-date fair value of options exercisable.

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The weighted average grant-date fair value of options vested and expected to vest.

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The number of shares into which fully or partially vested stock options outstanding as of the balance sheet date can be currently converted under the option plan.

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The weighted-average price as of the balance sheet date at which grantees can acquire the shares reserved for issuance on vested portions of options outstanding and currently exercisable under the stock option plan.

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The number of shares under options that were cancelled during the reporting period as a result of occurrence of a terminating event specified in contractual agreements pertaining to the stock option plan.

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Gross number of share options (or share units) granted during the period.

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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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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 grantees could have acquired the underlying shares with respect to stock options that were terminated.

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Weighted average per share amount at which grantees can acquire shares of common stock by exercise of options.

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Weighted average grant-date fair value of non-vested options forfeited.

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Weighted average grant-date fair value of non-vested options outstanding.

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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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Number of new stock issued during the period.

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