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SHARE-BASED COMPENSATION - Narrative (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands | 1 Months Ended | 3 Months Ended | 9 Months Ended |
--- | --- | --- | --- | ---
May 31, 2021 | Jun. 30, 2019 | Sep. 30, 2021 | Sep. 30, 2020 | Sep. 30, 2021 | Sep. 30, 2020 | Dec. 31, 2020
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |
Stock options outstanding (in shares) |  |  | 15,511 |  | 15,511 |  | 10,147
Share-based compensation expense |  |  | $ 1,168 | $ 453 | $ 3,385 | $ 1,351 |
Unrecognized compensation cost, non-vested stock options |  |  | $ 11,500 |  | $ 11,500 |  |
Weighted-average period to recognize non-vested stock options |  |  |  |  | 2 years 9 months 29 days |  |
Weighted-average grant-date fair value of stock options granted (in dollars per share) |  |  |  |  | $ 2.20 | $ 0.56 |
Employee |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |
Stock options outstanding (in shares) |  |  | 2,600 |  | 2,600 |  |
Stock options |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |
Shares of common stock available for issuance (in shares) |  |  | 15,511 |  | 15,511 |  | 10,147
2014 Stock Incentive Plan | Stock options |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |
Number of shares reserved for issuance (in shares) | 12,000 | 7,900 |  |  |  |  |
Shares of common stock available for issuance (in shares) |  |  | 12,000 |  | 12,000 |  |
Award vesting period |  |  |  |  | 4 years |  |
Period exercisable from date of grant |  |  |  |  | 10 years |  |
Stock options outstanding (in shares) |  |  | 12,700 |  | 12,700 |  |
2009 Stock Incentive Plan | Stock options |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |
Period exercisable from date of grant |  |  |  |  | 10 years |  |
Vested stock options outstanding (in shares) |  |  |  |  | 100 |  |
Vesting on the First Anniversary of Date of Grant | 2014 Stock Incentive Plan | Stock options |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |
Share-based compensation, vesting rate |  |  |  |  | 25.00% |  |
Vesting at End of Each Successive Three-Month Period Thereafter | 2014 Stock Incentive Plan | Stock options |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |
Share-based compensation, vesting rate |  |  |  |  | 6.25% |  |

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