STOCK-BASED COMPENSATION, Assumptions Used to Determine Fair Value of Share Options Granted (Details) - $ / shares		9 Months Ended	12 Months Ended
		Sep. 30, 2020	Dec. 31, 2019
Employee Share Option [Member] | Market Based [Member]
Fair Value of Options Granted [Abstract]
Fair value of common share (in dollars per share)	[1]	$ 22.07
Volatility	[2]	45.10%
Risk-free interest rate	[3]	2.68%
Expected dividend	[4]	0.00%
Employee Share Option [Member] | Market Based [Member] | Minimum [Member]
Fair Value of Options Granted [Abstract]
Expected term in years	[5]	9 years 2 months 12 days
Employee Share Option [Member] | Market Based [Member] | Maximum [Member]
Fair Value of Options Granted [Abstract]
Expected term in years	[5]	9 years 9 months 18 days
Employee and Non-employee Stock Option [Member] | Service Period Based [Member]
Fair Value of Options Granted [Abstract]
Expected term in years	[5]		6 years 3 months
Expected dividend	[4]	0.00%	0.00%
Employee and Non-employee Stock Option [Member] | Service Period Based [Member] | Minimum [Member]
Fair Value of Options Granted [Abstract]
Fair value of common share (in dollars per share)	[1]	$ 22.07	$ 13.64
Expected term in years	[5]	5 years 6 months
Volatility	[2]	42.17%	39.91%
Risk-free interest rate	[3]	0.27%	1.69%
Employee and Non-employee Stock Option [Member] | Service Period Based [Member] | Maximum [Member]
Fair Value of Options Granted [Abstract]
Fair value of common share (in dollars per share)	[1]	$ 85.27	$ 16.81
Expected term in years	[5]	6 years 3 months
Volatility	[2]	48.15%	40.35%
Risk-free interest rate	[3]	0.82%	2.46%

[1] Fair value of Class A common stock value was the closing market price of the Class A common stock on the grant date.

[2] Volatility is calculated based on the historical volatility of the stock of companies comparable to ACM in the period equal to the expected term of each grant.

[3] Risk-free interest rate is based on the yields of U.S. Treasury securities with maturities similar to the expected term of the share options in effect at the time of grant.

[4] Expected dividend is assumed to be 0% as ACM has no history or expectation of paying a dividend on its common stock.

[5] Expected term of share options is based on the average of the vesting period and the contractual term for each grant according to Staff Accounting Bulletin 110.