STOCK-BASED COMPENSATION, Share Option Activities (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019
Weighed Average Remaining Contractual Term [Abstract]
Stock-based compensation expense	$ 2,779	$ 1,557	$ 4,323	$ 2,919
Employee Share Option [Member]
Number of Option Share [Roll Forward]
Outstanding, beginning of period (in shares)			2,994,063
Granted (in shares)			778,399
Exercised (in shares)			(327,917)
Expired (in shares)			0
Forfeited/cancelled (in shares)			(40,515)
Outstanding, end of period (in shares)	3,404,030		3,404,030		2,994,063
Vested and exercisable (in shares)	2,061,793		2,061,793
Weighted Average Grant Date Fair Value [Abstract]
Outstanding at beginning of period (in dollars per share)			$ 2.59
Granted (in dollars per share)			11.98
Exercised (in dollars per share)			1.56
Expired (in dollars per share)			0
Forfeited/cancelled (in dollars per share)			4.94
Outstanding at end of period (in dollars per share)	$ 4.81		4.81		$ 2.59
Weighted Average Exercise Price [Abstract]
Outstanding, beginning of period (in dollars per share)			6.77
Granted (in dollars per share)			28.70
Exercised (in dollars per share)			4.33
Expired (in dollars per share)			0
Forfeited/cancelled (in dollars per share)			13.04
Outstanding, end of period (in dollars per share)	$ 11.95		$ 11.95		$ 6.77
Weighed Average Remaining Contractual Term [Abstract]
Outstanding weighed average remaining contractual term			7 years 1 month 20 days		7 years 18 days
Stock-based compensation expense	$ 2,651	1,329	$ 3,717	1,841
Unrecognized employee stock-based compensation expense	$ 9,697		$ 9,697		$ 4,712
Weighted-average period over which unrecognized compensation is expected to be recognized			1 year 11 months 15 days		1 year 5 months 19 days
Employee Share Option [Member] | ACM Shanghai [Member]
Number of Option Share [Roll Forward]
Outstanding, beginning of period (in shares)			0
Granted (in shares)			5,869,808
Exercised (in shares)			0
Expired (in shares)			0
Forfeited/cancelled (in shares)			(330,770)
Outstanding, end of period (in shares)	5,539,038		5,539,038		0
Vested and exercisable (in shares)	0		0
Weighted Average Grant Date Fair Value [Abstract]
Outstanding at beginning of period (in dollars per share)			$ 0
Granted (in dollars per share)			0.22
Exercised (in dollars per share)			0
Expired (in dollars per share)			0
Forfeited/cancelled (in dollars per share)			0.23
Outstanding at end of period (in dollars per share)	$ 0.22		0.22		$ 0
Weighted Average Exercise Price [Abstract]
Outstanding, beginning of period (in dollars per share)			0
Granted (in dollars per share)			1.87
Exercised (in dollars per share)			0
Expired (in dollars per share)			0
Forfeited/cancelled (in dollars per share)			1.87
Outstanding, end of period (in dollars per share)	$ 1.87		$ 1.87		$ 0
Weighed Average Remaining Contractual Term [Abstract]
Outstanding weighed average remaining contractual term			3 years 9 months 3 days		0 years
Stock-based compensation expense	$ 84	0	$ 250	0
Non-Employee Stock Option [Member]
Number of Option Share [Roll Forward]
Outstanding, beginning of period (in shares)			1,101,613
Granted (in shares)			20,000
Exercised (in shares)			(265,029)
Expired (in shares)			0
Forfeited/cancelled (in shares)			(111)
Outstanding, end of period (in shares)	856,473		856,473		1,101,613
Vested and exercisable (in shares)	819,819		819,819
Weighted Average Grant Date Fair Value [Abstract]
Outstanding at beginning of period (in dollars per share)			$ 0.82
Granted (in dollars per share)			10.29
Exercised (in dollars per share)			0.91
Forfeited/cancelled (in dollars per share)			0.30
Outstanding at end of period (in dollars per share)	$ 1.01		1.01		$ 0.82
Weighted Average Exercise Price [Abstract]
Outstanding, beginning of period (in dollars per share)			2.69
Granted (in dollars per share)			25.60
Exercised (in dollars per share)			3.34
Forfeited/cancelled (in dollars per share)			0.75
Outstanding, end of period (in dollars per share)	$ 3.02		$ 3.02		$ 2.69
Weighed Average Remaining Contractual Term [Abstract]
Outstanding weighed average remaining contractual term			5 years 1 month 9 days		5 years 10 months 6 days
Stock-based compensation expense	$ 44	$ 228	$ 356	$ 1,078
Unrecognized employee stock-based compensation expense	235		$ 235		$ 406
Weighted-average period over which unrecognized compensation is expected to be recognized			1 month 9 days		2 months 23 days
Non-Employee Stock Option [Member] | ACM Shanghai [Member]
Weighed Average Remaining Contractual Term [Abstract]
Unrecognized employee stock-based compensation expense	$ 911		$ 911
Weighted-average period over which unrecognized compensation is expected to be recognized			2 years 9 months