SHARE-BASED COMPENSATION - Stock Option Activity (Details) - Options to purchase Common Stock - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	6 Months Ended	12 Months Ended
Jun. 30, 2020	Dec. 31, 2019
Number of Options
Beginning balance (in shares)	3,497
Granted (in shares)	2,505
Exercised (in shares)	(17)
Forfeited or expired (in shares)	(826)
Ending balance (in shares)	5,159	3,497
Vested or expected to vest (in shares)	5,159
Exercisable (in shares)	2,328
Vested (in shares)	2,328
Weighted Average Exercise Price
Beginning balance (in usd per share)	$ 5.19
Granted (in usd per share)	0.89
Exercised (in usd per share)	0.76
Forfeited or expired (in usd per share)	4.84
Ending balance (in usd per share)	3.17	$ 5.19
Vested or expected to vest (in usd per share)	3.17
Exercisable (in usd per share)	4.70
Vested (in usd per share)	$ 4.70
Weighted average remaining contractual term and Aggregate intrinsic value
Outstanding (Weighted Average Remaining Contractual Term)	6 years 10 months 9 days	5 years 10 months 28 days
Vested or expected to vest (Weighted Average Remaining Contractual Term)	6 years 10 months 9 days	5 years 10 months 28 days
Outstanding (Aggregate Intrinsic Value)	$ 633,352	$ 0
Vested or expected to vest (Aggregate Intrinsic Value)	$ 633,352	$ 0
Exercisable (Weighted Average Remaining Contractual Term)	5 years 3 months 10 days
Vested (Weighted Average Remaining Contractual Term)	5 years 3 months 10 days
Exercisable (Aggregate Intrinsic Value)	$ 34,819
Vested (Aggregate Intrinsic Value)	$ 34,819

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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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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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Amount by which current fair value of underlying stock exceeds exercise price of 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 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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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 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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Weighted average remaining contractual term for fully vested and expected to vest exercisable or convertible options, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the 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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