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STOCK-BASED COMPENSATION, Assumptions Used to Determine Fair Value of Share Options Granted (Details) - Employee Share Option [Member] - Service Period Based [Member]

3 Months Ended

Mar. 31, 2021

$ / shares

Fair Value of Options Granted [Abstract]

Volatility

48.53%

^[1]

Risk-free interest rate

1.00%

^[2]

Expected dividend

0.00%

^[3]

Minimum [Member]

Fair Value of Options Granted [Abstract]

Fair value of common share (in dollars per share)

$ 48.25

^[4]

Expected term in years

5 years 6 months

^[5]

Maximum [Member]

Fair Value of Options Granted [Abstract]

Fair value of common share (in dollars per share)

$ 51.07

^[4]

Expected term in years

6 years 3 months

^[5]

[1]	Volatility is calculated based on the historical volatility of ACM’s comparable companies in the period equal to the expected term of each grant.
[2]	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.
[3]	Expected dividend is assumed to be 0% as ACM has no history or expectation of paying a dividend on its common stock.
[4]	Equal to closing value on the grant date.
[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.

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The estimated dividend rate (a percentage of the share price) to be paid (expected dividends) to holders of the underlying shares over the option's term.

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
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The estimated measure of the percentage by which a share price is expected to fluctuate during a period. Volatility also may be defined as a probability-weighted measure of the dispersion of returns about the mean. The volatility of a share price is the standard deviation of the continuously compounded rates of return on the share over a specified period. That is the same as the standard deviation of the differences in the natural logarithms of the stock prices plus dividends, if any, over the period.

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The risk-free interest rate assumption that is used in valuing an option on its own shares.

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Price of a single share of a number of saleable stocks of a company.

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Expected term of award under share-based payment arrangement, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days.

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