SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Cash and Cash Equivalents (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2026	Jun. 30, 2025	Jun. 30, 2026	Jun. 30, 2025	Dec. 31, 2025
Cash and Cash Equivalent [Abstract]
Cash, cash equivalents and restricted cash and time deposits	$ 969,229		$ 969,229		$ 757,373
ACM Shanghai
Cash and Cash Equivalent [Abstract]
Cash payments for procurement of goods and services	191	$ 3,509	3,449	$ 6,183
United States
Cash and Cash Equivalent [Abstract]
Cash, cash equivalents and restricted cash and time deposits	314,256		314,256		107,184
Mainland China
Cash and Cash Equivalent [Abstract]
Cash, cash equivalents and restricted cash and time deposits	223,718		223,718		228,777
China Hong Kong
Cash and Cash Equivalent [Abstract]
Cash, cash equivalents and restricted cash and time deposits	427,310		427,310		421,104
South Korea
Cash and Cash Equivalent [Abstract]
Cash, cash equivalents and restricted cash and time deposits	2,674		2,674		241
Singapore
Cash and Cash Equivalent [Abstract]
Cash, cash equivalents and restricted cash and time deposits	$ 1,271		$ 1,271		$ 67