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STOCKHOLDERS' EQUITY - SALES AGREEMENTS (Details) - USD ($) |  |  | 12 Months Ended
--- | --- | --- | ---
Sep. 04, 2020 | May 18, 2018 | Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018
At Market Issuance Sales Agreements [Line Items] |  |  |  |  |
Issuance of common stock in connection with at market offering, net of issuance costs |  |  | $ 4,075,000 | $ 19,295,000 | $ 86,017,000
2018 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, net of issuance costs (in shares) |  |  | 3,496,616 |  |
Issuance of common stock in connection with at market offering, net of issuance costs |  |  | $ 4,075,000 |  |
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% |  |  |  |

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