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STOCKHOLDERS' EQUITY (DEFICIT) - Equity Financing (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions |  | 1 Months Ended | 3 Months Ended | 9 Months Ended | 13 Months Ended |
--- | --- | --- | --- | --- | --- | ---
Jun. 01, 2021 | Feb. 28, 2021 | Nov. 30, 2019 | Sep. 30, 2021 | Jun. 30, 2021 | Mar. 31, 2021 | Sep. 30, 2020 | Jun. 30, 2020 | Mar. 31, 2020 | Sep. 30, 2021 | Sep. 30, 2020 | Oct. 31, 2021 | Dec. 31, 2020
Class of Stock [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Common stock, par value (in dollars per share) |  |  |  | $ 0.001 |  |  |  |  |  | $ 0.001 |  |  | $ 0.001
Aggregate sales price | $ 200.0 |  |  |  |  |  |  |  |  |  |  |  |
Issuance of common stock, issuance costs |  |  |  | $ 1.2 | $ 2.0 | $ 2.2 | $ 0.3 | $ 0.1 | $ 0.1 |  |  |  |
ATM Facility |  |  |  |  |  |  |  |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Common stock, par value (in dollars per share) |  |  | $ 0.001 |  |  |  |  |  |  |  |  |  |
Aggregate sales price |  |  | $ 35.0 |  |  |  |  |  |  |  |  |  |
Additional shares authorized for issuance (in shares) |  | $ 34.5 |  |  |  |  |  |  |  |  |  |  |
Commission fixed rate |  |  | 3.00% |  |  |  |  |  |  |  |  |  |
Proceeds from sale of stock |  |  |  | $ 38.2 |  |  | $ 8.2 |  |  | $ 175.0 | $ 16.2 |  |
Sale of stock, shares issued (in shares) |  |  |  | 9.8 |  |  | 7.0 |  |  | 56.9 | 16.8 |  |
Stock price per share (in dollars per share) |  |  |  | $ 4.01 |  |  | $ 1.21 |  |  | $ 3.17 | $ 0.99 |  |
Issuance of common stock, issuance costs |  |  |  | $ 1.2 |  |  |  |  |  | $ 5.4 |  |  |
ATM Facility | Forecast |  |  |  |  |  |  |  |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Additional shares authorized for issuance (in shares) |  |  |  |  |  |  |  |  |  |  |  | $ 50.0 |

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