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STOCKHOLDERS' EQUITY-Common stock and warrants (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions | Feb. 09, 2018 | Jun. 30, 2019 | Dec. 31, 2018 | Sep. 09, 2016
--- | --- | --- | --- | ---
Stockholders' equity |  |  |  |
Shares issued | 9.2 |  |  |
Common stock, par value (in dollars per share) | $ 0.0001 | $ 0.0001 | $ 0.0001 |
Share price (in dollars per share) | $ 6.90 |  |  |
Net proceeds issuance of common stock | $ 60.4 |  |  |
Equivalent number shares of the warrants |  |  |  | 4.7
Warrants Issued to Former Holders of Unsecured and Convertible Notes |  |  |  |
Stockholders' equity |  |  |  |
Equivalent number shares of the warrants |  |  |  | 4.7
Exercise price (in dollars per share) |  |  |  | $ 14.04

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