STOCK-BASED COMPENSATION - Option activity (Details) - 2016 Omnibus Incentive Plan - $ / shares	6 Months Ended
Jun. 30, 2020
Number of options
Options outstanding, beginning of period (in shares)	4,803,146
Granted (in shares)	96,765
Exercised (in shares)	(900,180)
Forfeited/Canceled (in shares)	(163,935)
Options outstanding, end of period (in shares)	3,835,796
Vested and expected to vest, end of period (in shares)	3,835,796
Exercisable, end of period (in shares)	2,393,293
Weighted-average exercise price
Options outstanding, beginning period (in USD per share)	$ 12.29
Granted (in USD per share)	7.58
Exercised (in USD per share)	1.41
Forfeited (in USD per share)	9.76
Options outstanding, end of period (in USD per share)	14.83
Vested and expected to vest, end of period (in USD per share)	14.83
Exercisable, end of period (in USD per share)	$ 13.11

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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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 exercisable options that may 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 exercisable or convertible options. 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 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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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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Number of share options (or share units) exercised during the current period.

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