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STOCKHOLDERS' EQUITY - AT MARKET ISSUANCE SALES AGREEMENT (Details) - USD ($) |  | 3 Months Ended
--- | --- | ---
Sep. 04, 2020 | Mar. 31, 2021 | Mar. 31, 2020
At Market Issuance Sales Agreements [Line Items] |  |  |
Net cash proceeds from issuance of common stock after deducting sales commissions and other offering costs |  | $ 16,234,000 | $ 686,000
2020 Sales Agreement |  |  |
At Market Issuance Sales Agreements [Line Items] |  |  |
Aggregate offering price of common stock | $ 100,000,000 |  |
Maximum commission rate (as a percent) | 3.00% |  |
Issuance of common stock in connection with at market offering (in shares) |  | 7,948,505 |
Net cash proceeds from issuance of common stock after deducting sales commissions and other offering costs |  | $ 16,234,000 |

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