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STOCKHOLDERS' EQUITY - SALES AGREEMENTS (Details) - USD ($) |  |  | 1 Months Ended | 12 Months Ended
--- | --- | --- | --- | ---
May 18, 2018 | Aug. 28, 2015 | Dec. 31, 2017 | Dec. 31, 2019 | Dec. 31, 2018 | Dec. 31, 2017
At Market Issuance Sales Agreements [Line Items] |  |  |  |  |  |
Net cash proceeds from issuance of common stock after deducting sales commissions and offering costs |  |  |  | $ 19,295,000 | $ 86,017,000 | $ 1,060,000
2015 Sales Agreement |  |  |  |  |  |
At Market Issuance Sales Agreements [Line Items] |  |  |  |  |  |
Aggregate offering price of common stock |  | $ 50,000,000 |  |  |  |
Maximum commission rate (as a percent) |  | 3.00% |  |  |  |
Issuance of common stock in connection with at market offering (in shares) |  |  | 614,230 |  | 13,195,106 |
Net cash proceeds from issuance of common stock after deducting sales commissions and offering costs |  |  | $ 1,060,000 |  | $ 47,651,000 |
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 (in shares) |  |  |  | 13,214,867 | 10,083,079 |
Net cash proceeds from issuance of common stock after deducting sales commissions and offering costs |  |  |  | $ 19,295,000 | $ 38,366,000 |

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