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STOCK-BASED COMPENSATION (Details) - USD ($) $ / shares in Units, $ in Thousands | 3 Months Ended | 9 Months Ended | 12 Months Ended
--- | --- | --- | ---
Sep. 30, 2019 | Sep. 30, 2018 | Sep. 30, 2019 | Sep. 30, 2018 | Dec. 31, 2018
ACM Shanghai [Member] |  |  |  |  |
Weighed Average Remaining Contractual Term [Abstract] |  |  |  |  |
Percentage of discount on purchase price for employee entities | 20.00% |  |  |  |
Employee Share Option [Member] |  |  |  |  |
Number of Option Share [Roll Forward] |  |  |  |  |
Outstanding, beginning of period (in shares) |  |  | 2,503,405 |  |
Granted (in shares) | 319,000 |  | 614,000 |  |
Exercised (in shares) |  |  | (105,113) |  |
Expired (in shares) |  |  | (628) |  |
Forfeited/cancelled (in shares) |  |  | (41,203) |  |
Outstanding, end of period (in shares) | 2,970,461 |  | 2,970,461 |  | 2,503,405
Vested and exercisable (in shares) | 1,673,780 |  | 1,673,780 |  |
Weighted Average Grant Date Fair Value [Abstract] |  |  |  |  |
Outstanding at beginning of period (in dollars per share) |  |  | $ 0.91 |  |
Granted (in dollars per share) |  |  | 6.30 |  |
Exercised (in dollars per share) |  |  | 0.60 |  |
Expired (in dollars per share) |  |  | 0.55 |  |
Forfeited/cancelled (in dollars per share) |  |  | 1.35 |  |
Outstanding at end of period (in dollars per share) | $ 2.55 |  | 2.55 |  | $ 0.91
Weighted Average Exercise Price [Abstract] |  |  |  |  |
Outstanding, beginning of period (in dollars per share) |  |  | 4.09 |  |
Granted (in dollars per share) |  |  | 16.39 |  |
Exercised (in dollars per share) |  |  | 2.08 |  |
Expired (in dollars per share) |  |  | 3.00 |  |
Forfeited/cancelled (in dollars per share) |  |  | 3.82 |  |
Outstanding, end of period (in dollars per share) | $ 6.71 |  | $ 6.71 |  | $ 4.09
Weighed Average Remaining Contractual Term [Abstract] |  |  |  |  |
Outstanding weighed average remaining contractual term |  |  | 7 years 3 months 11 days |  | 7 years 3 months 18 days
Stock-based compensation expense | $ 1,329 | $ 195 | $ 1,841 | $ 458 |
Unrecognized employee stock-based compensation expense | $ 5,009 |  | $ 5,009 |  | $ 2,424
Weighted-average period over which unrecognized compensation is expected to be recognized |  |  | 1 year 7 months 17 days |  | 1 year 7 months 13 days
Employee Share Option [Member] | Class A Common Stock [Member] |  |  |  |  |
Number of Option Share [Roll Forward] |  |  |  |  |
Forfeited/cancelled (in shares) | (31,339) |  | (31,339) |  |
Employee Share Option [Member] | ACM Shanghai [Member] |  |  |  |  |
Weighed Average Remaining Contractual Term [Abstract] |  |  |  |  |
Percentage of discount on purchase price for employee entities |  |  | 20.00% |  |
Stock-based compensation expense |  |  | $ 949 |  |
Non-Employee Share Option [Member] |  |  |  |  |
Number of Option Share [Roll Forward] |  |  |  |  |
Outstanding, beginning of period (in shares) |  |  | 1,212,374 |  |
Granted (in shares) |  |  | 0 |  |
Exercised (in shares) |  |  | (88,529) |  |
Expired (in shares) |  |  | 0 |  |
Forfeited/cancelled (in shares) |  |  | (22,232) |  |
Outstanding, end of period (in shares) | 1,101,613 |  | 1,101,613 |  | 1,212,374
Vested and exercisable (in shares) | 959,845 |  | 959,845 |  |
Weighted Average Grant Date Fair Value [Abstract] |  |  |  |  |
Outstanding at beginning of period (in dollars per share) |  |  | $ 0.78 |  |
Granted (in dollars per share) |  |  | 0 |  |
Exercised (in dollars per share) |  |  | 0.45 |  |
Expired (in dollars per share) |  |  | 0 |  |
Forfeited/cancelled (in dollars per share) |  |  | 0.55 |  |
Outstanding at end of period (in dollars per share) | $ 0.82 |  | 0.82 |  | $ 0.78
Weighted Average Exercise Price [Abstract] |  |  |  |  |
Outstanding, beginning of period (in dollars per share) |  |  | 2.57 |  |
Granted (in dollars per share) |  |  | 0 |  |
Exercised (in dollars per share) |  |  | 1.06 |  |
Expired (in dollars per share) |  |  | 0 |  |
Forfeited/cancelled (in dollars per share) |  |  | 3.00 |  |
Outstanding, end of period (in dollars per share) | $ 2.69 |  | $ 2.69 |  | $ 2.57
Weighed Average Remaining Contractual Term [Abstract] |  |  |  |  |
Outstanding weighed average remaining contractual term |  |  | 6 years 1 month 6 days |  | 6 years 7 months 28 days
Stock-based compensation expense | $ 228 | $ 216 | $ 1,078 | $ 2,313 |
Unrecognized employee stock-based compensation expense | $ 634 |  | $ 634 |  | $ 1,713
Weighted-average period over which unrecognized compensation is expected to be recognized |  |  | 3 months 29 days |  | 1 year 3 months 22 days
Non-Employee Share Option [Member] | Class A Common Stock [Member] | Director [Member] |  |  |  |  |
Number of Option Share [Roll Forward] |  |  |  |  |
Forfeited/cancelled (in shares) | (22,232) |  | (22,232) |  |

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