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STOCK-BASED COMPENSATION, Share Option Activities (Details) - USD ($) $ / shares in Units, $ in Thousands | 3 Months Ended | 12 Months Ended
--- | --- | ---
Mar. 31, 2021 | Mar. 31, 2020 | Dec. 31, 2020
Weighed Average Remaining Contractual Term [Abstract] |  |  |
Stock-based compensation expense | $ 1,210 | $ 689 |
Employee Share Option [Member] |  |  |
Number of Option Share [Roll Forward] |  |  |
Outstanding, beginning of period (in shares) | 3,191,411 |  |
Granted (in shares) | 106,400 |  |
Exercised (in shares) | (255,582) |  |
Forfeited/cancelled (in shares) | (11,989) |  |
Outstanding, end of period (in shares) | 3,030,240 |  | 3,191,411
Vested and exercisable (in shares) | 1,809,322 |  |
Weighted Average Grant Date Fair Value [Abstract] |  |  |
Outstanding at beginning of period (in dollars per share) | $ 5.13 |  |
Granted (in dollars per share) | 50.67 |  |
Exercised (in dollars per share) | 1.78 |  |
Forfeited/cancelled (in dollars per share) | 27.45 |  |
Outstanding at end of period (in dollars per share) | 6.93 |  | $ 5.13
Weighted Average Exercise Price [Abstract] |  |  |
Outstanding, beginning of period (in dollars per share) | 12.73 |  |
Granted (in dollars per share) | 111.98 |  |
Exercised (in dollars per share) | 4.55 |  |
Forfeited/cancelled (in dollars per share) | 62.58 |  |
Outstanding, end of period (in dollars per share) | $ 16.70 |  | $ 12.73
Weighed Average Remaining Contractual Term [Abstract] |  |  |
Outstanding weighed average remaining contractual term | 7 years 2 months 19 days |  | 7 years 1 month 17 days
Stock-based compensation expense | $ 1,085 | 431 |
Unrecognized employee stock-based compensation expense | $ 12,340 |  | $ 8,733
Weighted-average period over which unrecognized compensation is expected to be recognized | 1 year 11 months 1 day |  | 1 year 10 months 20 days
Employee Share Option [Member] | ACM Shanghai [Member] |  |  |
Number of Option Share [Roll Forward] |  |  |
Outstanding, beginning of period (in shares) | 5,423,654 |  |
Granted (in shares) | 0 |  |
Exercised (in shares) | 0 |  |
Expired (in shares) | 0 |  |
Forfeited/cancelled (in shares) | (46,154) |  |
Outstanding, end of period (in shares) | 5,377,500 |  | 5,423,654
Vested and exercisable (in shares) | 0 |  |
Weighted Average Grant Date Fair Value [Abstract] |  |  |
Outstanding at beginning of period (in dollars per share) | $ 0.23 |  |
Granted (in dollars per share) | 0 |  |
Exercised (in dollars per share) | 0 |  |
Expired (in dollars per share) | 0 |  |
Forfeited/cancelled (in dollars per share) | 0.24 |  |
Outstanding at end of period (in dollars per share) | 0.24 |  | $ 0.23
Weighted Average Exercise Price [Abstract] |  |  |
Outstanding, beginning of period (in dollars per share) | 1.89 |  |
Granted (in dollars per share) | 0 |  |
Exercised (in dollars per share) | 0 |  |
Expired (in dollars per share) | 0 |  |
Forfeited/cancelled (in dollars per share) | 2.00 |  |
Outstanding, end of period (in dollars per share) | $ 2.00 |  | $ 1.89
Weighed Average Remaining Contractual Term [Abstract] |  |  |
Outstanding weighed average remaining contractual term | 3 years 3 months 3 days |  | 3 years 6 months
Stock-based compensation expense | $ 85 | 86 |
Non-Employee Stock Option [Member] |  |  |
Number of Option Share [Roll Forward] |  |  |
Outstanding, beginning of period (in shares) | 836,038 |  |
Granted (in shares) | 0 |  |
Exercised (in shares) | (54,358) |  |
Expired (in shares) | 0 |  |
Forfeited/cancelled (in shares) | (76) |  |
Outstanding, end of period (in shares) | 781,604 |  | 836,038
Vested and exercisable (in shares) | 773,139 |  |
Weighted Average Grant Date Fair Value [Abstract] |  |  |
Outstanding at beginning of period (in dollars per share) | $ 1.02 |  |
Granted (in dollars per share) | 0 |  |
Exercised (in dollars per share) | 1.51 |  |
Expired (in dollars per share) | 0 |  |
Forfeited/cancelled (in dollars per share) | 0.30 |  |
Outstanding at end of period (in dollars per share) | 0.99 |  | $ 1.02
Weighted Average Exercise Price [Abstract] |  |  |
Outstanding, beginning of period (in dollars per share) | 3.07 |  |
Granted (in dollars per share) | 0 |  |
Exercised (in dollars per share) | 5.44 |  |
Expired (in dollars per share) | 0 |  |
Forfeited/cancelled (in dollars per share) | 0.75 |  |
Outstanding, end of period (in dollars per share) | $ 2.91 |  | $ 3.07
Weighed Average Remaining Contractual Term [Abstract] |  |  |
Outstanding weighed average remaining contractual term | 4 years 7 months 20 days |  | 4 years 11 months 1 day
Stock-based compensation expense | $ 40 | 172 |
Unrecognized employee stock-based compensation expense | $ 155 |  | $ 195
Weighted-average period over which unrecognized compensation is expected to be recognized | 29 days |  | 1 month 2 days
Non-Employee Stock Option [Member] | ACM Shanghai [Member] |  |  |
Weighed Average Remaining Contractual Term [Abstract] |  |  |
Unrecognized employee stock-based compensation expense | $ 779 | $ 822 |
Weighted-average period over which unrecognized compensation is expected to be recognized | 2 years 3 months 18 days | 2 years 6 months |

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