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

Weighted average grant-date fair value of non-vested options exercised.

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Amount of expense for award under share-based payment arrangement. Excludes amount capitalized.

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The number of shares into which fully or partially vested stock options outstanding as of the balance sheet date can be currently converted under the option plan.

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The number of shares under options that were cancelled during the reporting period as a result of occurrence of a terminating event specified in contractual agreements pertaining to the stock option plan.

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Gross number of share options (or share units) granted during the period.

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The weighted average grant-date fair value of options granted during the reporting period as calculated by applying the disclosed option pricing methodology.

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Number of options outstanding, including both vested and non-vested options.

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Weighted average price at which grantees can acquire the shares reserved for issuance under the stock option plan.

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Weighted average price at which option holders acquired shares when converting their stock options into shares.

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Weighted average price at which grantees could have acquired the underlying shares with respect to stock options of the plan that expired.

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Weighted average remaining contractual term for option awards outstanding, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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Number of share options (or share units) exercised during the current period.

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