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STOCKHOLDERS' EQUITY - AT MARKET ISSUANCE SALES AGREEMENT (Details) - USD ($) |  |  | 3 Months Ended | 6 Months Ended
--- | --- | --- | --- | ---
May 18, 2018 | Aug. 28, 2015 | Jun. 30, 2019 | Jun. 30, 2018 | Mar. 31, 2018 | Jun. 30, 2019 | Jun. 30, 2018
At Market Issuance Sales Agreements [Line Items] |  |  |  |  |  |  |
Net cash proceeds from issuance of common stock after deducting sales commissions and other offering costs |  |  | $ 101,000 | $ 82,306,000 | $ 1,553,000 |  |
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) |  |  |  |  |  |  | 13,195,106
Net cash proceeds from issuance of common stock after deducting sales commissions and other offering costs |  |  |  |  |  |  | $ 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) |  |  | 108,386 | 9,447,026 |  | 108,386 | 9,447,026
Net cash proceeds from issuance of common stock after deducting sales commissions and other offering costs |  |  | $ 101,000 | $ 36,208,000 |  | $ 101,000 | $ 36,208,000

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